九州大学 研究者情報
論文一覧
池添 竜也(いけぞえ りゆうや) データ更新日:2019.09.16

准教授 /  応用力学研究所 附属高温プラズマ理工学研究センター


原著論文
1. M. Fukuyama, Hiroshi Idei, T. I. Tsujimura, S. Kubo, S. Kobayashi, M. Yunoki, Takumi Onchi, Ryuya Ikezoe, Quasi-optical polarizer system for ECHCD experiments in the QUEST, Fusion Engineering and Design, 10.1016/j.fusengdes.2019.02.099, 2019.01, [URL], A new transmission line from 28 GHz gyrotron to QUEST spherical tokamak (ST) has been developed for the highly efficient electron cyclotron heating and current drive (ECHCD) experiments. The ECHCD effect depends on the incident mode or polarization states of the injected beam. Although two corrugated-polarizer miter bends were developed and assembled to transmission line in the QUEST, arcing events were frequently detected at the polarizer miter bend when the high-power millimeter-wave were transmitted. A new quasi-optical (QO) concept for the polarizer system composed of two corrugated plate and a QO mirror was proposed to avoid the arcing at polarizer. The QO mirror was designed for coupling of input and output HE
11
modes at the polarizer, its coupling property is discussed with Kirchhoff integral. Polarization-setting performance of the developed QO polarizer was examined in a low-power test system, and is discussed with grating-polarizer calculation..
2. Atsuto Tanaka, Makoto Ichimura, Mafumi Hirata, Ryuya Ikezoe, Shuhei Sumida, Seowon Jang, Koki Izumi, Yushi Kubota, Ryo Sekine, Hiroki Kayano, Mizuki Sakamoto, Yousuke Nakashima, Observation of potential increase in the central cell due to ICRF heating in the non-axisymmetric anchor cell on GAMMA 10/PDX, Plasma and Fusion Research, 10.1585/pfr.14.2401028, 14, Specialissue1, 2019.01, [URL], We used the additional Ion Cyclotron Range of Frequency (ICRF) heating in the anchor cell for the high density plasma production. When the ICRF waves are excited with antennas installed in the anchor cell, it is observed that the plasma potential increases significantly and forms non-axisymmetric potential distribution in the periphery of the central cell. Experiments with different frequencies were carried out to understand the physical mechanism of this potential change due to the ICRF waves. It is confirmed that the ion heating does not play an important role to the potential increase. The floating potentials on some tips set outside of the plasma in the anchor cell remarkably decrease during the additional heating and non-axisymmetric distribution of the floating potential in the central cell depends on the antenna location in the anchor cell. It is suggested that the radial transport of electrons near the antennas in the non-axisymmetric anchor cells with the ICRF antennas is a possible candidate of the potential increase in the central cell..
3. Ryo Sekine, Ryuya Ikezoe, Seowon Jang, Mafumi Hirata, Makoto Ichimura, Mizuki Sakamoto, Shuhei Sumida, Koki Izumi, Atsuto Tanaka, Yushi Kubota, Hiroki Kayano, Yousuke Nakashima, Observation Of Density Fluctuations Originated From RF waves with two-channel reflectometer in GAMMA 10/PDX, Plasma and Fusion Research, 10.1585/pfr.14.2402011, 14, Specialissue1, 2019.01, [URL], Slow Alfvén wave with a frequency just below an ion cyclotron frequency can be destabilized by the anisotropy of ion temperature and called as Alfvén-Ion-Cyclotron (AIC) wave. In the GAMMA 10/PDX central cell, several AIC waves simultaneously appear in an unstable frequency range. In addition, low-frequency waves with the difference frequencies (DF) between the AIC waves are observed, and their characteristics have to be clarified to reveal their unknown contributions to the ion transport suggested in GAMMA 10/PDX. In this study, simultaneous two-point measurements in the azimuthal and axial directions were conducted by using a two-channel microwave reflectometer with multi horn antennas. The azimuthal mode number (m) of the DF waves was confirmed at inner plasma region for the first time, and was zero, which satisfies azimuthal mode matching with the m = -1 AIC waves. In addition, it was shown that the DF waves were radially localized in an inner region, which is different from rather broad profiles of the AIC waves..
4. Koki Izumi, Ryuya Ikezoe, Makoto Ichimura, Mafumi Hirata, Mizuki Sakamoto, Shuhei Sumida, Seowon Jang, Atsuto Tanaka, Yushi Kubota, Ryo Sekine, Hiroki Kayano, Yousuke Nakashima, Measurement of end-loss ions originated from spontaneously excited high frequency waves by using an MCP detector on GAMMA 10/PDX, Plasma and Fusion Research, 10.1585/pfr.14.2402033, 14, Specialissue1, 2019.01, [URL], Effective ICRF heating creates a high ion-temperature plasma with strong ion-temperature anisotropy in the GAMMA 10/PDX central cell, where several Alfvén-ion-cyclotron (AIC) waves are spontaneously excited. It is clearly measured with a microwave reflectometer installed in the central cell that waves with the difference frequencies of the AIC waves are excited in a core region. In addition, it has been observed with an end-loss high-energy ion detector that high-energy ions of over 6 keV are axially transported along the magnetic field lines with the same frequencies as the difference frequencies of the AIC waves. In this study, in order to understand the related wave-particle interaction, another end-loss high-energy ion detector using a micro-channel plate has been developed. By changing the ion retarding grid voltage, which the new detector equips, it is found that an energy dependence exists in the loss mechanism; some frequencies that the axial transport includes disappear for high-energy ions of over 6 keV while they are significant for low-energy ions. In addition, by changing the radial position of the new detector, a radial localization of the axial transport to a core region, which is consistent with the profile of the difference frequencies waves, is indicated..
5. Yushi Kubota, Mafumi Hirata, Seowon Jang, Makoto Ichimura, Ryuya Ikezoe, Mizuki Sakamoto, Shuhei Sumida, Atsuto Tanaka, Koki Izumi, Ryo Sekine, Hiroki Kayano, Yousuke Nakashima, Atsushi Fukuyama, Investigation of ICRF heating efficiency in plug/barrier cell on GAMMA 10/PDX with a full-wave code, Plasma and Fusion Research, 10.1585/pfr.14.2401012, 14, Specialissue1, 2019.01, [URL], In the west end of GAMMA 10/PDX, the end-loss ion flux is utilized for divertor simulation experiments. It is important for controlling parameters of the end-loss ion flux to investigate the efficiency of ion cyclotron range of frequency (ICRF) heating in the west plug/barrier cell, which is next to the west end region. The ICRF heating experiments in the west plug/barrier cell have been performed. In this study, three-dimensional simulations of the ICRF wave propagation were carried out in the plug/barrier cell for the first time. Two types of antennas, a Double Half Turn (WB-DHT) and a Nagoya Type-III (WB-Type-III) antennas, are evaluated. As the results, it is clarified that the WB-Type-III antenna is more effective for the ion heating than the WB-DHT antenna when the frequency is near the ion cyclotron resonance frequency at the midplane of the plug/barrier cell. An additional experiment with the WB-Type-III antenna which is located near the midplane of the plug/barrier cell has been performed. The improvement of the power absorption has been observed. TheWB-Type-III antenna located near the midplane is more effective for the ion heating. The experimental results are consistent with the simulation of the wave propagation analysis..
