Faculty Profiles - SAKAI Osamu

Papers - SAKAI Osamu

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Skin Diagnostic Method Using Fontana-Masson Stained Images of Stratum Corneum Cells

Shuto Hasegawa, Koichiro Enomoto, Taeko Mizutani, Yuri Okano, Takenori Tanaka, Osamu Sakai

IEICE Transactions on Information and Systems E107.D ( 8 ) 1070 - 1078 2024.8

Publishing type：Research paper (scientific journal)

Melanin, which is responsible for the appearance of spots and freckles, is an important indicator in evaluating skin condition. To assess the efficacy of cosmetics, skin condition scoring is performed by analyzing the distribution and amount of melanin from microscopic images of the stratum corneum cells. However, the current practice of diagnosing skin condition using stratum corneum cells images relies heavily on visual evaluation by experts. The goal of this study is to develop a quantitative eval- uation system for skin condition based on melanin within unstained stratum corneum cells images. The proposed system utilizes principal component regression to perform five-level scoring, which is then compared with vi- sual evaluation scores to assess the system's usefulness. Additionally, we evaluated the impact of indicators related to melanin obtained from images on the scores, and verified which indicators are effective for evaluation. In conclusion, we confirmed that scoring is possible with an accuracy of more than 60% on a combination of several indicators, which is comparable to the accuracy of visual assessment.

DOI： 10.1587/transinf.2023EDP7256

Maze-solving in a plasma system based on functional analogies to reinforcementlearning model

Osamu Sakai, Toshifusa Karasaki, Tsuyohito Ito, Tomoyuki Murakami, Manabu Tanaka, Makoto Kambara, Satoshi Hirayama

PLoS ONE 19 ( 4 April ) 2024.4

Publishing type：Research paper (scientific journal)

Maze-solving is a classical mathematical task, and is recently analogously achieved using various eccentric media and devices, such as living tissues, chemotaxis, and memristors. Plasma generated in a labyrinth of narrow channels can also play a role as a route finder to the exit. In this study, we experimentally observe the function of maze-route findings in a plasma system based on a mixed discharge scheme of direct-current (DC) volume mode and alternative-current (AC) surface dielectric-barrier discharge, and computationally generalize this function in a reinforcement-learning model. In our plasma system, we install two electrodes at the entry and the exit in a square lattice configuration of narrow channels whose cross section is 1×1 mm2 with the total length around ten centimeters. Visible emissions in low-pressure Ar gas are observed after plasma ignition, and the plasma starting from a given entry location reaches the exit as the discharge voltage increases, whose route converging level is quantified by Shannon entropy. A similar short-path route is reproduced in a reinforcement- learning model in which electric potentials through the discharge voltage is replaced by rewards with positive and negative sign or polarity. The model is not rigorous numerical representation of plasma simulation, but it shares common points with the experiments along with a rough sketch of underlying processes (charges in experiments and rewards in modelling). This finding indicates that a plasma-channel network works in an analog computing function similar to a reinforcement-learning algorithm slightly modified in this study.

DOI： 10.1371/journal.pone.0300842

MACHINE LEARNING EVALUATION OF FRUIT RIPENESS WITH MULTIDIMENSIONAL SPARSE-DATASET CALIBRATION

Shinya Ueno, Osamu Sakai, Takuya Morimoto

Applied Engineering in Agriculture 40 ( 5 ) 501 - 514 2024

Publishing type：Research paper (scientific journal)

. In agriculture, harvests of fruits and vegetables depend on their ripeness, in which farmers should rely not only on the eye-sensed outlook but rigorously on constituents inside the targets, like sugar content. So far, detail measurements on the outlook color of fruits have been widely reported to detect suitable harvest times, where non-destructive instruments, which are costly in general agriculture fields, work well for color estimation whose resolution level is much higher than human eyes. In this study, we propose a scheme with a calibration procedure for sugar content prediction from outlook-color quantities, which is applicable even if a sensor includes systematic errors in output signals or the base dataset includes some datapoint sparsity. Most multidimensional data, which are both in agriculture and in other general cases, are imperfect in terms of several aspects, but they can be useful when we take appropriate sparsity detection and setting of rebalancing weights into account for machine learning procedures. We newly introduce a complex-network method for this purpose, and after the color calibration, we successfully obtain accurate hue and chroma, which are quantities representing a position on calibrated color coordinate, to predict °Brix sugar content in Japanese pears.

DOI： 10.13031/aea.15899

Formation of a Dirac Cone and Dynamic Control of its Half-Metallic Properties Using Double-Layer Ring Dipole Arrays Reviewed

Itami G., Sakai O.

Physical Review Applied 20 ( 3 ) 2023.9

Detection of Sparsity in Multidimensional Data Using Network Degree Distribution and Improved Supervised Learning with Correction of Data Weighting Reviewed

Ueno S., Sakai O.

Studies in Computational Intelligence 1077 SCI 390 - 401 2023.1

Language：English Publishing type：Research paper (scientific journal) Publisher：Studies in Computational Intelligence

Multidimensional data are representatives in a wide range of applications, from those in the latest state-of-the-art science and technology to specific social issues. And they have been subject to analysis using methods such as regression analysis and machine learning. However, they are rarely obtained as complete data and contain more or less biases and deficiencies. In this study, we form a network from a multidimensional dataset and use its degree distribution to detect data sparsity. Although model analysis based on the degree distribution has been conducted for many years, sparsity detection has not been a target of the degree distribution analysis. Furthermore, we attempt to increase the accuracy and precision of supervised learning by applying regressive weighting according to node grouping in the degree distribution spectrum. By making use of this algorithm, we can expand the range of utilization of incomplete data together with other promising progresses in complex networks.

DOI： 10.1007/978-3-031-21127-0_32

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Independent control method for plasmonic skin depth based on transformation from spoof surface plasmon polaritons to bound states in the continuum Reviewed

Itami G., Sakai O.

Physical Review B 106 ( 24 ) 2022.12

Language：English Publishing type：Research paper (scientific journal) Publisher：Physical Review B

A metal plate array (MPA) is the complementary structure of a metal hole array and forms spoof surface plasmon polaritons (SSPPs) supported by parallel plate modes in a unit cell. A ring dipole array, which is a periodic structure designed by providing an aperture in the conductive part of the unit cell of an MPA, has the potential to produce bound states in the continuum (BICs). In this paper, the transformation from SSPPs to BICs is confirmed using these two structures by broadening the aperture in the unit cell. Making use of this phenomenon, control of the skin depth of surface modes is numerically demonstrated while retaining the resonant frequency, and the fitting values of the plasmonic skin depth are in good agreement with theoretical values.

DOI： 10.1103/PhysRevB.106.245406

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Foundations of plasma photonics: lamps, lasers, and electromagnetic devices Reviewed International coauthorship

Nijdam S., Desai K.V., Park S.J., Sun P.P., Sakai O., Lister G., Eden J.G.

Plasma Sources Science and Technology 31 ( 12 ) 2022.12

Language：English Publishing type：Research paper (scientific journal) Publisher：Plasma Sources Science and Technology

The enduring contributions of low temperature plasmas to both technology and science are largely a result of the atomic, molecular, and electromagnetic (EM) products they generate efficiently such as electrons, ions, excited species, and photons. Among these, the production of light has arguably had the greatest commercial impact for more than a century, and plasma sources emitting photons over the portion of the EM spectrum extending from the microwave to soft x-ray regions are currently the workhorses of general lighting (outdoor and indoor), photolithography for micro- and nano-fabrication of electronic devices, disinfection, frequency standards (atomic clocks), lasers, and a host of other photonic applications. In several regions of the EM spectrum, plasma sources have no peer, and this article is devoted to an overview of the physics of several selected plasma light sources, with emphasis on thermal arc and fluorescent lamps and the more recently-developed microcavity plasma lamps in the visible and ultraviolet/vacuum ultraviolet regions. We also briefly review the physics of plasma-based metamaterials and plasma photonic crystals in which low temperature plasma tunes the EM properties of filters, resonators, mirrors, and other components in the microwave, mm, and sub-mm wavelength regions.

DOI： 10.1088/1361-6595/ac8448

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Complexity visualization, dataset acquisition, and machine-learning perspectives for low-temperature plasma: A review Reviewed

Sakai O., Kawaguchi S., Murakami T.

Japanese Journal of Applied Physics 61 ( 7 ) 2022.7

Language：English Publishing type：Research paper (scientific journal) Publisher：Japanese Journal of Applied Physics

Low-temperature plasma plays various roles in industrial material processing as well as provides a number of scientific targets, both from theoretical and experimental points of view. Such rich features in variety are based on its complexities, arising from diverse parameters in constituent gas-phase species, working gas pressure, input energy density, and spatial boundaries. When we consider causalities in these complexities, direct application of machine-learning methods is not always possible since levels of complexities are so high in comparison with other scientific research targets. To overcome this difficulty, progresses in plasma diagnostics and data acquisition systems are inevitable, and the handling of a large number of data elements is one of the key issues for this purpose. In this topical review, we summarize previous and current achievements of visualization, acquisition, and analysis methods for complex plasma datasets which may open a scientific and technological category mixed with rapid machine-learning advancements and their relevant outcomes. Although these research trends are ongoing, many reports published so far have already convinced us of various expanding aspects of low-temperature plasma leading to the potential for scientific progress as well as developments of intellectual design in industrial plasma processes.

DOI： 10.35848/1347-4065/ac76fa

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Data Driven Calibration of Color-Sensitive Optical Sensor by Supervised Learning for Botanical Application

Shinya Ueno, Osamu Sakai

IEEE Sensors Journal 22 ( 12 ) 11915 - 11927 2022.6

Publishing type：Research paper (scientific journal)

Color is a fundamental and informative element in botanical applications, as it is in our various daily activities. The conventional ways of evaluating colors based on human visibility are person-specific and thus likely to be biased, whereas color sensing equipment with high sensitivity and accuracy is often a very expensive product. Recently, nonlinear data conversions using machine learning, such as supervised learning, have become available in various fields, and such techniques may be useful for calibrations of low-cost, poor-accuracy sensors. In this study, we propose a method for color-value calibration applicable to low-cost sensors by supervised learning. Specifically, after a supervised learning procedure using training data based on a large number of color samples positioned on a two-dimensional color plane, we improve calibration accuracy over mixed-color areas containing red, green and blue values. We apply this calibrated sensor to estimating the ripeness level of Japanese pears, and the results indicate that the chroma shown in a color coordinate is a good measure for this purpose. In another botanical case, temporal changes in color values of autumn tree leaves in chroma and hue could be successfully detected. This technology is aimed at use with low-cost consumer electronic devices, and it is applicable to color sensing in other areas beyond in the botanical field, thus showing high potential for other kinds of low-cost sensors.

DOI： 10.1109/JSEN.2022.3171221

Synthesis of Electromagnetic Wave-Absorbing Co-Ni Alloys and Co-Ni Core-Shell Structured Nanoparticles Reviewed

Ishijima M., Takada T., Cuya Huaman J.L., Mizutomi T., Sakai O., Shinoda K., Uchikoshi M., Mamiya H., Suzuki K., Miyamura H., Balachandran J.

Inorganic Chemistry 2022.1

Language：English Publishing type：Research paper (scientific journal) Publisher：Inorganic Chemistry

Co-Ni alloy nanoparticles, a potential candidate for microwave absorption material, were successfully synthesized by tuning the reduction timing of Co and Ni ions by introducing oleylamine as a complexing agent and 1-heptanol as a reducing solvent. The formation mechanism elucidated using time-resolved sampling and in situ X-ray absorption spectroscopy (XAS) and ultraviolet-visible (UV-vis) spectrophotometry measurements suggested that the delay in the reduction of Co ions via complexation with oleylamine facilitated the co-reduction of Co with Ni ions and led to the formation of Co-Ni alloys. The successful synthesis of Co-Ni alloys experimentally confirmed the differences in magnetic properties between alloy and core-shell structured Co50Ni50 particles. Further, the syntheses of Co-Ni alloys with different compositions were also possible using the above technique. In addition, the microwave absorption properties were measured using the free-space method utilizing a vector network analyzer of Co50Ni50-polyethylene composite with different sheet thicknesses. A reflection loss (RL) value of -25.7 dB at 13.6 GHz for the alloy structure was more significant than the core-shell counterpart. The above values are high compared to results reported in the past. The validity of the measurements was confirmed by utilizing the parameter retrieval method to extract permittivity and permeability from the scattering parameter (S) and recalculation of the RL as a function of frequency.

