Faculty Profiles - SAKAI Osamu

Papers - SAKAI Osamu

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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

Scopus

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

Scopus

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

Scopus

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

Scopus

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

Web of Science

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.

Scopus

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

Web of Science

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

Web of Science

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

Web of Science

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

Web of Science

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

Web of Science

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

Web of Science

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

Scopus

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

Scopus

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

Scopus

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

Scopus

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

Scopus

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

Scopus

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

Scopus

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Papers - SAKAI Osamu