6. Mafumi Hirata, Junpei Itagaki, Ryuya Ikezoe, Makoto Ichimura, Shuhei Sumida, Seowon Jang, Koki Izumi, Atsuto Tanaka, Yushi Kubota, Ryo Sekine, Hiroki Kayano, Mizuki Sakamoto, Yousuke Nakashima, Investigation of ICRF heating effect in anchor region on GAMMA 10/PDX, Plasma and Fusion Research, 10.1585/pfr.14.2402055, 14, Specialissue1, 2019.01, [URL], To investigate end-loss plasma control, several heating operations were carried out using ion-cyclotron range of frequency (ICRF) heating systems in GAMMA 10/PDX. In anchor region, one of the important heating positions to control the end loss particle and heat fluxes, excited ICRF waves are measured as the density fluctuations using an upgraded reflectometer system and the detailed radial profiles are observed. The efficiency of anchor heating was changed using the phase control between two antennas installed in the anchor and/or the central region. The interference of two waves during a phase-control ICRF heating has been directly measured inside the plasma. In addition, several phase dependencies were observed using absorption methods with photodiode in anchor region..
7. Y. Nakashima, M. S. Islam, T. Iijima, M. Sakamoto, N. Ezumi, M. Yoshikawa, N. Asakura, M. Fukumoto, A. Hatayama, M. Hirata, M. Ichimura, Ryuya Ikezoe, T. Imai, M. M. Islam, T. Kariya, J. Kohagura, S. Masuzaki, R. Minami, T. Nakano, K. Nojiri, T. Numakura, K. Sawada, M. Shoji, A. Terakado, S. Togo, S. Yamashita, T. Yoshimoto, Impact of additional plasma heating on detached plasma formation in divertor simulation experiments using the GAMMA 10/PDX tandem mirror, Nuclear Materials and Energy, 10.1016/j.nme.2018.12.033, 18, 216-221, 2019.01, [URL], The transition of a detached to attached plasma experiment has been performed in GAMMA 10/PDX by applying an additional plasma heating pulse of electron cyclotron heating (ECH). In a plasma detachment experiment, a short pulse (25 ms) of ECH was applied at the east plug-cell to examine the effects of electron heating on the plasma parameters in the D-module. It was found that the ion flux increased significantly during ECH injection. In the ECH injection period, the ion flux increased with the increasing impurity injection, which indicates the impurity ion components are enhanced by ionization in the D-module due to the application of ECH. During ECH injection period, the increase of the ion flux near the corner of the target plate shows a clear dependence on the gas species. The 2D visible emission has been captured by the high-speed camera and a bright emission near the corner of the target has been observed. The spectroscopic measurement in the D-module shows that the impurity ion emission increases remarkably during ECH injection. These results indicate the detached plasma change to the attached state..
8. Takumi Onchi, Hiroshi Idei, K. Nakamura, T. Nagata, S. Kawasaki, R. Ashida, M. Fukuyama, M. Hasegawa, Ryuya Ikezoe, A. Higashijima, K. Kuroda, Y. Nagashima, K. Hanada, High voltage electrical system of 8.56 GHz CW klystron for electron cyclotron heating on QUEST spherical tokamak, Fusion Engineering and Design, 10.1016/j.fusengdes.2019.04.043, 2019.01, [URL], A high voltage DC power supply for the cathode of the 8.56 GHz CW klystron has been set up for electron cyclotron heating (ECH)in steady state tokamak operation on QUEST spherical tokamak. The power supply is equipped with an IGBT array and a reactor for fast shutoff of the voltage in 10 μs, where the influx of electric energy at the short circuit is limited to 5 J. AC switches also have been installed in the three-phase power lines. High voltage relays are useful to save electric energy consumption. Fast three-phase AC switching by IGBT-stack is applicable to reduce the electric load of the components of the klystron power supply..
9. Hatem Elserafy, Kazuaki Hanada, Kengoh Kuroda, Hiroshi Idei, Ryota Yoneda, Canbin Huang, Shinichiro Kojima, Makoto Hasegawa, Yoshihiko Nagashima, Takumi Onchi, Ryuya Ikezoe, Aki Higashijima, Takahiro Nagata, Shoji Kawasaki, Shun Shimabukuro, Nicola Bertelli, Masayuki Ono, HFS injection of X-mode for EBW conversion in QUEST, Plasma and Fusion Research, 10.1585/pfr.14.1205038, 14, 1, 2019.01, [URL], High field side (HFS) injection of eXtra-ordinary X-mode for electron Bernstein wave (EBW) conversion was conducted in the QUEST tokamak. Radio frequency (RF; 8.2 GHz) power was delivered from the low field side (LFS) to the high field side HFS through waveguides, and from the HFS placed 20 cm above the midplane of the vacuum vessel. The aim was to compare the RF launches from the LFS and HFS. The plasma brightness, measured by a fast camera, as well as the H
α
signal captured along the mid-plane, was noticeably higher in the HFS launch than in the LFS launch. The HFS injection achieved a plasma current of approximately 130 A, versus 35A in the LFS injection. The electron density n
e
predicted from the position of the upper hybrid resonance agreed with the line-averaged n
e
measured by an interferometer, confirming the effective conversion and subsequent damping of the EBW mode. The RF leakage of the HFS injection was less than one-sixth that of the LFS injection. These results indicate that HFS delivers better RF coupling and conversion efficiency to EBW than LFS injection. Such efficient plasma heating via EBW will significantly enhance the plasma production..
10. Ryo Sekine, Ryuya Ikezoe, Seowon Jang, Mafumi Hirata, Makoto Ichimura, Mizuki Sakamoto, Shuhei Sumida, Koki Izumi, Atsuto Tanaka, Yushi Kubota, Hiroki Kayano, Yousuke Nakashima, Erratum
"Observation of density fluctuations originated from RF waves with two-channel reflectometer in GAMMA 10/PDX" [Plasma Fusion Res. 14, (2019), (2402011)], Plasma and Fusion Research, 10.1585/pfr.14.2902065, 14, Specialissue1, 2019.01, [URL].
11. Seowon Jang, Makoto Ichimura, Mafumi Hirata, Ryuya Ikezoe, Mizuki Sakamoto, Shuhei Sumida, Koki Izumi, Atsuto Tanaka, Yushi Kubota, Ryo Sekine, Hiroki Kayano, Yousuke Nakashima, Effect of the potential of confined plasma on end-loss Ion in GAMMA 10/PDX, Plasma and Fusion Research, 10.1585/pfr.14.2402032, 14, Specialissue1, 2019.01, [URL], In the GAMMA 10/PDX tandem mirror, studies on divertor physics have been performed in the west end region by utilizing end-loss plasmas flowing from the confinement region. Since the plasma density in the end region is quite low (~ 1016 m-3), an increase of the end-loss ion flux is required. The increase of the end-loss ion flux has been obtained by increasing the density in the confinement region on previous experiments. When an additional ICRF heating using the antennas in the anchor cells has been performed, a significant increase of the ion flux has been observed with the increase of the potential in the central cell although the change of line densities in the confinement region is little. The effect of the potential on the ion flux has been examined on the GAMMA 10/PDX using about 10,000 discharges. The ion flux increases almost linearly with the potential and the density increase. By comparing with a simple calculation, the increase of the ion flux is explained by the expansion of loss cone boundary of ions and the decrease of the transport time of the end-loss ions from the confinement region to the end region..