DOI： 10.1021/acs.inorgchem.2c02633

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Logistics Route Planning in Agent-Based Simulation and Its Optimization Represented in Higher-Order Markov-Chain Networks Reviewed

Ikai R., Miyagi S., Sakai O.

Springer Proceedings in Complexity 38 - 50 2021.7

Language：English Publishing type：Research paper (scientific journal) Publisher：Springer Proceedings in Complexity

Route planning in logistics, in which multiple pickup and delivery positions exist in a road network, is a complicated task with many choices in a path selection and their influences on the following procedures. Solving this task by multi-agent simulations, we examine the route optimization process by monitoring motions in networks based on simple or higher-order Markov chains (MCs). Agent footprints in the networks, which spread over the entire network at the initial phase, converge on small number of edges as the transportation path gets shortened. When we increase the order of MCs in agent mobilities, the MC networks are enlarged and possess a large number of nodes and edges with structural regularity so that one node contains partial trace history, while the optimized route that frequently overlaps edge groups with high transition probabilities is equivalent to a smaller and more noticeable subgraph around a local optimal solution. In other words, this localization of the traces indicates a convergence level in optimization, which can be a measure for route planning in logistics.

DOI： 10.1007/978-3-030-81854-8_4

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In-vacuum active colour sensor and wireless communication across a vacuum-air interface Reviewed

Sakai O., Kitagawa T., Sakurai K., Itami G., Miyagi S., Noborio K., Taguchi K.

Scientific Reports 11 ( 1 ) 2021.1

Language：English Publishing type：Research paper (scientific journal) Publisher：Scientific Reports

© 2021, The Author(s). In situ sensing with wireless digital-data transfer is a potential processing scheme that works very closely to the location of an event monitored by a sensor and converts the sensor’s raw output into digitized and informative small-volume bits, as suggested by recent proposals for edge computing and the Internet of Things (IoT). Colour perception may be a target of in situ sensor data acquisition; however, in contrast to from other sensing devices, colour sensors that detect visible light signals are usually located away from light-emitting sources, collecting light transmitting through the space and attenuating it in some manner. For example, in a vacuum chamber whose gas pressure is much less than the ambient atmosphere in which the sensors usually work, there are many veiled light sources, such as discharge plasma, for various industrial purposes including nanoscale manufacturing. In this study, we designed an in-vacuum colour sensor that can work with analogue-to-digital conversion and transfer data by wireless communication; this sensor is active in a low-pressure plasma chamber, detecting light signals and transferring them to a personal computer located outside the vacuum chamber. In addition to detecting lights with controlled spectra from outside successfully, we achieved complete operation of our in-vacuum active sensor for plasma emissions generated at 100 Pa. Comparing the signals with data from simultaneous monitoring by a monochromator, we established that the recorded signals arose from the plasma, confirming successful direct detection of low-pressure plasma emissions without any filtering effects between the sensor and the target object.

DOI： 10.1038/s41598-020-80501-z

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Order Estimation of Markov-Chain Processes in Complex Mobility Network Embedded in Vehicle Traces Reviewed

Yamamoto K., Miyagi S., Sakai O.

Studies in Computational Intelligence 944 231 - 242 2021.1

Language：English Publishing type：Research paper (scientific journal) Publisher：Studies in Computational Intelligence

Vehicle mobility in urban traffic systems is complex, partly because it reflects mobility of a human who drives a vehicle, and partly because it depends on many roles which the vehicle plays. Previous studies on human mobility revealed that it includes Lévy-flights-like motions and memoryless deterministic walks as well as random walks, but the mobility of vehicles may be more biased due to their functions. Focusing our research target on a sightseeing vehicle with sufficiently limited functions, we show a method to measure regularity of visitation patterns, quantified by order(s) of Markov chains in their mobility. Graphs of higher-order Markov chains, which are representatives of mobility in a network style, possess statistical properties; in our observation dataset, they include degree distributions similar to scale-free networks. The detection of mobility in real social experiments, which is also assumed on these graphs, yields the order of Markov chains inside it with its comparison with the results of agent-based simulations. Centrality indices of the mobility networks well coincide with prediction of these analytical and numerical results.

DOI： 10.1007/978-3-030-65351-4_19

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Statistics of Growing Chemical Network Originating from One Molecule Species and Activated by Low-Temperature Plasma Reviewed

Mizui Y., Miyagi S., Sakai O.

Studies in Computational Intelligence 944 398 - 409 2021.1

Language：English Publishing type：Research paper (scientific journal) Publisher：Studies in Computational Intelligence

Chemistry in plasma is complicated because it has so many reactions in parallel and in series. A complex network is suitable for the visualization and the analysis of its complexity. A numerical calculation based on hundreds of rate equations is a typical tool for plasma chemistry, but such a computational process does not clarify the undergoing physical and chemical properties that stabilize many industrial plasma processes for a number of applications. In this study, we focus on low-temperature plasma in which high-energy electrons are activators for chemical reactions, and investigate the origin of the stability by examining the statistical properties of networks for silane (SiH ) plasma. There is only one seed species in the initial space, SiH , which is surrounded by high-energy electrons. SiH is decomposed into several fragments composed of Si and/or H atoms with possible charges, and such radical and ion species are decomposed or synthesized into other species, leading to the formation of temporal reaction networks in chemistry. With the effects of rate constants that determine chemical reaction rates, we create temporal networks and observe preferential attachments that induce a new reaction in a transient state. The centrality indices for participant species and degree distributions reveal what is occurring in this complex system, and during the sequential process we observe an exponential-tail degree distribution, which is a significant source of reaction stability. 4 4 4

DOI： 10.1007/978-3-030-65351-4_32

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Low-Cost Color-Sensitive Optical Sensor Calibrated by Sparse Training Data

Shinya Ueno, Osamu Sakai

2021 IEEE 10th Global Conference on Consumer Electronics, GCCE 2021 371 - 372 2021

Publishing type：Research paper (international conference proceedings)

Color is one of the important practical indices as an aid in a typical daily life. To detect colors qualitatively, inexpensive sensors are effective for promoting applicable fields, which include tests of products in factories and leveling of agriculture harvest. In our previous study, we have achieved low-cost but accurate sensors by calibrated sensors using a simple supervised learning, although, its training data has relatively sparse parameter spaces as compared with other areas, which is a factor of lowering the accuracy after calibration. In this study, when there are sparse areas in the two-dimensional training data, we show that the sparse areas can be reinforced accurately by weighting the data according to its density profile. We confirmed that this method is effective for supervised learning in calibration of color sensors.

DOI： 10.1109/GCCE53005.2021.9621774

Analysis of Visualized Complex Reaction Network in Low-Temperature Molecular Plasma

Osamu Sakai, Yasutaka Mizui, Kyosuke Nobuto, Shigeyuki Miyagi

IEEE International Symposium on Semiconductor Manufacturing Conference Proceedings 2020-December 2020.12

Publishing type：Research paper (international conference proceedings)

It is quite frequent that a factorial fabrication process includes very complex systems, leading to difficulties in its regulation, estimation and prediction. To overcome these difficulties, causality in the complex systems is a key issue, which has not been frequently stressed but is of importance for effective performance of machine learning. One of the examples with such complexity is plasma and its chemistry, where they are in processes of dry etching and plasma chemical vapor deposition. So far, we successfully visualized the complexities using graphs or networks, where nodes represent elements and edges imply interactions between them. In this study, focusing on silane (SiH4) and methane (CH4) low-Temperature molecular plasma chemistry, we clarify roles of species in the chemical reaction network, like reactants, intermediates and products, where a species is a node in this species network and a reactant-product pair is an edge. This distinction is straightforward for selection of reactants as input and products as output variables. We also show and discuss another network, reaction network, in which a reactant-product pair is an edge and its size is so huge that its network statistics is categorized by complex network science. By visualizing and analyzing a complex chemical reaction network in molecular plasma, we obtain useful information for parameter regulation in real processes and also identification of input/output variables for machine learning of a given process.

DOI： 10.1109/ISSM51728.2020.9377518

Analysis and observation of the breakdown of Babinet’s principle in complementary spoof surface plasmon polariton structures Reviewed

Itami G., Sakai O.

Scientific Reports 10 ( 1 ) 2020.12

Language：English Publishing type：Research paper (scientific journal) Publisher：Scientific Reports

© 2020, The Author(s). A metal plate array (MPA) which is a structure complimentary to a metal hole array (MHA), supports spoof surface plasmon polaritons (SSPP) as well as an MHA does. Babinet’s principle attributes the phenomenon of duality to transmission characteristics of the complimentary impedance surfaces because of the symmetry of the behaviors of electric and magnetic fields. However, it is also a fact that the complimentary structures do not follow this principle if they have wavelength-size thickness, because electromagnetic waves do not treat such thick structures as a boundary surface but as propagation spaces with the specific boundaries such as a waveguide which shows SSPP modes. If the thickness is so small that it is negligible, Babinet’s principle is still valid, while it has been uncertain how the layer thickness works to break the principle as it is increased. The unconfirmed transformation is revealed analytically and experimentally with the use of MPAs and MHAs of varying thicknesses.

DOI： 10.1038/s41598-020-67923-5

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Analysis and observation of the breakdown of Babinet’s principle in complementary spoof surface plasmon polariton structures

Go Itami, Osamu Sakai

Scientific Reports 10 ( 1 ) 2020.12

Publishing type：Research paper (scientific journal)

A metal plate array (MPA) which is a structure complimentary to a metal hole array (MHA), supports spoof surface plasmon polaritons (SSPP) as well as an MHA does. Babinet’s principle attributes the phenomenon of duality to transmission characteristics of the complimentary impedance surfaces because of the symmetry of the behaviors of electric and magnetic fields. However, it is also a fact that the complimentary structures do not follow this principle if they have wavelength-size thickness, because electromagnetic waves do not treat such thick structures as a boundary surface but as propagation spaces with the specific boundaries such as a waveguide which shows SSPP modes. If the thickness is so small that it is negligible, Babinet’s principle is still valid, while it has been uncertain how the layer thickness works to break the principle as it is increased. The unconfirmed transformation is revealed analytically and experimentally with the use of MPAs and MHAs of varying thicknesses.

DOI： 10.1038/s41598-020-67923-5

Rescaling the complex network of low-temperature plasma chemistry through graph-theoretical analysis Reviewed

Murakami T., Sakai O.

Plasma Sources Science and Technology 29 ( 11 ) 2020.11

Language：English Publishing type：Research paper (scientific journal) Publisher：Plasma Sources Science and Technology

© 2020 IOP Publishing Ltd. We propose graph-theoretical analysis for extracting inherent information from complex plasma chemistry and devise a systematic way to rescale the network under the following key criteria: (1) maintain the scale-freeness and self-similarity in the network topology and (2) select the primary species considering its topological centrality. Network analysis of reaction sets clarifies that the scale-freeness emerging from a weak preferential mechanism reflects the uniqueness of plasma-induced chemistry. The effect of chemistry rescaling on the dynamics and chemistry of the He + O2 plasma is quantified through numerical simulations. The present chemical compression dramatically reduces the computational load, whereas the concentration profiles of reactive oxygen species (ROS) remain largely unchanged across a broad range of time, space and oxygen admixture fraction. The proposed analytical approach enables us to exploit the full potential of expansive chemical reaction data and would serve as a guideline for creating chemical reaction models.

DOI： 10.1088/1361-6595/abbdca

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Classifying dysphagic swallowing sounds with support vector machines Reviewed

Miyagi S., Sugiyama S., Kozawa K., Moritani S., Sakamoto S.I., Sakai O.

Healthcare (Switzerland) 8 ( 2 ) 2020.6

Language：English Publishing type：Research paper (scientific journal) Publisher：Healthcare (Switzerland)

Swallowing sounds from cervical auscultation include information related to the swallowing function. Several studies have been conducted on the screening tests of dysphagia. The literature shows a significant difference between the characteristics of swallowing sounds obtained from different subjects (e.g., healthy and dysphagic subjects; young and old adults). These studies demonstrate the usefulness of swallowing sounds during dysphagic screening. However, the degree of classification for dysphagia based on swallowing sounds has not been thoroughly studied. In this study, we investigate the use of machine learning for classifying swallowing sounds into various types, such as normal swallowing or mild, moderate, and severe dysphagia. In particular, swallowing sounds were recorded from patients with dysphagia. Support vector machines (SVMs) were trained using some features extracted from the obtained swallowing sounds. Moreover, the accuracy of the classification of swallowing sounds using the trained SVMs was evaluated via cross-validation techniques. In the two-class scenario, wherein the swallowing sounds were divided into two categories (viz. normal and dysphagic subjects), the maximum F-measure was 78.9%. In the four-class scenario, where the swallowing sounds were divided into four categories (viz. normal subject, and mild, moderate, and severe dysphagic subjects), the F-measure values for the classes were 65.6%, 53.1%, 51.1%, and 37.1%, respectively.

DOI： 10.3390/healthcare8020103

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A tunable double negative device consisting of a plasma array and a negativepermeability metamaterial Reviewed International coauthorship

Iwai A., Righetti F., Wang B., Sakai O., Mark A. Cappelli, M. A.

Physics of Plasmas 27 2020.2

Mobilities in network topology and simulation reproducibility of sightseeing vehicle detected by low-power wide-area positioning system Reviewed

Yamamoto K., Yoshida J., Miyagi S., Minami S., Minami D., Sakai O.

Electronics (Switzerland) 9 ( 1 ) 2020.1

Language：English Publishing type：Research paper (scientific journal) Publisher：Electronics (Switzerland)

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. Vehicle mobilities for passengers in a city’s downtown area or in the countryside are significant points to characterize their functions and outputs. We focus on commercial sightseeing vehicles in a Japanese city where many tourists enjoy sightseeing. Such mobilities and their visualizations make tourist activities smoother and richer. We design and install a low-power, wide-area positioning system on a rickshaw, which is a human-pulled, two-or three-wheeled cart, and monitor its mobility in Hikone City. All the spatial locations, which are recorded in a time sequence on a cloud server, are currently available as open data on the internet. We analyze such sequential data using graph topology, which reflects the information of corresponding geographical maps, and reproduce it in cyberspace using an agent-based model with similar probabilities to the accumulated rickshaw records from one spatial node to another. Although the numerical results of the agent traced in a simulated city are partially consistent with the rickshaw’s record, we identify some significant differences. We conclude that the rickshaw’s mobility observed at the actual sightseeing sites is partially in the random motion; some cases are strongly biased by memory routes. Such non-randomness in the rickshaw’s mobility indicates the existence of specific features in tourism sources that are identified for each sightseeing activity and affected by local sightseeing resources.

DOI： 10.3390/electronics9010116

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Localized breaking of parameter uniformity by macroscopically-negative-permeability metamaterial in low-pressure microwave plasma

Akinori Iwai, Yoshihiro Nakamura, Osamu Sakai, Yoshiharu Omura

Plasma Sources Science and Technology 29 ( 3 ) 2020

Publishing type：Research paper (scientific journal)

Measurements of plasma parameters and transmitted microwaves are performed in a composite of double-split-ring resonators (DSRRs) making magnetic permeability negative and plasmas excited by a 2.45 GHz microwave. We launch the microwave at 100-200 W, modulated by a pulse wave at a low frequency and a low duty ratio to detect time evolutions of plasma parameters. We perform the measurements at different positions with varying distance from the supporting plate of the DSRRs, and find enhanced wave transmission in the composite with non-uniform profiles of electron temperature. To reinforce the measurements of electron temperature, we evaluate the electron energy distribution function (EEDF) by deriving the second deviation of electron current, upon which data smoothing methods are applied, and the estimated EEDF becomes spatially non-uniform with a similar profile to electron temperature. In terms of the validity of results on the enhanced wave transmission, one-dimensional electromagnetic particle simulations reveal sneaking microwaves inside the composite. In comparison with cases of the negative-permeability and the passive (positive-permeability) DSRRs, we confirm that the non-uniformity is attributed to the magnetic resonance of the DSRRs, which is a microscopic effect of the DSRRs to plasma generation, whereas their macroscopic effect makes permeability negative and enhances wave energy transfer inside the composite.

DOI： 10.1088/1361-6595/ab2d74

Detection of bending motion at waist of kitchen workers by using a single commercial accelerometer

Ryosuke Tsuji, Katsumi Furuta, Osamu Sakai, Shigeyuki Miyagi

2019 IEEE 8th Global Conference on Consumer Electronics, GCCE 2019 1172 - 1173 2019.10

Publishing type：Research paper (international conference proceedings)

Investigating the motion of kitchen workers is one of the important elements for reducing the time for serving food to customers in fast food restaurants. We show that a single 3-axis accelerometer attached to kitchen workers is useful for detecting their back bending motion at the waist and estimating time or time period by using only a simple technique. The introduced technique was applied to the practical data acquired from a real situation in a fast food restaurant. The precision of the detected back bending motion reached approximately 70%.

DOI： 10.1109/GCCE46687.2019.9015606

Detection of swallowing times using a commercial RGB-D camera

Jun Yoshida, Keiko Kozawa, Sueyoshi Moritani, Shin Ichi Sakamoto, Osamu Sakai, Shigeyuki Miyagi

2019 IEEE 8th Global Conference on Consumer Electronics, GCCE 2019 1154 - 1155 2019.10

Publishing type：Research paper (international conference proceedings)

For assisting early detection and treatment for dysphagic patients, an assessment system for swallowing disorders is necessary. Detecting the swallowing time is one of the key components for developing such an assessment system. Hence, we propose a swallowing detection algorithm using a commercial depth camera. The proposed method considers movements in the front of the neck, before and during swallowing, to determine the swallowing time. We then experimentally demonstrate the performance of the algorithm. The results show that the proposed algorithm achieves a recall and precision of 0.958 and 0.813, respectively. Thus, this algorithm can assist in the early detection of dysphagia.

DOI： 10.1109/GCCE46687.2019.9015325

Low-cost color-sensitive optical sensor and its calibration by neural network

Shinya Ueno, Osamu Sakai

2019 IEEE 8th Global Conference on Consumer Electronics, GCCE 2019 1156 - 1157 2019.10

Publishing type：Research paper (international conference proceedings)

Although sensor systems play various roles in consumer electronics, high-precision sensors with high-quality calibration are not suitable for all cases in industrial processes and in our daily life uses because of their price and size. Otherwise, one relies on subjective and personal evaluations when judgments are required. In particular, color data is one of the essential information in many industrial and daily cases, and, for example, color sensitive optical sensors are useful for estimations of fruits and vegetables in agriculture and in purchases of such products. In this research, in order to lower the price of the sensors with sufficient precision, which is one of the obstacles to extend the application fields, we propose a method to make software calibration for color spectra detected by color sensors using a neural network. When the low-cost sensors with adequate calibration are available, they work well for not only engineers in industrial sites and in agriculture fields but also general consumer customers.

DOI： 10.1109/GCCE46687.2019.9015596

Analytical formulation for radiation characteristics of a surface wave sustained plasma antenna

T. Naito, O. Sakai

Physics of Plasmas 26 ( 7 ) 2019.7

Publishing type：Research paper (scientific journal)

This paper reports the analytical formulation of a surface wave sustained plasma antenna based on the antenna theory and the theory for a surface wave sustained plasma. The antenna (tube) structure is a basic quarter-wavelength monopole antenna, comparable to a wavelength of a surface wave. The theory for a surface wave sustained plasma in a semi-infinitely long tube is extended to a finite length plasma. The electron density distribution is calculated based on the extended theory, and antenna characteristics are obtained from the electron density distribution. The analytical results obtained from the formulation show good agreement with the numerical results. The electron density in the finite length tube depends on the existence of the interference between forward- and backward-traveling waves and the boundary condition at the tube end. The electron density distribution is determined based on the balance between the forward- and backward-traveling waves. In the case that the plasma does not reach the tube end, which is equivalent to the case of a semi-infinitely long tube, the antenna gain is constant at a low value due to high plasma resistance and low radiation resistance, without the propagation of the backward-traveling wave. Once the plasma reaches the tube end, due to the coexistence of the forward- and backward-traveling waves with their interference, the electron density is elevated fairly beyond the critical density for sustaining surface wave propagation. Consequently, the antenna gain increases with the synergistic effect of the decrease in plasma resistance and the increase in radiation resistance, and the surface wave sustained plasma works as an antenna.

DOI： 10.1063/1.5088855

Symmetrical estimation method for skin depth control of spoof surface plasmon polaritons using dispersed waves from a metallic hole array

Go Itami, Osamu Sakai

Journal of Applied Physics 125 ( 21 ) 2019.6

Publishing type：Research paper (scientific journal)

A metallic hole array (MHA) supports the propagation of spoof surface plasmon polariton (SSPP) modes on its surface. We observe enhancement of dispersed waves at a specific angle when SSPP modes propagate. We propose a symmetrical estimation method for SSPP skin depth using waves dispersed from an MHA. The proposed method provides an experimental demonstration of the skin depth of the electric fields of SSPP modes. Using the dispersed waves emitted from the plane of the MHA, we are able to detect confined electric fields with decay lengths that resemble those theoretically predicted around a specific frequency of SSPP mode excitation.

DOI： 10.1063/1.5088116

Direction-of-arrival estimation from scattering patterns in a subwavelength periodic structure of a conductive scatterer

G. Itami, T. Iizuka, Y. Toriumi, J. Kato, O. Sakai

2019 URSI International Symposium on Electromagnetic Theory, EMTS 2019 2019.5

Publishing type：Research paper (international conference proceedings)

A novel direction-of-arrival (DOA) estimation method that uses subwavelength periodic structures of conductive scatterers (SPCS) is proposed. There is a possibility that scattering patterns with frequency and angular responses can be generated by using subwavelength structures on the basis of metamaterials. Therefore, the frequency and angular dependencies of scattering patterns generated by SPCS are investigated analytically and experimentally. In the analytical studies, the 2D spatial distributions of electric field intensities in SPCS are analyzed. And it is confirmed that the patterns have frequency and angular dependencies. In the experimental studies, the 1D spatial distributions of the scattering patterns in SPCS were measured, confirming the frequency and angular dependence of their spatial distribution. Thus, the proposed method has the potential for application in an advanced DOA estimation device.

DOI： 10.23919/URSI-EMTS.2019.8931530

Two-dimensional imaging of permittivity distribution by an activated meta-structure with a functional scanning defect Reviewed

Itami G., Sakai O., Harada Y.

Electronics (Switzerland) 8 ( 2 ) 2019.2

Language：English Publishing type：Research paper (scientific journal) Publisher：Electronics (Switzerland)

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. A novel 2D imaging method for permittivity imaging using a meta-structure with a functional scanning defect is proposed, working in the millimeter wave-range. The meta-structure we used here is composed of a perforated metal plate with subwavelength-holes and a needle-like conductor that can scan two-dimensionally just beneath the plate. The metal plate, which is referred to as a metal hole array (MHA) in this study, is known as a structure supporting propagation of spoof surface plasmon polaritons (SSPPs). High-frequency waves with frequencies higher than microwaves, including SSPPs, have the potential to detect signals from inner parts embedded beneath solid surfaces such as living cells or organs under the skin, without physical invasion, because of the larger skin depth penetration of millimeter wave-bands than optical wave-bands. Focused on activated SSPPs, the localized distortion of SSPP modes on an MHA is used in the proposed method to scan the electromagnetic properties of the MHA with a needle-like conductor (conductive probe), which is a kind of active defect-initiator. To show the validity of the proposed method, electromagnetic analyses of the localized distortions of wave fields were performed, and one-and two-dimensional imaging experiments were conducted with the aim of detecting both conductive and dielectric samples. The analytical results confirmed the localized distortion of the electric field distribution of SSPP modes and also indicated that the proposed method has scanning ability. In experimental studies, the detection of conductive and dielectric samples was successful, where the detected dielectrics contained pseudo-biological materials, with an accuracy on the order of millimeters. Finally, a biomedical diagnosis in the case of a rat lung is demonstrated by using the experimental system. These results indicate that the proposed method may be usable for non-invasive and low-risk biomedical diagnosis.

DOI： 10.3390/electronics8020239

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Functional Composites of Discharge Plasmas and Solid Metamaterials Reviewed

Sakai O., Iwai A.