12. Ryuya Ikezoe, Yushi Kubota, Makoto Ichimura, Mafumi Hirata, Shuhei Sumida, Seowon Jang, Koki Izumi, Atsuto Tanaka, Ryo Sekine, Hiroki Kayano, Yoriko Shima, Junko Kohagura, Masayuki Yoshikawa, Mizuki Sakamoto, Yousuke Nakashima, Atsushi Fukuyama, A full wave simulation on the density dependence of a slow wave excitation in the GAMMA 10/PDX central cell with TASK/WF3D, Plasma and Fusion Research, 10.1585/pfr.14.2402003, 14, Specialissue1, 2019.01, [URL], Beach heating using a slow Alfvén wave in ion cyclotron range of frequencies would be the first candidate for ion heating in a DEMO-relevant divertor testing linear plasma device if it is applicable to a high-density regime. To clarify its availability, the density dependence of a slow wave excitation is investigated using a full wave simulation with TASK/WF3D code in the GAMMA 10/PDX central cell configuration, where there is an extensive track record of a beach heating. A shielding effect is successfully demonstrated and well understood under a three-dimensional configuration in the limit of cold plasma approximation. As the density increases, excitable left-handed electric field, which contributes to ion cyclotron heating, degrades more and more from a core region, and resultantly the ion absorption region goes outwards with reducing its power. For core densities above 1020 m-3, the wave field exists only at a very edge, and ion heating becomes negligible unless the wave frequency is much increased with a correspondent magnetic field enhancement..
13. Shuhei Sumida, Kouji Shinohara, Ryuya Ikezoe, Makoto Ichimura, Mizuki Sakamoto, Mafumi Hirata, Shunsuke Ide, Characteristics of fast 3He ion velocity distribution exciting ion cyclotron emission on JT-60U, Plasma Physics and Controlled Fusion, 10.1088/1361-6587/aaf184, 61, 2, 2019.02, [URL], Ion cyclotron emission (ICE) is probably excited by non-thermal ion velocity distribution. Identifying characteristics of the ion velocity distribution that excites the ICE can contribute to understanding its emission mechanism. The characteristics of fast helium-3 (3He) ion velocity distribution that excites 3He ICE [ICE(3He)] on JT-60U are investigated. First, fast deuterium ion distributions, which mainly contribute to deuterium-deuterium fusion reactions, are calculated with a fast ion orbit following Monte-Carlo code (OFMC code) under realistic conditions to evaluate birth spatial and velocity distributions of the 3He ions. Then, the fast 3He ion distributions are evaluated with the OFMC code using the birth distributions and compared between cases when the ICE(3He) is observed and not. The evaluated fast 3He ion distributions at the midplane edge of the plasma on the low field side have strong pitch-angle anisotropy in both cases. In the case with the ICE(3He) excitation, a relatively peaked bump-on tail structure in the energy direction is formed in the distributions. The formation of the relatively peaked bump-on tail structure is localized at the plasma edge on the low field side. On the other hand, the distributions at the plasma edge on the low field side have a broader structure in the energy direction in the case without the excitation. The comparison results show that the formation of the relatively peaked bump-on tail structure in the fast 3He ion distribution makes a key contribution to the excitation of the ICE(3He) on JT-60U..
14. S. Sumida, K. Shinohara, Ryuya Ikezoe, M. Ichimura, M. Sakamoto, M. Hirata, S. Ide, Study on ion cyclotron emission excited by DD fusion produced ions on JT-60U, 45th EPS Conference on Plasma Physics, EPS 2018
45th EPS Conference on Plasma Physics, EPS 2018
, 589-592, 2018.01.
15. Md Maidul Islam, Yousuke Nakashima, Shinji Kobayashi, Nobuhiro Nishino, Kazuya Ichimura, Takaaki Iijima, Md Shahinul Islam, Takayuki Yokodo, Guanyi Lee, Tsubasa Yoshimoto, Sotaro Yamashita, Masayuki Yoshikawa, Junko Kohagura, Mafumi Hirata, Ryutaro Minami, Tsuyoshi Kariya, Ryuya Ikezoe, Makoto Ichimura, Mizuki Sakamoto, Tsuyoshi Imai, Study of Plasma Behavior during ECRH Injection in the GAMMA 10 SMBI Experiments, 2017 International Conferences on Research and Applications of Plasmas, PLASMA 2017
Journal of Physics: Conference Series
, 10.1088/1742-6596/959/1/012007, 959, 1, 2018.02, [URL], Establishment of fueling system is one of the critical issues for the future fusion reactors. Fueling experiment supersonic molecular beam injection (SMBI) have been carried out in the central-cell of GAMMA 10. In GAMMA 10, electron cyclotron resonance heating (ECRH) is used at plug/barrier-cells for the formation of the axial confining potential. Recently, ECRH was applied during SMBI to plug the loss particles and increased the plasma density in the central-cell compared to without ECRH. This result suggests that the particles are confined during SMBI due to the injection of ECRH at plug/barrier-cells in GAMMA 10..
16. Shuhei Sumida, Kouji Shinohara, Ryuya Ikezoe, Makoto Ichimura, Mizuki Sakamoto, Mafumi Hirata, Shunsuke Ide, Comparison of dispersion model of magneto-acoustic cyclotron instability with experimental observation of3He ion cyclotron emission on JT-60U, Journal of the Physical Society of Japan, 10.7566/JPSJ.86.124501, 86, 12, 2017.12, [URL], The Magneto-acoustic Cyclotron Instability (MCI) is a possible emission mechanism for Ion Cyclotron Emissions (ICEs). A dispersion model of the MCI driven by a drifting-ring-type ion velocity distribution has been proposed. In this study, the model was compared with the experimental observations of 3He ICEs [ICEs(3He)] on JT-60U. For this purpose, at first, velocity distributions of deuterium–deuterium fusion produced fast 3He ions at the time of an appearance of the ICE(3He) were evaluated by using a fast ion orbit following code under a realistic condition. The calculated distribution at the edge of the plasma on the midplane on the low field side is shown to have an inverted population and strong anisotropy. This distribution can be reasonably approximated by the drifting-ring-type distribution. Next, dispersions of the MCIs driven by the drifting-ring-type distribution were compared with those of observed ICEs(3He). The comparison shows that toroidal wavenumbers and frequencies of the calculated MCIs agree with those of the observed ICEs(3He)..
17. Ryuya Ikezoe, M. Ichimura, J. Itagaki, M. Hirata, S. Sumida, S. Jang, K. Izumi, A. Tanaka, R. Sekine, Y. Kubota, Y. Shima, J. Kohagura, M. Yoshikawa, M. Sakamoto, Y. Nakashima, Measurement of ICRF wave propagation using a microwave reflectometer with fast antenna switching on GAMMA 10, Journal of Instrumentation, 10.1088/1748-0221/12/12/C12017, 12, 12, 2017.12, [URL], Slow Alfvén wave in ion cyclotron range of frequency (ICRF) is a powerful tool to heat ions confined in a mirror field. In spite of its efficient heating effect that has been attained in the central cell of GAMMA 10, there are still unknown characteristics concerning boundary condition, transient variation of heating effect, exact picture of cyclotron damping, and so on. To study these characteristics in detail, a multi-point measurement of the waves inside the hot plasma has been recently developed by using a microwave reflectometer. In addition to a radial profile measurement that is available by a usual reflectometer, an axial measurement has been achieved by arraying transmitting and receiving horn antennas in the axial direction, which are repeatedly switched in time during a discharge with PIN diode switches. Another transmitting and receiving horn antenna pair was newly added to the system and probing at five cross sections was achieved in a single discharge with time resolution of about 1 ms at each antenna pair position. With the upgraded reflectometer system, axial and radial distributions of wave-induced fluctuations and those temporal behavior were clearly observed, offering valuable data on wave physics in a hot mirror plasma..