Springer Series in Materials Science 287 151 - 167 2019.1

Language：English Publishing type：Research paper (scientific journal) Publisher：Springer Series in Materials Science

© Springer Nature Singapore Pte Ltd 2019. Discharge plasmas are composed of electrons and ions, and their permittivity is dynamic and tunable. Conventional metamaterials are composed of designed functional microstructures of solid materials, and become extraordinary wave media such as negative-permeability materials. The composites of the plasmas and the metamaterials are well mixed to show dynamic properties coming from plasmas and extraordinary outputs based on metamaterials. Here, we describe their theoretical basis and topical features observed in microwave experiments. Beyond properties of tunability, such composite “plasma metamaterials” work well as nonlinear and high-energy-carrier metamaterials, unlike conventional solid-state metamaterials.

DOI： 10.1007/978-981-13-8649-7_10

Other Link： https://www.scopus.com/inward/record.uri?partnerID=HzOxMe3b&scp=85072821574&origin=inward

Topological network properties of fractal-like metallic nanoparticle patterns and their effects on optical resonances Reviewed

Kihara N., Sakai O.

Applied Sciences (Switzerland) 8 ( 8 ) 2018.8

Language：English Publishing type：Research paper (scientific journal) Publisher：Applied Sciences (Switzerland)

© 2018 by the authors. Fractal-like nanoparticle two-dimensional patterns forming in diffusion-limited aggregation show variant spatial patterns. However, they have invariant statistical properties in their network topologies, even though their formation is completely in self-assembled processes. One of the outputs from these topological properties is optical resonances at invariant frequencies, which is a required feature of a metamaterial alternative. Fractal-like metallic patterns studied here in both experiments and theoretical models exhibit similar resonance frequencies in the infrared-ray range, and they depend on the unit length of nanoparticles composing arbitrary fractal-like structures. The scheme of analysis applied here using complex network theory does not only reveal the topological properties of the nanoparticle network, but points out their optical and possibly other physical potentials arising from their geometrical properties.

DOI： 10.3390/app8081310

Other Link： https://www.scopus.com/inward/record.uri?partnerID=HzOxMe3b&scp=85075768667&origin=inward

Invariance of parameter identification in multiscales of meta-atoms in metamaterials Reviewed

Sakai O., Iwai A., Omura Y.

Advances in Physics: X 3 ( 1 ) 339 - 356 2018.1

Language：English Publishing type：Research paper (scientific journal) Publisher：Advances in Physics: X

© 2018 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group. The concept of metamaterials has led to extraordinary schemes of wave propagation, which has been verified using various meta-atoms, constituent units of metamaterials, as well as applied to a number of categories in physics. Although its definition clarifies the maximum size of meta-atoms as a fraction of one wavelength, the size may vary by several orders, like from millimeters (‘macroscopic level’) to atomic scales (‘microscopic level’) for microwaves. This review surveys several patterns of parameter combinations, like permittivity and permeability in electromagnetic metamaterials, which have been achieved at either macroscopic or microscopic levels, with the similar analogy under the concept of metamaterials. Various experimental and theoretical efforts reported so far and shown here verify that the parameter identification of these values (permittivity, permeability, and refractive index) is independent of meta-atom sizes, with importance of spatial integration procedure on the order of a wavelength.

DOI： 10.1080/23746149.2018.1433551

Other Link： https://www.scopus.com/inward/record.uri?partnerID=HzOxMe3b&scp=85064523651&origin=inward

Spectroscopic ellipsometry analysis of perovskite manganite films for resistance switching devices

Masaki Yamada, Osamu Sakai, Toshihiro Nakamura

Thin Solid Films 571 ( P3 ) 597 - 600 2014.11

Publishing type：Research paper (international conference proceedings)

Pr0.5Ca0.5MnO3 (PCMO) films were deposited on LaAlO3 (100) substrates changing process pressure from 1.33 to 5.33 Pa by RF magnetron sputtering. Current-voltage characteristic measurements and spectroscopic ellipsometry measurements were carried out to investigate the mechanism of resistance switching in PCMO films. Resistance switching was observed in the devices composed of the PCMO films deposited at low pressures of 1.33 and 2.67 Pa. The deposition pressure dependence of the electronic structure of PCMO films was detected as a difference in dielectric functions by spectroscopic ellipsometry. Spectroscopic ellipsometry data indicated that the PCMO films exhibiting resistance switching had large oscillator strength of the electric dipole charge transition in (MnO6)9 - and (MnO6)8 - octahedral complexes, small oscillator strength of d-d transitions in Mn3 + and Mn4 + ions, and large high frequency dielectric constant. The formation of (MnO6)9 - and (MnO6)8 - octahedral complexes and oxygen vacancies might be required for obtaining large resistance switching.

DOI： 10.1016/j.tsf.2013.11.145

Localized electromagnetic distortion in 2D metal hole array and its application to imaging of permittivity distribution

G. Itami, T. Akiyama, O. Sakai, Y. Harada

2014 Asia-Pacific Microwave Conference Proceedings, APMC 2014 1253 - 1255 2014.3

Publishing type：Research paper (international conference proceedings)

We performed two-dimensional imaging of conductive and dielectric samples above a metal plate with a two-dimensional array of subwavelength holes (metal hole array) with electromagnetic distortion formed by inserting a needle-like conductive probe into holes in it. A metal hole array has a special electromagnetic response such as localization of electromagnetic waves at a specific frequency near it. Generally, the phenomenon described above is called 'spoof surface plasmon resonance.' And such special media can be regarded as 'metamaterial' from a point of view that it realizes a macroscopically uniform medium by the subwavelength-size holes in the metal plate. We conducted numerical analyses of electric field profiles in order to clarify how electric field distributions near the metamaterial change by the electromagnetic distortion, and confirmed the phenomenon that these uniform electric fields around the 'metamaterial' are distorted locally by using the needle-like conductive probe. Electromagnetic waves response to the permittivity at the space, so that it is considered that the change of the permittivity at the space is reflected as that of the electromagnetic distribution at the space. Based on the above, the formation of the electromagnetic distortion can be a detecting method of samples which have dielectric responses. Therefore, we consider that this two-dimensional imaging system is applicable to detecting something which has abnormal permittivity inside bulk material, such as a cancer in biomaterial tissues.

Control of work function of indium tin oxide: A surface treatment by atmospheric-pressure plasma layer on fabric-type electrodes

Yoshihiko Ueda, Junichi Abe, Hideyuki Murata, Yasuhito Gotoh, Osamu Sakai

Japanese Journal of Applied Physics 53 ( 3 SPEC. ISSUE 2 ) 2014.3

Publishing type：Research paper (scientific journal)

The surface electronic properties of metal oxides, which affect the performances of layered electronic devices, are controlled by non-thermal atmospheric-pressure plasmas generated using a fabric-type electrode and gas flow induced through this plasma layer. We specify a method to control the work function of indium tin oxide (ITO) films by this plasma, in which such reactive species as oxidant radicals are generated. These oxidants are spatially transported in the gas flow to the ITO surface and increase the ITO's work function, as verified in our experimental results. To examine the effects of the increased work function in a specific electronic device, we treat the surface of an ITO electrode in an organic lightemitting diode and observe the enhancement of the light-emitting efficiency. © 2014 The Japan Society of Applied Physics.

DOI： 10.7567/JJAP.53.03DG03

High-density microwave plasma source using negative-permeability metamaterial with tuned wave attenuation

Yoshihiro Nakamura, Osamu Sakai

Japanese Journal of Applied Physics 53 ( 3 SPEC. ISSUE 2 ) 2014.3

Publishing type：Research paper (scientific journal)

High-density plasma generated by waves with efficient wave power consumption is suitable for various dry processes. However, when we use microwaves as power sources, the electron density in a conventional microwave plasma source is limited within the range in which the permittivity of the plasma is positive. To overcome this drawback, a negative refractive index phenomenon is applicable, in which the permittivity and permeability are both negative and microwaves can propagate even in a negative-permittivity space with high electron density. In this report, we demonstrate a promising scheme for the efficient generation of high-density plasma by propagating microwaves in a negative-permeability space, and show the experimental observation of the enhancement of plasma generation using the lower imaginary part of the permeability. © 2014 The Japan Society of Applied Physics.

DOI： 10.7567/JJAP.53.03DB04

Correlation of resistance switching behaviors with dielectric functions of manganite films: A study by spectroscopic ellipsometry

Masaki Yamada, Toshihiro Nakamura, Osamu Sakai

Materials Research Society Symposium Proceedings 1633 2014

Publishing type：Research paper (international conference proceedings)

Pr0.5Ca0.5MnO3 (PCMO) films were deposited on LaAlO3 (100) substrates under pressure from 1.33 to 5.33 Pa by RF magnetron sputtering. Resistance switching and dielectric functions of PCMO films were studied by DC current-voltage characteristic measurements and spectroscopic ellipsometry (SE) measurements. Resistance switching was observed in the devices composed of PCMO films deposited under low pressures of 1.33 and 2.67 Pa. SE measurements revealed that dielectric functions also depended on deposition pressure. PCMO films deposited under lower pressure had larger high-frequency dielectric constant, larger oscillator strength of the electric dipole charge transitions in MnO6 octahedral complexes, and lower oscillator strength of d-d transitions in Mn3+ and Mn4+ ions. SE measurements suggested that oxygen vacancies and MnO6 octahedral complexes play an important role in resistance switching in PCMO films. Copyright © Materials Research Society 2014.

DOI： 10.1557/opl.2014.85

Multiheterodyne interference spectroscopy using a probing optical frequency comb and a reference single-frequency laser Reviewed

Keiichiro Urabe, Sakai Osamu

PHYSICAL REVIEW A 88 ( 2 ) 2013.8

Functional composites of plasmas and metamaterials: Flexible waveguides, and variable attenuators with controllable phase shift Reviewed

Sakai Osamu, Maeda Jun, Shimomura Takuya, Keiichiro Urabe

PHYSICS OF PLASMAS 20 ( 7 ) 2013.7

Fractal-like ray trajectories traced in three-dimensional array of photoelectrochemical cells

Osamu Sakai, Yoshihiko Ueda, Tomohide Akiyama, Josuke Nakata

Journal of Physics D: Applied Physics 46 ( 18 ) 2013.5

Publishing type：Research paper (scientific journal)

Underwater spherical photoelectrochemical cells aligned in three-dimensional electrolyte space convert injected sunlight into driving force for water splitting, which is analysed by a model calculation and verified in experiments. Sunlight is well absorbed via a number of bounces on Si, and trajectories of its rays are in a fractal-like scale-free system. Similar systems are applicable to general photoelectrochemical cells. © 2013 IOP Publishing Ltd.

DOI： 10.1088/0022-3727/46/18/182001

Hydrazine generation for the reduction process using small-scale plasmas in an argon/ammonia mixed gas flow Reviewed

Keiichiro Urabe, Hiraoka Yu, Sakai Osamu

PLASMA SOURCES SCIENCE & TECHNOLOGY 22 ( 3 ) 2013.5

Diagnostic of electrophoretic particles by dielectric dispersion spectra

Ayumi Ando, Ryuichi Kiyoe, Yuusuke Nishio, Hiroshi Inoue, Osamu Sakai

Proceedings of the International Display Workshops 2 1318 - 1321 2013

Two-dimensional metamaterial for surface wave propagation with scanning metallic defect for permittivity imaging

Osamu Sakai, Go Itami, Tomohide Akiyama, Yoshinori Harada

2013 International Symposium on Electromagnetic Theory, EMTS 2013 - Proceedings 719 - 721 2013

Publishing type：Research paper (international conference proceedings)

A perforated metal plate, working as a two-dimensional metamaterial to support surface wave propagation, plays a role of an imaging head of permittivity mapping at millimeter-wave ranges. One noninvasive metallic probe is swept two-dimensionally along the metamaterial surface to induce near field to detect the corresponding local permittivity. This configuration is applicable to detection of disease tissues for medical uses. © 2013 IEICE.

Absorption spectroscopy using interference between optical frequency comb and single-wavelength laser Reviewed

Keiichiro Urabe, Osamu Sakai

APPLIED PHYSICS LETTERS 101 ( 5 ) 2012.7

Microcavity array plasma system for remote chemical processing at atmospheric pressure

Dae Sung Lee, Satoshi Hamaguchi, Osamu Sakai, Sung Jin Park, J. Gary Eden

Journal of Physics D: Applied Physics 45 ( 22 ) 2012.6

Publishing type：Research paper (scientific journal)

A microplasma system designed for chemical processing at atmospheric pressure is fabricated and characterized with flowing He/O 2 gas mixtures. At the heart of this microcavity dielectric barrier discharge (MDBD) system are two arrays of half-ellipsoidal microcavities engraved by micropowder blasting into dielectric surfaces facing a flowing, low-temperature plasma. Experiments demonstrate that the ignition voltage is reduced, and the spatially averaged optical emission is doubled, for an MDBD flowing plasma array relative to an equivalent system having no microcavities. As an example of the potential of flowing atmospheric microplasma systems for chemical processing, the decomposition of methylene blue (as evidenced by decoloration at 650.2nm) is shown to proceed at a rate as much as a factor of two greater than that for a non-microcavity equivalent. © 2012 IOP Publishing Ltd.

DOI： 10.1088/0022-3727/45/22/222001

Temporally and spectrally resolved observation of a crossed-flow DBD plasma jet using pure helium and argon/acetone mixed gases Reviewed

Keiichiro Urabe, Brian L. Sands, Biswa N. Ganguly, Osamu Sakai

PLASMA SOURCES SCIENCE & TECHNOLOGY 21 ( 3 ) 2012.6

Plasma microchannel and jet enhanced by an array of ellipsoidal microcavities

D. S. Lee, O. Sakai, S. J. Park, J. G. Eden, S. Hamaguchi

IEEE Transactions on Plasma Science 39 ( 11 PART 1 ) 2690 - 2691 2011.11

Publishing type：Research paper (scientific journal)

Plasma jet discharges are shown to be enhanced significantly, with respect to emission intensity, in the presence of ellipsoidal microcavities dispersed at uniform intervals along the gas flow channel. In these experiments, 25 microcavities were fabricated in the top and bottom faces of a gas flow channel having a rectangular cross section. Conducting film electrodes exterior to the glass structure, powered by a 20-kHz sinusoidal voltage, produced plasma along the entire channel as well as within the microcavities. Stronger optical emission and longer plasma jets were observed in this system than in an equivalent plasma jet system without microcavities. © 2006 IEEE.

DOI： 10.1109/TPS.2011.2165561

Temporally Resolved Imaging of Jet-Type Dielectric Barrier Discharge Using He and Ar/Acetone Crossed Gas Flows Reviewed

Keiichiro Urabe, Brian L. Sands, Osamu Sakai, Biswa N. Ganguly

IEEE TRANSACTIONS ON PLASMA SCIENCE 39 ( 11 ) 2011.11

Discharge-Mode Transition in Jet-Type Dielectric Barrier Discharge Using Argon/Acetone Gas Flow Ignited by Small Helium Plasma Jet Reviewed

Keiichiro Urabe, Keitaro Yamada, Osamu Sakai

JAPANESE JOURNAL OF APPLIED PHYSICS 50 ( 11R ) 2011.10

Chemical filters by non-thermal atmospheric pressure plasmas for reactive fields Reviewed

Osamu Sakai, Tadasuke Morita, Yoshihiko Ueda, Noriaki Sano, Kunihide Tachibana

THIN SOLID FILMS 519 ( 20 ) 6999 - 7004 2011.8

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE SA

Non-thermal plasmas in electrode configurations are designed in this study and investigated at atmospheric pressure for generation of reactive fields. A combination of insulated wire electrodes and bare metal wire electrodes makes it possible to obtain a filter-like assembly of microplasmas, even in ambient air or in aqueous solutions. Oxidation and reduction fields are obtained by controlling the gas supply of hydrogen, oxygen, and water vapor, and these fields benefit various functional processes such as recycling CO(2), decomposing organic molecules, and increasing the work function on metal surfaces. (C) 2011 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.tsf.2011.01.225

Transition between positive and negative permittivity in field-dependent metamaterial

Osamu Sakai

Journal of Applied Physics 109 ( 8 ) 2011.4

Publishing type：Research paper (scientific journal)

Permittivity in a metamaterial, which depends on the electric field of a propagating electromagnetic wave, induces anomalous responses against electromagnetic waves. When we consider discharge plasma an example of a metamaterial with field-dependent permittivity and negative permeability, a transition takes place between positive and negative permittivity, leading to reflection of waves in a low-field branch and transmission by the creation of a negative-refractive-index medium in a high-field branch. Nonlinearity in the phase space of the permittivity and the electric field in this system trigger this dynamic transition, which is suggested in the numerical results and clarified in an analytical model. © 2011 American Institute of Physics.

DOI： 10.1063/1.3574921

Combined spectroscopic methods for electron-density diagnostics inside atmospheric-pressure glow discharge using He/N-2 gas mixture Reviewed

Keiichiro Urabe, Osamu Sakai, Kunihide Tachibana

JOURNAL OF PHYSICS D-APPLIED PHYSICS 44 ( 11 ) 2011.3

Enhancement of visible He<inf>2</inf> emission by infrared laser excitation of He metastable atoms in atmospheric-pressure plasma jet

Keiichiro Urabe, Hideki Motomura, Osamu Sakai, Kunihide Tachibana

Journal of Physics D: Applied Physics 44 ( 4 ) 2011.2

Publishing type：Research paper (scientific journal)

We observed bright red emission at an intersection of an infrared (IR) laser beam and an atmospheric-pressure plasma jet (APPJ). The APPJ had a coaxial dielectric barrier discharge configuration and generated a plasma plume in a He gas flow ejected into air from a glass capillary. Although the laser-beam wavelength was 1083 nm corresponding to He I (23P-23S) atomic transition, the enhanced red emission peaked in the visible region at 640 nm and had broad bandwidth corresponding to He2 (d 3Σ+u -b 3IIg) molecular transition. This result indicates that the IR laser excitation of He metastable (23S) atoms leads to selective formation of excited He2 (d 3Σ+u ) molecules from excited He (23P) atoms. The 640 nm emission intensity decreased as the laser beam moved downstream in the plasma plume, because of the quenching effects on the excited He atoms and the excited He2 molecules by air entrainment. We believe that the He2 (d 3Σ+u ) molecular formation follows two potential curves of He 2 (f 3Σu or f 3IIu) and (d 3Σ+u ) states, overcoming a potential hump by three-body collision processes. © 2011 IOP Publishing Ltd.

DOI： 10.1088/0022-3727/44/4/042001

Emerging aspects in a plasma-metamaterial composite

Osamu Sakai

2011 30th URSI General Assembly and Scientific Symposium, URSIGASS 2011 2011

Publishing type：Research paper (international conference proceedings)

When plasmas are immersed in a metamaterial structure, such a composite shows various properties which are not expected in an ordinary metamaterial. Plasmas are either generated by external powers or present in nature as a space plasma, and composites of metamaterials and such plasmas serve as elements of electromagnetic wave controllers as well as negative-refractive-index materials. This report describes the fundamental properties of the composites with specific examples, and some fields of potential applications for human space activities are surveyed. © 2011 IEEE.

DOI： 10.1109/URSIGASS.2011.6051083

High speed deposition of oxide materials by using plasma jet at atmospheric pressure

Yosuke Ito, Osamu Sakai, Kunihide Tachibana

AIP Conference Proceedings 1393 11 - 14 2011

Publishing type：Research paper (international conference proceedings)

We carried out plasma enhanced chemical vapor deposition using a plasma jet at atmospheric pressure for one-dimensional deposition of SiO2 films and ZnO compounds, and measured the film quality by Fourier transform infrared spectroscopy. In the case of SiO2, the deposition rate was about 400 nm/s and the breakdown voltage was 5×105 V/cm. On the other hand, in the case of ZnO, the deposition rate was about 160 nm/s. However, as-prepared films contained much carbon contamination, so it is necessary to anneal the compounds in order to obtain high-quality films. After 1-hour annealing at 500°C in air, the transparency was more than 80% in visible light range and the drop of the transparency at around 370 nm corresponding to ZnO band-edge energy. Moreover, the resistivity of the annealed ZnO compounds was estimated to be about 106 Ωcm, so we could obtain the transparent and semiconductor-like ZnO compounds successfully. © 2011 American Institute of Physics.

DOI： 10.1063/1.3653596

Negative refractive index designed in a periodic composite of lossy microplasmas and microresonators

Osamu Sakai, Takuya Shimomura, Kunihide Tachibana

Physics of Plasmas 17 ( 12 ) 2010.12

Publishing type：Research paper (scientific journal)

A novel metamaterial with negative refractive index was designed using a spatially periodic composite of microplasmas and microresonators. Microresonators, which are double-helix metal wires in this report, work as an origin of macroscopic negative permeability material, as well as discharge electrodes. If a resonance frequency of the resonators is well below electron plasma frequency of the microplasmas where their permittivity is negative in the real part with a certain imaginary part, the macroscopic refractive index becomes negative just above the resonance frequency where the macroscopic permeability is negative, even if microplasmas are fairly lossy; due to such a loss or conductive component in permittivity, electromagnetic waves are not evanescent but propagating. This result indicates that plasmas can play important roles in parameter control of a metamaterial with a complex refractive index. © 2010 American Institute of Physics.

DOI： 10.1063/1.3524561

FTIR study of methylene blue plasma degradation products through plasma treatment on water Reviewed

T. Shirafuji, J. Hieda, O. Takai, N. Saito, T. Morita, O. Sakai, K. Tachibana

Proc. 2010 IEEE Region 10 Conf. 2010.9

Effect of Series Capacitance and Accumulated Charge on a Substrate in a Deposition Process with an Atmospheric-Pressure Plasma Jet Reviewed

Yosuke Ito, Yutaka Fukui, Keiichiro Urabe, Osamu Sakai, Kunihide Tachibana

JAPANESE JOURNAL OF APPLIED PHYSICS 49 ( 6 ) 2010.6

Language：English Publishing type：Research paper (scientific journal) Publisher：JAPAN SOC APPLIED PHYSICS

In order to investigate the effect of accumulated charge on a substrate surface in the deposition of a SiO(2) film with an atmospheric-pressure plasma jet, we have measured the discharge current flowing into a copper substrate, which was placed 20 mm from the exit of the plasma jet and connected to a variable capacitor in series. We found that the discharge current decreased markedly when the capacitance of the substrate was below 100 pF, and the deposition rate of SiO(2) traced the variation of the capacitance. To analyze the behavior of the plasma jet, we considered an equivalent circuit of our system and verified the validity of our supposition that the accumulated charge restricts the deposition rate. Thereby, we found that the discharge current and deposition rate were determined by the capacitance of both the glass tube wall and the substrate. (C) 2010 The Japan Society of Applied Physics

DOI： 10.1143/JJAP.49.066201

Experimental and numerical verification of microplasma assembly for novel electromagnetic media Reviewed

Osamu Sakai, Teruki Naito, Kunihide Tachibana

PHYSICS OF PLASMAS 17 ( 5 ) 2010.5

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER INST PHYSICS

Microplasmas have a number of potential roles to control propagating electromagnetic waves. This report focuses on novel physics of periodic microplasma assembly for electromagnetic media, which is verified by experimental results and analyzed by numerical methods. Using an assembly composed of microplasmas, novel functions are expected due to its complex dielectric function arising from dielectric and lossy properties. The dielectric property creates photonic band gaps, and the lossy property drastically changes transmittance around the photonic band gaps, leading to an attenuation gap. As a result, a "complex" dispersion relation or a band diagram in the three-dimensional space of real and imaginary wavenumbers and wave frequency will open new possibilities to control electromagnetic waves by complex-variable filters composed of microplasma assemblies. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3314334]

DOI： 10.1063/1.3314334

Measurement of electron density in a microdischarge-integrated device operated in nitrogen at atmospheric pressure using a millimetre-wave transmission method Reviewed

Yosuke Ito, Osamu Sakai, Kunihide Tachibana

PLASMA SOURCES SCIENCE & TECHNOLOGY 19 ( 2 ) 2010.4

Language：English Publishing type：Research paper (scientific journal) Publisher：IOP PUBLISHING LTD

We have diagnosed the electron density in nitrogen plasma generated in a device composed of coaxial-hollow microdielectric-barrier discharges using a millimetre-wave transmission method. At atmospheric pressure the generated plasma occupied almost the entire hollow-electrode space, and the electron density was estimated as 10(12)-10(13) cm(-3). This value is in good agreement with theoretical prediction in a simple particle-balance model. We also studied the effect of small admixtures of H(2)O and O(2) into nitrogen discharges at atmospheric pressure. In the case of H(2)O addition, the measured electron density was lower than that in pure nitrogen, in particular at high reduced electric fields, due to the increase in the loss processes of electrons. In the case of O(2) addition, the electron density decreased with an increase in O(2) concentration due to the additional loss process through the formation of negative ions.