18. Y. Nakashima, K. Ichimura, M. S. Islam, M. Sakamoto, N. Ezumi, M. Hirata, M. Ichimura, Ryuya Ikezoe, T. Imai, T. Kariya, I. Katanuma, J. Kohagura, R. Minami, T. Numakura, M. Yoshikawa, T. Iijima, M. M. Islam, K. Nojiri, K. Shimizu, A. Terakado, S. Togo, N. Asakura, M. Fukumoto, A. Hatayama, Y. Hirooka, S. Kado, H. Kubo, S. Masuzaki, H. Matsuura, T. Nakano, S. Nagata, N. Nishino, N. Ohno, A. Sagara, K. Sawada, M. Shoji, A. Tonegawa, Y. Ueda, Recent progress of divertor simulation research using the GAMMA 10/PDX tandem mirror, Nuclear Fusion, 10.1088/1741-4326/aa7cb4, 57, 11, 2017.08, [URL], This paper describes the recent progress in divertor simulation research using the GAMMA 10/PDX tandem mirror towards the development of divertors in fusion reactors. During a plasma flow generation experiment in the end cell of the GAMMA 10/PDX, ICRF heating in the anchor cell successfully extended the particle flux up to 3.3 1023 m2 s-1. Superimposing the short pulse of the ECH also attained a maximum heat flux of ∼30 MW m-2. We have succeeded in achieving and characterizing the detachment of the high-temperature plasma, which is equivalent to the SOL plasma of tokamaks, by using the divertor simulation experimental module (D-module) in the GAMMA 10/PDX end cell, in spite of using a linear device with a short magnetic field line connection length. Various gases (Ar, Xe, Ne and N2) are examined to evaluate the effect of radiation cooling against the plasma flow at the MW m-2 level in the divertor simulation region and the following results are obtained: (i) Xe gas was most effective in the reduction of heat and particle fluxes (1%, 3%, respectively) and has a stronger effect on electron cooling (down to ∼1.6 eV) in the used gas species. (ii) Ne gas was less effective. On the other hand, (iii) N2 gas showed more favorable effects than Ar in the lower pressure range. These results will contribute to the progress in detached plasma operation and in clarifying the radiation cooling mechanism towards the development of future divertors..
19. Ryuya Ikezoe, M. Ichimura, T. Okada, J. Itagaki, M. Hirata, S. Sumida, S. Jang, K. Izumi, A. Tanaka, M. Yoshikawa, J. Kohagura, M. Sakamoto, Y. Nakashima, Multi-point measurement using two-channel reflectometer with antenna switching for study of high-frequency fluctuations in GAMMA 10, Review of Scientific Instruments, 10.1063/1.4978323, 88, 3, 2017.03, [URL], A two-channel microwave reflectometer system with fast microwave antenna switching capability was developed and applied to the GAMMA 10 tandem mirror device to study high-frequency small-amplitude fluctuations in a hot mirror plasma. The fast switching of the antennas is controlled using PIN diode switches, which offers the significant advantage of reducing the number of high-cost microwave components and digitizers with high bandwidths and large memory that are required to measure the spatiotemporal behavior of the high-frequency fluctuations. The use of two channels rather than one adds the important function of a simultaneous two-point measurement in either the radial direction or the direction of the antenna array to measure the phase profile of the fluctuations along with the normal amplitude profile. The density fluctuations measured using this system clearly showed the high-frequency coherent fluctuations that are associated with Alfvén-ion-cyclotron (AIC) waves in GAMMA 10. A correlation analysis applied to simultaneously measured density fluctuations showed that the phase component that was included in a reflected microwave provided both high coherence and a clear phase difference for the AIC waves, while the amplitude component showed neither significant coherence nor clear phase difference. The axial phase differences of the AIC waves measured inside the hot plasma confirmed the formation of a standing wave structure. The axial variation of the radial profiles was evaluated and a clear difference was found among the AIC waves for the first time, which would be a key to clarify the unknown boundary conditions of the AIC waves..
20. M. M. Islam, Y. Nakashima, S. Kobayashi, N. Nishino, K. Hosoi, K. Ichimura, M. S. Islam, K. Fukui, K. Shimizu, M. Ohuchi, M. Arai, T. Yokodo, G. Lee, M. Yoshikawa, J. Kohagura, M. Hirata, Ryuya Ikezoe, M. Ichimura, M. Sakamoto, T. Imai, Characteristics of SMBI fueling with laval nozzle in GAMMA 10 based on experimental and simulation results, 11th International Conference on Open Magnetic Systems for Plasma Confinement, OS 2016
Open Magnetic Systems for Plasma Confinement, OS 2016
Proceedings of the 11th International Conference on Open Magnetic Systems for Plasma Confinement
, 10.1063/1.4964174, 1771, 2016.10, [URL], Results of supersonic molecular bean injection (SMBI) with Laval nozzle has been carried out in the GAMMA 10 tandem mirror. The neutral transport during SMBI was investigated by the two-dimensional image of the light emission captured by the high-speed camera. We used the value of full width at half maximum (FWHM) of the distribution of emission intensity as an index of neutral transport. The FWHM value decreases with the increase of the plenum pressure which indicates that the particle diffusion was more convergent at high plenum pressure. Fully three-dimensional neutral transport simulation using a Monte-Carlo code DEGAS is applied to SMBI experiment and simulation results qualitatively explained the experimental results..
21. Ryuya Ikezoe, M. Ichimura, J. Itagaki, M. Hirata, S. Sumida, S. Jang, M. Yoshikawa, J. Kohagura, M. Sakamoto, Y. Nakashima, Development of internal ICRF wave detection using microwave reflectometry on GAMMA 10, 11th International Conference on Open Magnetic Systems for Plasma Confinement, OS 2016
Open Magnetic Systems for Plasma Confinement, OS 2016
Proceedings of the 11th International Conference on Open Magnetic Systems for Plasma Confinement
, 10.1063/1.4964196, 1771, 2016.10, [URL], The microwave reflectometer system in the GAMMA 10 central cell has been recently upgraded using PIN diode switches and an axial array of transmitting and receiving horn antennas to investigate internal structures of ICRF waves. The system successfully worked to measure high-frequency density fluctuations accompanied by ICRF waves at multi axial locations in a single discharge. By using several discharges for radial scan with the axial antenna switching, radial and axial measurement of the density fluctuations was achieved. Obtained profiles of 6.36 MHz ICRF wave for ion heating show axial variation in those intensity, suggesting the effect of cyclotron damping. Furthermore, in accordance with the unexplained variation of diamagnetism before the main ramp-up, the intensity near the ICRF antenna is found to also vary in time. The developed system is useful to study the behavior of ICRF waves inside a hot plasma..