DOI： 10.1088/0963-0252/19/2/025006

Study of plasma enhanced chemical vapor deposition of ZnO films by non-thermal plasma jet at atmospheric pressure Reviewed

Yosuke Ito, Osamu Sakai, Kunihide Tachibana

THIN SOLID FILMS 518 ( 13 ) 3513 - 3516 2010.4

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE SA

Plasma enhanced chemical vapor deposition using a non-thermal plasma jet was applied to deposition of ZnO films. Using vaporized bis(octane-2,4-dionato)zinc flow crossed by the plasma jet, the deposition rate was as high as several tens of nm/s. From the results of infrared spectra, the films deposited at the substrate temperature T(sub) = 100 degrees C contained a significant amount of carbon residue, while the films prepared at T(sub)=250 degrees C showed less carbon fraction. The experimental results confirmed that the plasma jet decomposed bis(octane-2,4-dionato)zinc in the gaseous phase and on the substrate, and that there should be the critical Tsub to form high-quality ZnO films in the range from 100 to 250 degrees C. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.tsf.2009.11.034

Microplasma array with metamaterial effects Reviewed

Osamu Sakai, Teruki Naito, Takuya Shimomura, Kunihide Tachibana

THIN SOLID FILMS 518 ( 13 ) 3444 - 3448 2010.4

Language：English Publishing type：Research paper (scientific journal) Publisher：ELSEVIER SCIENCE SA

Metamaterial effects emerging from a spatially-arranged microplasma array was investigated at microwave frequencies. With elaborated spatial designs of periodic columnar microplasmas, they serve as a converter of microwaves in the complex plane with various functions. Composites of microplasmas and metal wires show extraordinary properties for electromagnetic waves, such as negative-refractive index, which have not been naturally available. These two aspects of microplasma arrangement will open novel features beyond the conventional materials, which can be referred to as plasma metamaterial effects. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.tsf.2009.11.040

円筒プラズマ列による表面波導波路の動的形成

前田潤, 内藤皓貴, 酒井道

電気学会研究会資料. EMT, 電磁界理論研究会,2010 (1), pp. 179-183 2010.1

円筒プラズマ列による表面波導波路の動的形成 (レーザ・量子エレクトロニクス)

前田潤, 内藤皓貴, 酒井道

電子情報通信学会技術研究報告,109 (403), pp. 179-183 2010.1

円筒プラズマ列による表面波導波路の動的形成 (フォトニックネットワーク)

前田潤, 内藤皓貴, 酒井道

電子情報通信学会技術研究報告,109 (401), pp. 179-183 2010.1

円筒プラズマ列による表面波導波路の動的形成 (光エレクトロニクス)

前田潤, 内藤皓貴, 酒井道

電子情報通信学会技術研究報告,109 (402), pp. 179-183 2010.1

Interaction between Dielectric Barrier Discharge and Positive Streamer in Helium Plasma Jet at Atmospheric Pressure Reviewed

Keiichiro Urabe, Yosuke Ito, Osamu Sakai, Kunihide Tachibana

JAPANESE JOURNAL OF APPLIED PHYSICS 49 ( 10 ) 2010

Language：English Publishing type：Research paper (scientific journal) Publisher：JAPAN SOC APPLIED PHYSICS

We have investigated the discharge mechanisms in a helium plasma jet with a coaxial dielectric barrier discharge (DBD) configuration, which can generate low-temperature plasma plume mainly by positive corona streamer propagation. In this study, we focused on the relationships between the coaxial DBD inside a glass tube and the positive streamer propagating in the plasma plume, using novel types of electrode configurations composed of insulated electric wires. The discharge characteristics were investigated by measuring plume length, discharge currents of the DBD, and the line-integrated density of helium metastable atoms in the plume. The experimental results indicate that the excited-species density in the plume is largely dependent on surface-charge density accumulated by the DBD. Moreover, we analyzed the spatiotemporal distribution of electrical potential around the tube exit by the finite element method, and discussed the mechanism determining plume length. (C) 2010 The Japan Society of Applied Physics

DOI： 10.1143/JJAP.49.106001

プラズマアレイのメタマテリアル効果を用いた電磁波の動的複素制御

下村卓也, 内藤皓貴, 前田潤, 酒井道, 橘邦英

電気学会研究会資料. EMT, 電磁界理論研究会,2009 (102), pp. 97-102 2009.11

Mode Change Observed on Spatial Distribution of Microplasma Emission in a Microdischarge Cell with a Floating Electrode Reviewed

Dae-Sung Lee, Osamu Sakai, Kunihide Tachibana

JAPANESE JOURNAL OF APPLIED PHYSICS 48 ( 10 ) 2009.10

Language：English Publishing type：Research paper (scientific journal) Publisher：JAPAN SOCIETY APPLIED PHYSICS

A microplasma device fabricated on a silicon substrate with a floating electrode inside the microdischarge space was investigated-to determine the dependences of discharge properties in neon gas on the gas pressure p and the characteristic-dimension D. With a decrease in p towards the condition of the minimum ignition voltage, the spatial emission distribution in the negative phase of the applied bipolar pulsed voltage changed from a hollow pattern with a bright periphery to a concentrated bright pattern in the central area above the floating electrode, which was similar to that in the positive phase. This observation encourages us to make this device structure to provide a brighter light source at the microscales for various photonic applications. (C) 2009 The Japan Society of Applied Physics

DOI： 10.1143/JJAP.48.106002

Reduction of CO2 solute by hydrogen microplasmas in an electrolyte Reviewed

Osamu Sakai, Tadasuke Morita, Noriaki Sano, Tatsuru Shirafuji, Tomohiro Nozaki, Kunihide Tachibana

JOURNAL OF PHYSICS D-APPLIED PHYSICS 42 ( 20 ) 2009.10

Language：English Publishing type：Research paper (scientific journal) Publisher：IOP PUBLISHING LTD

CO2 as a solute of aqueous solution was deoxidized by hydrogen microplasmas generated in an electrolyte. Dielectric barrier discharges were generated in H-2 microbubbles created by electrolysis, and optical emission spectra included carbon-oriented lines as well as an atomic H line. From a chromatogram of a gas in a trap on the electrolyte surface, CO was detected, which was a product of the reduction reaction of CO2 induced by underwater hydrogen microplasmas.

DOI： 10.1088/0022-3727/42/20/202004

大気圧プラズマ－基礎と応用 Reviewed

橘邦英, 小田哲治, 堀勝, 渡辺隆行, 高井治, 白藤立, 明石治朗, 赤塚洋, 秋津哲也, 秋山秀典, 内野喜一郎, 大竹尚登, 大参宏昌, 岡崎健, 岡本幸雄, 小田昭紀, 小野亮, 垣内弘章, 河野明廣, 小駒益弘, 児玉英之, 酒井道, 櫻井彪, 佐藤岳彦, 佐藤正之, 佐野泰久, 白井肇, 神野雅文, 高木浩一, 高島成剛, 竹田圭吾, 寺嶋和夫, 栃久保文嘉, 永津雅章, 中野俊樹, 中村圭二, 西脇彰, 野崎智洋, 水野彰, 安武潔, 山内和人, 山村和也, 湯浅基和

大気圧プラズマ－基礎と応用 2009.10

Microplasma-Induced Deformation of an Anomalous Response Spectrum of Electromagnetic Waves Propagating along Periodically Perforated Metal Plates Reviewed

Dae-Sung Lee, Osamu Sakai, Kunihide Tachibana

JAPANESE JOURNAL OF APPLIED PHYSICS 48 ( 6 ) 2009.6

Language：English Publishing type：Research paper (scientific journal) Publisher：JAPAN SOCIETY APPLIED PHYSICS

Metal plates with two-dimensional hole arrays showed unusual responses of electromagnetic waves, and in this study microplasmas were generated in the holes to deform their properties dynamically and to induce novel frequency-dependent permittivity. Transmitted electromagnetic waves in millimeter wavelength range exhibited attenuation of anomalous transmittance around cutoff frequency of equivalent waveguide when microplasmas were present. On the contrary, the reflected waves were significantly enhanced around such a frequency, where reflectance is at minimum in the case without microplasma generation. These experimental results indicated resonance phenomenon which is usually called spoof surface plasmons, and dynamic change of dielectric constant in the medium inside the holes induced by microplasmas shifts their resonance frequency. (C) 2009 The Japan Society of Applied Physics

DOI： 10.1143/JJAP.48.062004

マイクロプラズマによる電磁波メタマテリアル Reviewed

酒井道, 橘邦英

應用物理,78 (5), pp. 437-441 2009.5

金属の周期的微細構造によるテラヘルツ波の伝搬制御

古川龍, 酒井道, 橘邦英

電気学会研究会資料. EMT, 電磁界理論研究会,2009 (22), pp. 63-66 2009.1

金属の周期的微細構造によるテラヘルツ波の伝搬制御(フォトニックNWシステム・デバイス,フォトニック結晶・ファイバとその応用,光集積回路,光導波路素子,光スイッチング,導波路解析,及び一般)

古川龍, 酒井道, 橘邦英

電子情報通信学会技術研究報告. LQE, レーザ・量子エレクトロニクス,108 (419), pp. 163-166 2009.1

Underwater microdischarge in arranged microbubbles produced by electrolysis in electrolyte solution using fabric-type electrode Reviewed

Osamu Sakai, Masaru Kimura, Tatsuru Shirafuji, Kunihide Tachibana

APPLIED PHYSICS LETTERS 93 ( 23 ) 2008.12

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER INST PHYSICS

Pulsed microdischarge was generated in microbubbles produced by electrolysis in an electrolyte solution without external gas feed by using a fabric-type electrode. The electrode structure not only allowed low-voltage ignition of the atmospheric-pressure discharge in hydrogen or oxygen containing microbubbles but also worked effectively in producing and holding the bubbles on its surface. The generation of reactive species was verified by optical emissions from the produced microplasmas, and their transport into the solution was monitored by the change in hydrogen concentration.

DOI： 10.1063/1.3006348

Experimental verification of complex dispersion relation in lossy photonic crystals Reviewed

Teruki Naito, Osamu Sakai, Kunihide Tachibana

APPLIED PHYSICS EXPRESS 1 ( 6 ) 2008.6

Language：English Publishing type：Research paper (scientific journal) Publisher：JAPAN SOC APPLIED PHYSICS

The complex wave number in lossy photonic crystals composed of microplasmas was verified experimentally by measuring the refractive index and the transmittance in a microwave range. These experimental results coincide with the dispersion relation obtained by a modified plan-wave expansion method. In particular, an extraordinary reduction of attenuation was observed in a certain frequency band, which is attributed to a loss term in the dielectric function and periodicity in the structure. These results indicate that a method of dynamic control for electromagnetic waves can be realized using of the dispersion relation in complex space. (C) 2008 The Japan Society of Applied Physics.

DOI： 10.1143/APEX.1.066003

Characteristics of metamaterials composed of microplasma arrays Reviewed

Osamu Sakai, Takui Sakaguchi, Teruki Naito, Dae-Sung Lee, Kunihide Tachibana

PLASMA PHYSICS AND CONTROLLED FUSION 49 ( 12B ) B453 - B463 2007.12

Language：English Publishing type：Research paper (scientific journal) Publisher：IOP PUBLISHING LTD

Functional arrays of microplasmas were designed for the purpose of controlling electromagnetic waves in the range 10 - 100GHz. A two-dimensional square-lattice array of microplasma columns was proved to have photonic-crystal-like properties showing the presence of photonic band gaps and flat bands. A one-dimensional linear array of cold cathode fluorescent lamps (CCFLs) embedded in between the truncated strip lines showed the resonant transmission characteristics due to the surface-wave modes corresponding to the flat-band frequencies. Another one-dimensional configuration of CCFLs vertically aligned along both sides of a coplanar strip line showed peculiar behavior at the lower and higher edges of the band gap. These unique wave-propagation properties are attributed to the periodic structures of which pitches are comparable to or smaller than the wavelengths of electromagnetic waves. Therefore, these artificial arrays are potential metamaterials, which can be used for plasma devices controlling electromagnetic waves.