22. S. Jang, M. Ichimura, S. Sumida, M. Hirata, Ryuya Ikezoe, M. Sakamoto, T. Okada, Y. Iwamoto, Y. Onodera, J. Itagaki, K. Ichimura, Y. Nakashima, ICRF heating in the plug/barrier region to control end-loss ions on GAMMA 10/PDX, 11th International Conference on Open Magnetic Systems for Plasma Confinement, OS 2016
Open Magnetic Systems for Plasma Confinement, OS 2016
Proceedings of the 11th International Conference on Open Magnetic Systems for Plasma Confinement
, 10.1063/1.4964167, 1771, 2016.10, [URL], On the GAMMA 10/PDX tandem mirror machine, divertor simulation experiments are carried out in the west end region by utilizing the particle flux from the confinement region. In order to control the particle flux and the ion temperature on the west end region, we have tried ICRF heating experiments with two types of antennas, double half turn (DHT) and Nagoya Type-III antennas, located in the west barrier cell. The ICRF heating in the barrier cell is effective for increasing the ion temperature at the end region. On the other hand, the particle flux is decreased due to the trapping of the ions from the central cell. This effect is more explicit by use of the Type-III antenna than the DHT antenna. The direct heating of the end-loss ions in the end region has been demonstrated to be effective for the increase of both ion temperature and particle flux..
23. Y. Nakashima, T. Imai, M. Sakamoto, I. Katanuma, T. Kariya, M. Yoshikawa, N. Ezumi, R. Minami, M. Hirata, J. Kohagura, T. Numakura, Ryuya Ikezoe, K. Ichimura, X. Wang, M. Ichimura, Overview of recent progress and future in GAMMA 10/PDX project, 11th International Conference on Open Magnetic Systems for Plasma Confinement, OS 2016
Open Magnetic Systems for Plasma Confinement, OS 2016
Proceedings of the 11th International Conference on Open Magnetic Systems for Plasma Confinement
, 10.1063/1.4964155, 1771, 2016.10, [URL], This paper presents an overview of the recent progress of GAMMA 10/PDX project. In the GAMMA 10/PDX project, development of fusion reactor relevant research related to magnetic mirror devices has been conducted. One of the main objectives is divertor simulation studies under the similar condition of actual fusion devices, which contributes to comprehensive development strategy towards the divertor plasma physics and plasma wall interaction (PWI) studies. The recent progress of this research is that an additional plasma heating (ICRF and ECH) significantly increased the ion flux up to 3.3 × 1023 /m2 s at the end-mirror exit, which proved an effectiveness of additional plasma heating for generating high ion flux from the mirror end. Superimposing a short pulse of ECH produces the maximum heat flux of 21 MW/m2 by improving the west plug ECH antenna system, which exceeds the heat-load of ITER divertor plates. In detached plasma formation experiments using the divertor simulation experimental module (D-module), comparison of various radiator gases (Xe, Ar, Ne, N2) injected into D-module showed that Xe was the most effective gas on electron cooling and reduction of heat and ion fluxes. In the development of high-power gyrotrons, a number of electron cyclotron heating experiments using high-power gyrotrons have been performed under strong collaboration and we made a remarkable contribution to LHD and QUEST. The first 28 GHz MW tube has attained 1.38 MW and new MW-level dual frequency gyrotron test is also in progress and 1.22 MW at 28GHz is achieved. Multi-channel/multi-pass Thomson scattering system has been developed and simultaneous measurement of radially 6 points with 10 Hz can be available. The multi-pass function of this system also improved the measurement accuracy and time resolution. Microwave interferometer system has been developed in the GAMMA 10/PDX end-cell, which contributes to detailed diagnostics for divertor simulation plasmas in D-module..
24. Masayuki Yoshikawa, Ryuya Ikezoe, Koichi Ohta, Xiaolong Wang, Junko Kohagura, Yoriko Shima, Masayuki Chikatsu, Kazuya Ichimura, Mizuki Sakamoto, Makoto Ichimura, Tsuyoshi Imai, Ryutaro Minami, Kohei Tsumura, Yousuke Nakashima, Direct observation of electron heating by electron Landau damping of Alfvén ion cyclotron waves using Thomson scattering system in the Tandem mirror GAMMA 10/PDX, Plasma and Fusion Research, 10.1585/pfr.11.2402051, 11, Specialissue1, 2016.01, [URL], In the hot ion mode of the tandem mirror GAMMA 10/PDX experiments, increases in the flux and temperature of end-loss electrons were observed after the excitation of Alfvén ion cyclotron (AIC) modes. The increase of the electron temperature in the central cell is directly observed using a Thomson scattering system, confirming electron heating due to the electron Landau damping of AIC modes..
25. Md Maidul Islam, Yousuke Nakashima, Shinji Kobayashi, Nobuhiro Nishino, Yuichiro Nakano, Katsuhiro Hosoi, Kazuya Ichimura, Md Shahinul Islam, Keita Shimizu, Kazuma Fukui, Masato Ohuchi, Akihiro Terakado, Masayuki Yoshikawa, Junko Kohagura, Mafumi Hirata, Ryuya Ikezoe, Xiaolong Wang, Makoto Ichimura, Mizuki Sakamoto, Tsuyoshi Imai, Effect of laval nozzle in the GAMMA 10 SMBI experiments, Plasma and Fusion Research, 10.1585/pfr.11.2402053, 11, Specialissue1, 2016.01, [URL], Gas fueling by supersonic molecular beam injection (SMBI) has been performed at the central-cell in GAMMA 10. The results of SMBI experiment with plenum pressure from 0.3 MPa to 2.0 MPa were obtained using a laval nozzle. The distribution of the emission intensity during SMBI was investigated by the 2-dimensional image captured by the fast camera as an index of the neutral transport. The experimental results indicates that the effect of the laval nozzle reduces the dispersion of injected hydrogen molecules in the peripheral region. In this paper, we also studied the line density at the central-cell (NLcc) and Ha emission intensity during SMBI. Comparison between SMBI and conventional gas puffing is also discussed from the view point of the fueling efficiency..
26. Kazuya Ichimura, Yousuke Nakashima, Md Maidul Islam, Md Shahinul Islam, Keita Shimizu, Kazuma Fukui, Masato Ohuchi, Mafumi Hirata, Ryuya Ikezoe, Shuhei Sumida, Masayuki Yoshikawa, Naomichi Ezumi, Mizuki Sakamoto, Makoto Ichimura, Tsuyoshi Imai, Study of the axial ion confinement time for high particle flux operations of GAMMA 10/PDX, Plasma and Fusion Research, 10.1585/pfr.11.2405045, 11, Specialissue1, 2016.01, [URL], Characteristics of axial plasma confinement in recent high-density mode of GAMMA 10/PDX tandem mirror is reported in this paper. The parameter of the end-loss ion flux is an important issue in the divertor simulation studies in GAMMA 10/PDX. With an ICRF heating in the anchor-cells of the machine, large increase in the electron line-density in the central-cell (NLCC) and the end-loss particle flux were observed. The behavior of the electron line density, plasma temperature and the amount of end-loss ion flux during the experiments suggested that the mechanism of the axial ion loss was changed due to the increase of ion collision frequency. In contrast with the standard operations of GAMMA 10/PDX, plasma parameters in recent high-density operations stays in the range of high collision frequency. In such collisional plasma, the end-loss ion particle flux show linear dependence on the plasma density instead of the parabolic dependence expected in plasmas of low collisionfrequency. In the high collision frequency plasma, it is notable that plasma potential φ largely changes the dependence of the particle flux on the plasma density..
27. X. Wang, M. Yoshikawa, J. Kohagura, Y. Shima, Ryuya Ikezoe, M. Sakamoto, T. Imai, Y. Nakashima, Z. Ma, I. Sakagami, A. Mase, A narrow band-pass filter type Wilkinson power divider for I-Q demodulator in microwave interferometer system, Journal of Instrumentation, 10.1088/1748-0221/10/11/C11005, 10, 11, 2015.11, [URL], I-Q (In-phase Quadrature) demodulator is one of key components in microwave interferometer system application. Normally, I-Q demodulator consists of amplifiers, mixers, 90 degree phase shifter, power divider and band-pass filters, and it is widely used in various microwave communication systems and measurement systems. In this paper, power divider and band-pass filters are newly designed as one single passive component, therefore, I-Q demodulator topology becomes simplified. The novel narrow band-pass filter type Wilkinson power divider not only provides extremely miniaturized circuit size, but also maintains the band-pass filter performance and power division function as well. One experimental circuit shows good agreement with the theoretical simulation..
28. Ryuya Ikezoe, M. Ichimura, T. Okada, M. Hirata, T. Yokoyama, Y. Iwamoto, S. Sumida, S. Jang, K. Takeyama, M. Yoshikawa, J. Kohagura, Y. Shima, X. Wang, Wave excitation by nonlinear coupling among shear Alfvén waves in a mirror-confined plasma, Physics of Plasmas, 10.1063/1.4930216, 22, 9, 2015.09, [URL], A shear Alfvén wave at slightly below the ion-cyclotron frequency overcomes the ion-cyclotron damping and grows because of the strong anisotropy of the ion temperature in the magnetic mirror configuration, and is called the Alfvén ion-cyclotron (AIC) wave. Density fluctuations caused by the AIC waves and the ion-cyclotron range of frequencies (ICRF) waves used for ion heating have been detected using a reflectometer in a wide radial region of the GAMMA 10 tandem mirror plasma. Various wave-wave couplings are clearly observed in the density fluctuations in the interior of the plasma, but these couplings are not so clear in the magnetic fluctuations at the plasma edge when measured using a pick-up coil. A radial dependence of the nonlinearity is found, particularly in waves with the difference frequencies of the AIC waves; bispectral analysis shows that such wave-wave coupling is significant near the core, but is not so evident at the periphery. In contrast, nonlinear coupling with the low-frequency background turbulence is quite distinct at the periphery. Nonlinear coupling associated with the AIC waves may play a significant role in the beta- and anisotropy-limits of a mirror-confined plasma through decay of the ICRF heating power and degradation of the plasma confinement by nonlinearly generated waves..
29. T. Yokoyama, M. Ichimura, A. Fukuyama, S. Sumida, M. Hirata, Ryuya Ikezoe, Y. Iwamoto, T. Okada, K. Takeyama, S. Jang, Analysis Of wave excitation of the phase-control ICRF antennas with three-dimensional full wave code on gamma 10, Fusion Science and Technology, 10.13182/FST14-899, 68, 1, 185-189, 2015.07, [URL], On the GAMMA 10 tandem mirror, divertor simulation experiments progress with high-temperature plasmas produced by waves in ion-cyclotron range of frequency (ICRF) and open magnetic fields. In these experiments, high-temperature and high-density plasmas are required to be produced and controlled. In order to enhance the ion heating in the anchor cell, phase-control experiments, which use two ICRF antennas installed in the central cell and the anchor cell, are introduced. In these experiments, ICRF waves in the same frequency (10 MHz) are excited. Ion heating in the anchor cell is performed more effectively with phase control. In order to analyze wave propagation in the phase-control experiments, a three-dimensional full wave code (TASK/WF), in which parallel processing has been implemented recently, is introduced. In this technical note, we discuss ICRF wave excitation and propagation in the phase-control experiments. It is clearly observed by the experiments and wave analysis that the enhancement of ion heating in the anchor cell is performed by phasecontrol experiments..
30. Ryuya Ikezoe, M. Ichimura, M. Hirata, T. Yokoyama, Y. Iwamoto, T. Okada, S. Sumida, K. Takeyama, S. Jang, T. Oi, K. Ichimura, Y. Nakashima, Extension Of operational regimes with ICRF heating on gamma 10/PDX, Fusion Science and Technology, 10.13182/FST14-853, 68, 1, 63-69, 2015.07, [URL], Recent ICRF heating experiments performed in GAMMA 10/PDX are reported. Owing to antennaphasing technique and four ICRF antennas installed in the mirror cells other than the central cell, the operational regime of GAMMA 10/PDX is significantly extended. Several ICRF heating schemes to increase the parameters (plasma density in the central cell, particle flux flowing to the machine end and its ion temperature parallel to the field lines) are demonstrated. These results present positive progress in the ongoing divertor simulation experiments on GAMMA 10/PDX..
31. S. Sumida, M. Ichimura, T. Yokoyama, M. Hirata, Ryuya Ikezoe, Y. Iwamoto, T. Okada, K. Takeyama, S. Jang, M. Sakamoto, Y. Nakashima, M. Yoshikawa, R. Minami, K. Oki, M. Mizuguchi, K. Ichimura, High-Density plasma production in the Gamma 10 central cell with ICRF heating on both anchor cells, Fusion Science and Technology, 10.13182/FST14-890, 68, 1, 136-141, 2015.07, [URL], In the GAMMA 10 tandem mirror, divertor simulation experiments that utilize particle flux toward the west end region (called end-loss flux) have been implemented. Since a positive correlation has been reported between the end-loss flux and the central-cell density, an increase of the central-cell density is important for obtaining a higher end-loss flux on the divertor simulation experiments. By arranging the ion cyclotron range of frequency (ICRF) systems so as to excite strong ICRF waves in both anchor cells simultaneously, we have succeeded in producing high-density plasmas (line density of 1.2 × 1014 cm-2) in both anchor cells. As a result, a higher central-cell density of 4.4 × 1012 cm-3and a higher end-loss flux of more than 1023 m-2s-1have been obtained. One of the possible mechanisms of the high density production is a formation of positive potentials on both anchor cells. Plasmas in the central cell are confined due to those potentials..
32. T. Okada, Ryuya Ikezoe, M. Ichimura, M. Hirata, T. Yokoyama, Y. Iwamoto, S. Sumida, K. Takeyama, S. Jang, M. Yoshikawa, J. Kohagura, Y. Shima, Internal measurement of propagation of ICRF waves by using reflectometers on GAMMA 10, Fusion Science and Technology, 10.13182/FST14-898, 68, 1, 161-165, 2015.07, [URL], In GAMMA 10/PDX, the divertor simulation experiment has been performed recently. Ion cyclotron range of frequency (ICRF) waves are used for plasma production and ion heating. It is necessary for obtaining better controllability and extending the operational regime to understand the excitation, propagation, and absorption of the waves. The density fluctuations accompanied by the ICRF waves propagating in the inner region of plasma has been measured by using a microwave reflectometer on the antenna-phasing experiments, where the propagation of the ICRF waves is actively controlled. The phase difference between two axially separated ICRF antennas remarkably affects the global plasma parameters. The density fluctuation caused by the interfered ICRF waves is shown to depend clearly on the phase difference between the waves excited from two antennas. The availability of a reflectometer for the evaluation of the internal wave propagation is shown..