DOI： 10.1088/0741-3335/49/12B/S43

Properties of electromagnetic wave propagation emerging in 2-D periodic plasma structures

Osamu Sakai, Kunihide Tachibana

IEEE Transactions on Plasma Science 35 ( 5 I ) 1267 - 1273 2007.10

Publishing type：Research paper (scientific journal)

Periodic structures in plasmas form unique dispersions of electromagnetic waves, which are examined in numerical calculation. Above electron plasma frequency, where waves equivalently propagate in dielectric media, plasma arrays with 2-D periodicity produce frequency regions of forbidden propagation, like band gaps in a photonic crystal. Below electron plasma frequency, where waves are usually forbidden to propagate due to cutoff phenomenon, bulk plasmas with periodic holes can become wave-propagating media in which localized surface modes play an important role. Such features give rise to concepts of dynamic wave controllers that change their inner electron density in time. © 2007 IEEE.

DOI： 10.1109/TPS.2007.906133

Generations and applications of atmospheric pressure glow discharge by integration of microplasmas

O. Sakai, K. Tachibana

Journal of Physics: Conference Series 86 ( 1 ) 2007.6

Publishing type：Research paper (scientific journal)

Integration of microplasmas enables us to obtain atmospheric pressure discharges with good macroscopic uniformity and various functions. We demonstrate two specific electrodes of microplasmas for generations of dielectric barrier discharges at atmospheric pressure, and integrated microplasmas on large two-dimensional area are sustained with electron density more than 1012cm-3. Especially "fabric" type electrode has a promising structure to provide various flexible discharge space and plasma processing on winding surface. Integrated microplasmas in array structure will serve as electromagnetic-wave control devices as well as processing tools, and two aspects are discussed in terms of equivalent dielectrics and metals. © 2007 IOP Publishing Ltd.

DOI： 10.1088/1742-6596/86/1/012015

Influence of sustaining frequency on the production efficiency of excited Xe atoms studied in unit cell microplasma for ACPDPs using spectroscopic diagnostics Reviewed

J.-S. Oh, O. Sakai, K. Tachibana

Journal of the Society for Information Display 15 297 2007

Plasma photonic crystals in two-dimensional arrays of microplasmas

O. Sakai, T. Sakaguchi, K. Tachibana

Contributions to Plasma Physics 47 ( 1-2 ) 96 - 102 2007

Publishing type：Research paper (international conference proceedings)

Two-dimensional arrays of microplasmas form "plasma photonic crystals", which have new functions that have not been present in bulk plasma. Two features of abnormal electromagnetic wave propagation on the first photonic band above the electron plasma frequency are demonstrated. Near the electron plasma frequency, unidirectional propagation takes place, and divergent propagation occurs near the band gap in the Γ-X direction. These phenomena are verified in experiments, and the details are confirmed by equi-frequency contours on a two-dimensional wavenumber plane. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA.

DOI： 10.1002/ctpp.200710014

Photonic bands in two-dimensional microplasma arrays. II. Band gaps observed in millimeter and subterahertz ranges

Takui Sakaguchi, Osamu Sakai, Kunihide Tachibana

Journal of Applied Physics 101 ( 7 ) 2007

Publishing type：Research paper (scientific journal)

Plasma photonic band gaps have been observed in a two-dimensional microplasma array, and we have characterized their properties by both experimental and theoretical results. Microplasma columns ignited in helium near atmospheric pressure formed crystal-like structures in a square lattice with a lattice constant from 1.5 to 2.5 mm. Microwaves in the millimeter range transmitting through the array region attenuated at frequencies of photonic band gap in the Γ-X direction, as predicted by the modified plane-wave expansion method. Frequency dependence around the band gap was clarified in the numerical analysis of electromagnetic wave propagation and agreed with experimental results. Electron density in microplasmas was estimated to be 1 × 1013 cm-3 from the attenuation rate at the band gap in the Γ-X direction. Variation of the lattice constant induced frequency shift of the band gap in the millimeter and subterahertz regions, and so plasma photonic crystal can perform as a dynamically controllable band-stop filter. © 2007 American Institute of Physics.

DOI： 10.1063/1.2713940

Photonic bands in two-dimensional microplasma arrays. I. Theoretical derivation of band structures of electromagnetic waves

Osamu Sakai, Takui Sakaguchi, Kunihide Tachibana

Journal of Applied Physics 101 ( 7 ) 2007

Publishing type：Research paper (scientific journal)

Two theoretical approaches appropriate for two-dimensional plasma photonic crystals reveal dispersions of propagating waves including photonic (electromagnetic) band gaps and multiflatbands. A modified plane-wave expansion method yields dispersions of collisional periodical plasmas, and the complex-value solution of a wave equation by a finite difference method enables us to obtain dispersions with structure effects in an individual microplasma. Periodical plasma arrays form band gaps as well as normal photonic crystals, and multiflatbands are present below the electron plasma frequency in the transverse electric field mode. Electron elastic collisions lower the top frequency of the multiflatbands but have little effect on band gap properties. The spatial gradient of the local dielectric constant resulting from an electron density profile widens the frequency region of the multiflatbands, as demonstrated by the change of surface wave distributions. Propagation properties described in dispersions including band gaps and flatbands agree with experimental observations of microplasma arrays. © 2007 American Institute of Physics.

DOI： 10.1063/1.2713939

Dynamic control of propagating electromagnetic waves using tailored millimeter plasmas on microstrip structures

O Sakai, K Tachibana

IEEE TRANSACTIONS ON PLASMA SCIENCE 34 ( 1 ) 80 - 87 2006.2

Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC

Millimeter plasmas were tailored to play a role as dynamic devices of microwave components, that is, to control electromagnetic wave propagation dynamically on a microstrip line. The generation of millimeter plasmas with their relatively long discharge channel (similar to 3 mm) was in high-pressure (20-200 torr) Ne, and they were successfully arranged near and on the conductor of microstrip lines. When such a plasma was set perpendicularly to the conductor to form a T junction, a significant reduction of electromagnetic wave transmission along the metal conductor was observed. The reduction rate depended on the discharge current and the number of T junctions. These experimental results are compared with the case of 2 MHz launching for crude electron density measurements and the numerical results of propagating electric fields in a two-dimensional model.

DOI： 10.1109/TPS.2005.863594

Spatiotemporal behaviors of excited xenon atoms in a three-dimensional diagnosable microplasma unit cell with high sustain frequency for plasma display panels

Jun Seok Oh, Osamu Sakai, Masaki Hashimoto, Kunihide Tachibana

Proceedings of International Meeting on Information Display 2006 925 - 928 2006

Publishing type：Research paper (international conference proceedings)

With the high sustain frequency at 250 kHz, a three-dimensional diagnostics of excited xenon atoms was investigated by the emission and absorption microscopic spectroscopy for plasma display panels, spatiotemporally. As a typical feature of the emissions, a broad discharge peak was observed on the temporal cathode and a sharp one was monitored on the anode, at the high frequency. However, the discharge starts from the cathode edge, actually, that is a discharge space on the floated address (or data) electrode. Spatially, the dense emission and absorption peak were observed in the discharge space between the sustain electrodes gap. Overall, the microdischarge at high sustain frequency showed priming dominant characteristics.

Interaction and control of millimetre-waves with microplasma arrays

O Sakai, T Sakaguchi, Y Ito, K Tachibana

PLASMA PHYSICS AND CONTROLLED FUSION 47 B617 - B627 2005.12

Language：English Publishing type：Research paper (scientific journal) Publisher：IOP PUBLISHING LTD

Propagation of electromagnetic waves in several types of microplasmas has been examined experimentally in a frequency range 10-75 GHz. Firstly, the fundamental characteristics of the propagation were investigated using a planar geometry of microplasma assembly, and the electron density was derived by a comparison of the transmittance with the theoretical analyses using a Drude type model with collisional effects. Secondly, an extraordinary propagation phenomenon such as the focusing effect was observed in a two-dimensional periodical microplasma array. This kind of anomalous refraction cannot be interpreted only by predictions based on the dielectric property of bulk plasma, and it is suggested that a photonic-crystal-like periodical dielectric structure may play a significant role. Thirdly, it was demonstrated that the T-junction formed by a microplasma connected to a microstrip line can control the transmission of microwaves. An attenuation (or modulation) depth of about 35% was obtained with a series of two T -junctions connected to the strip line at the right-angled corners. All the above features come from (a) the relatively high electron density of the microplasmas near 10(13) cm(-3), (b) the complex dispersion relation with collisional effects and (c) the spatial arrangement with a characteristic scale of the same order of the wavelength of microwaves.

DOI： 10.1088/0741-3335/47/12B/S45

Verification of a plasma photonic crystal for microwaves of millimeter wavelength range using two-dimensional array of columnar microplasmas

O Sakai, T Sakaguchi, K Tachibana

APPLIED PHYSICS LETTERS 87 ( 24 ) 2005.12

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER INST PHYSICS

We experimentally verified that a microplasma assembly can create a functional dielectric layer for the propagation of electromagnetic waves as a "plasma photonic crystal." A two-dimensional array in a square lattice was composed of columnar plasmas of about 2 mm in diameter, and the transmitted microwaves at 70-75 GHz showed a change of energy flow direction. This result is attributed to the fact that periodical structure is composed of individual plasma columns with a different dispersion than the ambient part and the experimental frequency range lies in the vicinity of the lowest band gap of the photonic crystal calculated theoretically. (c) 2005 American Institute of Physics.

DOI： 10.1063/1.2147709

Effects of self-erasing discharges on the uniformity of the dielectric barrier discharge

T Somekawa, T Shirafuji, O Sakai, K Tachibana, K Matsunaga

JOURNAL OF PHYSICS D-APPLIED PHYSICS 38 ( 12 ) 1910 - 1917 2005.6

Language：English Publishing type：Research paper (scientific journal) Publisher：IOP PUBLISHING LTD

Atmospheric pressure discharges in Ne, N-2 and N-2/O-2 gases have been investigated with a novel high-voltage pulse power supply. Three characteristic types of discharge modes were observed: (1) diffuse glow discharge seen in Ne gas both with the eye and high-speed camera; (2) diffuse glow-like discharge in N-2 gas observed with the eye, but seen as a vague filamentary discharge with a high-speed camera; and (3) clear filamentary discharge in N-2/O-2 gas mixture both with the eye and high-speed camera. Characteristics in the current and voltage waveforms revealed that the existence of the second voltage with opposite polarity following the first impulse voltage was important to have a diffuse glow-like discharge. This suggests that elimination of the memory effect by non-uniformly distributed charges on the surface is important for obtaining the diffuse mode because the non-uniformity triggers a positive feedback to the current constriction at the points with higher charge density, and results in filamentation of the discharge.

DOI： 10.1088/0022-3727/38/12/010

Measurement of metastable He-*(2(3)S(1)) density in dielectric barrier discharges with two different configurations operating at around atmospheric pressure

K Tachibana, Y Kishimoto, O Sakai

JOURNAL OF APPLIED PHYSICS 97 ( 12 ) 2005.6

Language：English Publishing type：Research paper (scientific journal) Publisher：AMER INST PHYSICS

We have measured the density of metastable He atoms in the lowest triplet state (2(3)S(1)) with a diode-laser absorption spectroscopic technique in atmospheric pressure plasmas produced by dielectric barrier discharge schemes. Two different types of electrode configuration are employed: one is a conventional parallel-plate system and the other is a microdischarge integrated system with stacked metal-mesh electrodes covered by insulating films. We have analyzed the pressure-broadened spectral line corresponding to the 2(3)S(1)-> 2(3)P(J) (J=0-2) transition to derive the broadening coefficient and to calibrate absolute densities. The measured density ranges from 10(11) to 10(12) cm(-3), but the values in the mesh-type system are larger than those in the parallel-plate system by about one order of magnitude. The density, however, depends strongly on the gas flow rate, showing the influence of quenching by the Penning-ionization process with impurities. Those behaviors are consistent with the variation of the electron density estimated by millimeter-wave transmittance measurement.(c) 2005 American Institute of Physics.

DOI： 10.1063/1.1922581

Diagnostics of microdischarge-integrated plasma sources for display and materials processing

K Tachibana, Y Kishimoto, S Kawai, T Sakaguchi, O Sakai

PLASMA PHYSICS AND CONTROLLED FUSION 47 A167 - A177 2005.5

Language：English Publishing type：Research paper (scientific journal) Publisher：IOP PUBLISHING LTD

Two different types of microdischarge-integrated plasma sources have been operated at around the atmospheric pressure range. The discharge characteristics were diagnosed by optical emission spectroscopy (OES), laser absorption spectroscopy (LAS) and microwave transmission (MT) techniques. The dynamic spatiotemporal behaviour of excited atoms was analysed using OES and LAS and the temporal behaviour of the electron density was estimated using the MT method. In Ar and Xe/Ne gases, waveforms of the NIT signal followed the current waveform in the rise period and lasted longer according to the recombination losses. However, in He the waveform followed the density of metastable atoms, reflecting the production of a large amount of electrons by the Penning ionization process with impurities. The estimated peak electron density in those plasma sources is of the order of 10(12) cm(-3), and the metastable atom density can reach 10(13) cm(-3). Thus, it is suggested that these sources can be potentially applied to convenient material processing tools of large area operated stably at atmospheric pressure.