33. Y. Nakashima, H. Takeda, K. Ichimura, K. Hosoi, K. Oki, M. Sakamoto, M. Hirata, M. Ichimura, Ryuya Ikezoe, T. Imai, M. Iwamoto, Y. Hosoda, I. Katanuma, T. Kariya, S. Kigure, J. Kohagura, R. Minami, T. Numakura, S. Takahashi, M. Yoshikawa, N. Asakura, M. Fukumoto, A. Hatayama, Y. Hirooka, S. Kado, H. Kubo, S. Masuzaki, H. Matsuura, S. Nagata, N. Nishino, N. Ohno, A. Sagara, T. Shikama, M. Shoji, A. Tonegawa, Y. Ueda, Progress of divertor simulation research toward the realization of detached plasma using a large tandem mirror device, Journal of Nuclear Materials, 10.1016/j.jnucmat.2014.12.063, 463, 537-540, 2015.07, [URL], Abstract This paper describes the results of the experiments performed on Tandem Mirror device GAMMA 10/PDX mainly using a new "divertor simulation experimental module (D-module)" installed on one of the end mirror exits which is specially designed to investigate the physics of plasma detachment. The additional ICRF heating in the anchor-cells, connected to both ends of the central-cell, significantly increases the density in the both cells, which attained the generation of the highest particle flux up to 1023 particles/s m2 at the end-mirror exit. H2 and noble gas injection to enhance the radiation cooling in D-module was performed and a remarkable reduction of the electron temperature (from few tens eV to <3 eV) on the target plate were successfully achieved associated with the strong reduction of particle and heat flux. A significant effect of simultaneous injection with hydrogen and noble gases for detached plasma formation was recognized for the first time..
34. Y. Nakashima, M. Sakamoto, H. Takeda, K. Ichimura, Y. Hosoda, M. Iwamoto, K. Shimizu, K. Hosoi, K. Oki, M. Yoshikawa, M. Hirata, Ryuya Ikezoe, T. Imai, T. Kariya, I. Katanuma, J. Kohagura, R. Minami, T. Numakura, X. Wang, M. Ichimura, Recent results of divertor simulation experiments using D-Module in the GAMMA 10/PDX tandem mirror, Fusion Science and Technology, 10.13182/FST14-882, 68, 1, 28-35, 2015.07, [URL], This paper describes the recent results of divertor simulation research toward the realization of the detached plasma using the end-mirror of a large tandem mirror device. The additional ion cyclotron range of frequency heating in the anchor-cells for higher particle flux generation significantly increases the density, which attained the highest particle flux up to 1.7 × 1023 particles/szm2 at the end-mirror exit. Massive gas injection (H2 and noble gases) into the divertor simulation experimental module (D-module) was performed, and a remarkable reduction of the electron temperature on the target plate was successfully achieved associated with the strong reduction of particle and heat fluxes in D-module. Two-dimensional images of Ha emission in D-module observed with a high-speed camera showed strong emission in the upstream region and significant reduction near the target plate. These results clarified the effect of radiation cooling and formation of detached plasma due to gas injection. It is also found that Xe gas is much more effective in achieving detached plasma than Ar gas. Numerical simulation studies also have been performed toward the understanding of the cooling mechanism of divertor plasma. The above results will contribute to establishment of detached plasma control and clarification of the radiation cooling mechanism toward the development of future divertor systems..
35. Ryutaro Minami, Tsuyoshi Imai, Tsuyoshi Kariya, Tomoharu Numakura, Taku Eguchi, Ryo Kawarasaki, Kazuhiro Nakazawa, Takaki Kato, Fumiya Sato, Hirotaka Nanzai, Makoto Uehara, Yoichi Endo, Mafumi Hirata, Makoto Ichimura, Ryuya Ikezoe, Isao Katanuma, Junko Kohagura, Yousuke Nakashima, Kensuke Oki, Mizuki Sakamoto, Masayuki Yoshikawa, Generation and control of high intermittent heat load pattern for divertor simulation studies in GAMMA 10 tandem mirror, Plasma and Fusion Research, 10.1585/pfr.9.3402116, 9, SpecialIssue2, 2014.01, [URL], Development of high power gyrotrons and electron cyclotron heating (ECH) systems for the power modulation experiments in GAMMA 10 have been started in order to generate and control the high heat flux and to make the ELM (edge localized mode) like intermittent heat load pattern for divertor simulation studies. ECH for potential formation at plug region (P-ECH) produces electron flow with high energy along the magnetic filed line. By modulating the ECH power, we can obtain arbitrary pulse heat load patterns. By changing the on/off timing, we can simulate the ELM intermittent heat pulses. The heat flux factor increases almost linearly with ECH power. An intense axial electron flow with energy from hundreds of eV to a few keV generated by fundamental P-ECH is observed. When ECH is turned off, a short burst appears in the end loss ion current due to the axial drain of the confined plasma..
36. Kazuya Ichimura, Yousuke Nakashima, Hisato Takeda, Katsuhiro Hosoi, Satoru Kigure, Shigehito Takahashi, Miki Iwamoto, Yasunari Hosoda, Mafumi Hirata, Ryuya Ikezoe, Kensuke Oki, Masayuki Yoshikawa, Mizuki Sakamoto, Tsuyoshi Imai, Makoto Ichimura, The first experiment of MPD Jet injection into GAMMA 10 plasma, Plasma and Fusion Research, 10.1585/pfr.9.3406098, 9, SpecialIssue2, 2014.01, [URL], Results of the first experiment of short pulse plasma injection by MPD (magneto plasma dynamic) Jet into GAMMA 10/PDX's longer pulse plasma are reported. In the experiment, a new method for plasma start-up without using plasma guns was applied. In this method, the main plasma of GAMMA 10/PDX was produced by ECRH (electron cyclotron resonance heating) and ICRF (ion cyclotron range of frequency). Then, MPD Jet plasma was injected into the main plasma along magnetic field line. As a result, density of the main plasma was increased and the end-loss flux was doubled. Flow velocity of the plasmoid injected by the MPD Jet was evaluated from the change of plasma density in each cell of the tandem mirror. The result indicated that the flow speed is several km/s. It is found that the plasmoid worked as strong fueling device which dramatically raises the density of plasma. Therefore injection of MPD Jet plasma into tandem mirror can be a useful tool to study physical phenomena of divertor and PWI..
37. Ryuya Ikezoe, M. Ichimura, M. Hirata, T. Iwai, T. Yokoyama, Y. Ugajin, T. Sato, T. Iimura, Y. Saito, M. Yoshikawa, J. Kohagura, Y. Shima, T. Imai, Coupling of ICRF waves and axial transport of high-energy ions owing to spontaneously excited waves in the GAMMA 10 tandem mirror, Nuclear Fusion, 10.1088/0029-5515/53/7/073040, 53, 7, 2013.07, [URL], Plasmas with high ion temperature of several kiloelectronvolts and a strong temperature anisotropy of greater than 10 were produced by ion cyclotron range of frequency (ICRF) heating in the GAMMA 10 tandem mirror. In such high-performance plasmas with strong anisotropy, high-frequency fluctuations, so-called Alfvén-ion-cyclotron (AIC) waves, are excited spontaneously. These AIC waves have several discrete peaks in the frequency spectrum. Coupling of the ICRF heating waves and the excited AIC waves was clearly observed in the density fluctuations measured with a newly developed reflectometer. Parametric decay from the heating ICRF waves to the AIC waves and low-frequency waves was also indicated. Alfvén waves with difference frequencies between the discrete peaks of the AIC waves were detected in a signal that measured the number of axially transported high-energy ions (over 6 keV) at the machine end, indicating pitch-angle scattering caused by the low-frequency waves. Energy transport along the magnetic field line is an important consideration when ICRF power is injected in the perpendicular direction to a magnetic field line. The importance of the spontaneously excited AIC waves for axial confinement of a tandem mirror through wave-wave couplings was demonstrated..