DOI： 10.1088/0741-3335/47/5A/012

Integrated coaxial-hollow micro dielectric-barrier-discharges for a large-area plasma source operating at around atmospheric pressure

O Sakai, Y Kishimoto, K Tachibana

JOURNAL OF PHYSICS D-APPLIED PHYSICS 38 ( 3 ) 431 - 441 2005.2

Language：English Publishing type：Research paper (scientific journal) Publisher：IOP PUBLISHING LTD

An integrated structure of coaxial-hollow micro dielectric-barrier-discharges has been developed by stacking two metal meshes covered with insulating material. The test panel has an effective area of 50 mm diameter in which hundreds of hollow structures are assembled with a unit hollow area of 0.2 x 1.7 mm(2). He or N-2 was used as the discharge gas in the pressure range from 20 to 100 kPa and the firing voltage was less than 2kV, even at the maximum pressure. The intensity of each microdischarge was observed to be uniform over the whole area throughout the pressure range, and its time evolution during one discharge pulse was analysed through two-dimensional microscopic observation with a gated CCD camera. In a gas flow regime through the coaxial hollow structures, several significant changes of the discharge properties were observed, such as impurity reduction, decrease in discharge voltage and cooling of the neutral gas. The fundamental plasma parameters were measured using a single probe in the downstream region of microdischarges using an auxiliary flat electrode set apart from the mesh electrode plane. The occurrence of an extended glow with a length of some millimetres was observed in He but not in N-2. The electron density derived by the probe data in He at 100 kPa was similar to3 x 10(11) cm(-3), suggesting a value of more than 10(12) cm(-3) in the active microdischarge region.

DOI： 10.1088/0022-3727/38/3/012

Two-dimensional analysis of data writing on a dielectric layer surface by a microdischarge working as a regulated output/storage device

Osamu Sakai, Kazuhiko Inoguchi

Journal of Physics D: Applied Physics 36 ( 23 ) 2891 - 2897 2003.12

Publishing type：Research paper (scientific journal)

Data writing on a dielectric layer surface by a microdischarge is investigated using a two-dimensional numerical analysis. A microdischarge, working as a regulated output/storage device, accumulates a regulated amount of charge on the surface of an adjacent dielectric layer. The charge density depends on the voltage applied on a writing electrode, located externally on the opposite side of the dielectric layer. Data writing as a charge accumulation is completed not during the discharge but in the phase of afterglow. As a result, a regulated electric field is imposed on the dielectric layer. The charge density dependence on its polarity is also investigated.

DOI： 10.1088/0022-3727/36/23/005

Flat triode structure with a pulsed coplanar discharge channel representing similar features to a field-effect transistor

Osamu Sakai, Satoshi Hashimoto, Akitsugu Hatano

Applied Physics Letters 82 ( 17 ) 2781 - 2783 2003.4

Publishing type：Research paper (scientific journal)

The generation of a regulated coplanar discharge in a pulse operation by a completely flat configuration of three electrodes was analyzed. The discharge current was monotonously controlled by the third electrode which keeps its isolation without any insulator layer. The current regulation mechanism shows the effects of electrode separation distance, gas pressure, and distortion of a three dimensional potential profile.

DOI： 10.1063/1.1570510

Switching and regulation of a pulsed discharge channel in a coplanar plasma tetrode

Osamu Sakai, Satoshi Hashimoto, Akitsugu Hatano

Applied Physics Letters 82 ( 15 ) 2392 - 2394 2003.4

Publishing type：Research paper (scientific journal)

The control of a pulsed coplanar discharge by two-dimensional potential tailoring in a flat tetrode was investigated. To realize, a tetrode configuration, tens of millimeter-size electrodes made of stainless steel with 2 nm thickness was used on a insulator plate surface. One pair of electrodes generated a coplanar discharge, and the other electrodes played roles of switching and controlling the discharge current by changing their electrical potential, where their electrical isolation from the discharge channel was kept without an insulator layer.

DOI： 10.1063/1.1567826

Comparative investigation of end plate and limiter biasing in the HIEI tandem mirror

Y. Yasaka, O. Sakai, Y. Ohe

Fusion Technology 39 ( 1 T ) 91 - 96 2001.1

Publishing type：Research paper (scientific journal)

It is demonstrated that the transition leading to the improved radial confinement is induced by the end plate biasing in the HIEI tandem mirror, in which the plasma is MHD stabilized, heated, and axially confined by the ICRF waves. On application of the bias voltage, the fluctuation level decreases, the density increases to over 1013 cm-3, and the plasma rotation changes significantly indicating a formation of the internal transport barrier. The observed strong nonlinear phenomena; the hysteresis change of the fluctuation level and the neutral line emission intensity, demonstrate bifurcation feature, which is similar to that in the previous limiter biasing experiment. This bifurcation feature may be explained by the model from the azimuthal momentum balance including the flow shear stabilization of the drift wave instabilities.

DOI： 10.13182/fst01-a11963419

Poloidal phasing effects on particle transport during ion Bernstein heating

H. Takeno, Y. Yasaka, O. Sakai, R. Itatani

Plasma Physics and Controlled Fusion 41 ( 5 ) 613 - 623 1999.5

Publishing type：Research paper (scientific journal)

Ion Bernstein wave (IBW) heating experiments are performed with a poloidally phased antenna. Between two kinds of poloidal phasing, no difference is found with the heating effect which is expected from the mechanism of heating presented previously. The significant differences due to the difference of poloidal phasing are found in observations of electron density variation, emission intensities of impurity lines and scrape-off layer plasma. All the results show that the poloidal phasing brings about variations of particle transport, and a ponderomotive effect of the antenna near-field might be the most probable mechanism. The phasing effect should be considered on an intense radio frequency (RF) injection by poloidally aligned antenna elements.

DOI： 10.1088/0741-3335/41/5/304

High-beta operation with enhanced axial and radial confinement in the HIEI tandem mirror

Osamu Sakai, Kenichi Iwami, Yasuyoshi Yasaka

Japanese Journal of Applied Physics, Part 1: Regular Papers and Short Notes and Review Papers 35 ( 7 ) 4056 - 4063 1996.7

Publishing type：Research paper (scientific journal)

The operation of a tandem mirror using Ion Cyclotron Range of Frequencies (ICRF) waves and limiter biasing is performed in the HIEI tandem mirror. An ion confining potential is formed in the plug cell by high-field-side injection of the fast magnetosonic wave. Limiter biasing is simultaneously imposed with formation of a confining potential and minority heating in the central cell, and both axial and radial confinement are enhanced by a factor of 4. As a result, a high-beta plasma (∼14%) is produced in the central cell. The high-beta plasma is sustained magnetohydrodynamically by ponderomotive force of ICRF electric fields.

DOI： 10.1143/jjap.35.4056

High density central cell plasma production and ion heating by helicon wave damping and its mode conversion into a slow Alfven wave

H. Takeno, Y. Yasaka, O. Sakai, R. Itatani

Nuclear Fusion 35 ( 1 ) 75 - 85 1995

Publishing type：Research paper (scientific journal)

A new scheme of plasma production and ion heating in the central cell of the tandem mirror is demonstrated. Helicon mode plasma production in combination with two ion species plasma heating by mode conversion of a fast Alfven wave into a slow Alfven wave realizes ion heating of a high density plasma, and could remove the density limitation caused by the conventional scheme using slow ion cyclotron waves. The achieved value of products of electron density and average ion temperature is 9*1020 eV.m-3 for an RF power of 165 kW. Analysis of wave damping indicates that matching the radiation spectrum of the antenna with the damping spectrum results in efficient plasma production. The phased dual double-half-turn antenna is suitable for helicon wave spectrum control.

DOI： 10.1088/0029-5515/35/1/I06

Mode conversion of fast magnetosonic waves and minority ion heating in a longitudinally inhomogeneous magnetic field

O. Sakai, Y. Yasaka

Physics of Plasmas 1 ( 12 ) 3896 - 3904 1994

Publishing type：Research paper (scientific journal)

Mode conversion process in ICRF (Ion Cyclotron Range of Frequencies) in a longitudinally inhomogeneous two-ion-species plasma is clarified in the HIEI tandem mirror experiments. Measurements of wave number parallel to the magnetic field reveal that the fast magnetosonic wave of low-field-side incidence converts into the slow ion-cyclotron wave near the ion-ion hybrid resonance. The propagation of the slow wave is detected between the ion-ion hybrid resonance and minority ion cyclotron resonance layers, which shows good agreement with the calculated dispersion relation based on one- and two-dimensional plasma models. Simultaneous minority ion heating is observed, and it is due to the presence of the slow wave that resonates with minority ions at the minority ion cyclotron resonance. © 1994 American Institute of Physics.

DOI： 10.1063/1.870861

Selective ion and electron heating and potential formation using mode conversions of ICRF waves in HIEI

Y. Yasaka, O. Sakai, H. Takeno, H. Hojo

Nuclear Fusion 34 ( 9 ) 1263 - 1269 1994

Publishing type：Research paper (scientific journal)

Plasma heating and potential formation by ion cyclotron range of frequencies (ICRF) waves are investigated in the HIEI tandem mirror. It is demonstrated that ions and electrons are selectively heated by the appropriate waves that have been mode converted from the fast magnetosonic wave in a two-ion-species plasma. In the central cell, the minority ions can be heated up to 0.3 keV with the presence of the mode-converted slow ion cyclotron wave. Plug cell electrons are heated by the mode-converted electrostatic slow wave resulting in the formation of an ion confining potential. The dependence of the ion confining potential on the plug electron temperature is in qualitative agreement with values predicted by the improved modified Boltzmann relation which includes velocity space diffusion of electrons due to the ICRF wave. The axial confinement of the core plasma is improved by the ICRF produced potential.

DOI： 10.1088/0029-5515/34/9/I07

High radial confinement mode induced by dc limiter biasing in the HIEI tandem mirror

O. Sakai, Y. Yasaka, R. Itatani

Physical Review Letters 70 ( 26 ) 4071 - 4074 1993

Publishing type：Research paper (scientific journal)

A transition from low to high radial confinement mode is triggered by biasing a limiter in the HIEI tandem mirror. Positive dc biasing gives rise to reduction of fluctuation level in density and potential in the periphery, drop of neutral line emission, and bifurcation in limiter current. After the transition, the plasma exhibits density rise in the bulk and steepening of the density gradient. Significant radial rotational shear is observed when the edge turbulence is suppressed by the biasing. The total feature shows similar characteristics of L- to H-mode transition observed in tokamak devices. © 1993 The American Physical Society.

DOI： 10.1103/PhysRevLett.70.4071

Nonlinear ion Bernstein wave heating experiment in the WT-3 tokamak

Y. Yasaka, H. Takeno, M. Sakka, O. Sakai, R. Itatani, T. Fujimoto, H. Suemitsu, K. Takahata, M. Fukao, S. Tanaka, Y. Terumichi

Plasma Physics and Controlled Fusion 35 ( 3 ) 379 - 389 1993

Publishing type：Research paper (scientific journal)

Ion Bernstein wave (IBW) heating at ion cyclotron half-harmonic frequencies was performed in hydrogen (H) discharges in the WT-3 tokamak. The bulk ion temperature increase was measured by a charge exchange analyser and a spatially scanning polychrometer. The latter diagnostics can provide radial ion-temperature profiles. The incremental ion temperature has shown a peak at the exact location where the IBW frequency omega equals 3/2 of the ion cyclotron frequency, indicating the nonlinear direct acceleration of H ions. The wave electric field was measured by a Langmuir probe in the scrape-off layer. The electric field at the second harmonic of the excited IBW frequency was observed to vary depending on the power level and the toroidal magnetic field. These experimental results are compared with the theoretical prediction of nonlinear IBW damping. The measured dependences of the ion heating and the wave amplitude at 2 omega on the toroidal magnetic field are in reasonable agreement with the prediction.

DOI： 10.1088/0741-3335/35/3/007

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