38. M. Yoshikawa, M. Sakamoto, Y. Miyata, M. Aoyama, M. Mizuguchi, M. Hirata, M. Ichimura, T. Imai, T. Kariya, I. Katanuma, J. Kohagura, R. Minami, Y. Nakashima, T. Numakura, Ryuya Ikezoe, K. Ichimura, K. Furutachi, H. Nakanishi, Y. Nagayama, Simultaneous evaluation of potential fluctuation from the core plasma to the end region in GAMMA 10, Nuclear Fusion, 10.1088/0029-5515/53/7/073031, 53, 7, 2013.07, [URL], A correlation between drift-type fluctuations and anomalous radial transport was observed in GAMMA 10, and these fluctuations were suppressed by the electron-cyclotron-heating-driven radial electric field. We developed new diagnostics to conduct these studies: a simultaneous two-point-measurement gold neutral beam probe (GNBP) for the radial electric field and potential fluctuations, and a high-speed end-plate potential fluctuation measurement system. The electric field and its fluctuations were successfully obtained in a single plasma shot for the first time. Coherence between the drift-type potential fluctuations measured in the core plasma by the GNBP and those obtained by end-plate measurements was clearly observed. These potential and electric field fluctuations were clearly suppressed by the positive electric fields measured by two-point potential measurements using the GNBP and the end-plate system. We can obtain a powerful tool for probing the radial electric field and potential fluctuations in the core plasma because the end-plate potential fluctuation is the same as that in the core plasma measured using the new GNBP..
39. Ryuya Ikezoe, S. Masamune, K. Oki, A. Sanpei, H. Himura, Takumi Onchi, A. Hirose, Asymmetric toroidal flux generation due to phase locking of internally resonant tearing modes in the RELAX reversed-field pinch, Plasma Physics and Controlled Fusion, 10.1088/0741-3335/55/1/015005, 55, 1, 2013.01, [URL], Self-generation and sustainment of toroidal flux via a dynamo is one of the unique characteristics of the reversed-field pinch (RFP) configuration. In a small aspect ratio RFP machine RELAX, it has been observed that a sudden dynamo usually occurs at a localized toroidal location. This localized dynamo evolves in time, causing a toroidally asymmetric flux distribution. The degree of asymmetry depends on the location and propagation of the localized dynamo. It has been observed that the position where the localized dynamo occurs coincides with the position where the phases of the internally resonant tearing modes align. The phase alignment plays an important role in triggering a localized dynamo activity and in the formation of asymmetric toroidal flux distribution especially in deep-reversal plasmas in RELAX..
40. Y. Nakashima, H. Takeda, K. Ichimura, K. Hosoi, M. Sakamoto, M. Hirata, M. Ichimura, Ryuya Ikezoe, T. Imai, T. Ishii, T. Kariya, I. Katanuma, J. Kohagura, R. Minami, T. Numakura, H. Ueda, M. Yoshikawa, N. Asakura, T. Furuta, A. Hatayama, Y. Hirooka, S. Kado, S. Masuzaki, H. Matsuura, S. Nagata, N. Nishino, N. Ohno, A. Sagara, T. Shikama, M. Shoji, M. Toma, A. Tonegawa, Y. Ueda, Recent results of divertor simulation research using an end-cell of a large tandem mirror device, Journal of Nuclear Materials, 10.1016/j.jnucmat.2013.01.157, 438, SUPPL, 2013.01, [URL], This paper describes the results of characterization of high heat and particle fluxes produced at the end-cell of the large tandem mirror GAMMA 10 and of the initial plasma-irradiation experiments. In the case of ICRF plasmas, the heat flux of 0.8 MW/m2 and the particle flux of 4 × 10 22/m2 s were achieved at the end-mirror exit. The heat flux increases with the ICRF power and has a linear relationship with the stored energy. Direct ion energy analysis clarified that the parallel ion temperature can be controlled from 100 eV to 400 eV by changing the ICRF power. Additional plasma heating using another ICRF system in the anchor-cell significantly increases the particle flux, which gives a clear prospect of generating the higher particle flux by applying additional ICRF heating in the neighboring cells. The initial results of the plasma-gas-material interactions on a new V-shaped tungsten target were also reported..
41. Kazuya Ichimura, Yousuke Nakashima, Katsuhiro Hosoi, Hisato Takeda, Takashi Ishii, Hideaki Ueda, Satoshi Kigure, Shigehito Takahashi, Satoru Hotaka, Makoto Ichimura, Ryuya Ikezoe, Masayuki Yoshikawa, Mizuki Sakamoto, Tsuyoshi Imai, Analysis of end-loss ion flux for application studies of the plasma flow from the end mirror exit of GAMMA 10, Plasma and Fusion Research, 10.1585/pfr.7.2405147, 7, SPL.ISS.1, 2012.12, [URL], The end-loss ion flux in GAMMA 10 is measured with a view to use it for a divertor simulation experiment or other studies that require high-performance plasma flux. First, the basic parameters of the end-loss ion flux, such as its energy and current density, were measured in typical plasma shots in GAMMA 10. A diagnostic device, the end loss ion energy analyzer (ELIEA), was used to the measure these parameters. An investigation of the relationship between the parameters of the end-loss ion flux and the plasma parameters in the central- cell revealed linear-like relationships between these parameters. We also analyzed the effects of plasma heating and fueling by using devices installed in GAMMA 10 (ion cyclotron resonance frequency (ICRF), electron cyclotron resonance heating (ECRH) and supersonic molecular beam injection (SMBI)) in order to generate more intense ion flux. The results showed that the energy distribution of the ion flux is more closely resembles a double component Maxwellian than a simple Maxwellian. Plasma heating schemes such as ECRH and ICRF are found to be effective for the generation of a more intense ion flux..
42. Takuro Yokoyama, Makoto Ichimura, Ryuya Ikezoe, Yumeto Ugajin, Toru Iwai, Tatsunori Sato, Takuma Iimura, Yu Ki Saito, Mafumi Hirata, Yuusuke Yamaguchi, Atsushi Fukuyama, Tsuyoshi Imai, Improvement of ICRF antenna loading in the minimum-B configuration on GAMMA 10, Plasma and Fusion Research, 10.1585/pfr.7.2402136, 7, SPL.ISS.1, 2012.12, [URL], On GAMMA10, waves in Ion-Cyclotron Range of Frequency (ICRF) are used to produce initial plasmas, heat plasmas and keep magneto-hydrodynamic (MHD) stability. High performance plasmas in the minimum- B configuration supply MHD stability. In order to enhance plasma heating, we introduce an ICRF system for heating plasmas in the anchor cell and evaluate its antenna loading. Changing the antenna shape is expected to improve antenna loading. In the experiment, we confirm to increase the antenna loading by changing the antenna shape from simple bar-type to the double elliptic arc type..
43. Ryuya Ikezoe, Sadao Masamune, Kensuke Oki, Akio Sanpei, Haruhiko Himura, Takumi Onchi, Akira Hirose, Phase locking and unlocking associated with transition to quasi-single helicity state in the RELAX reversed-field pinch, Journal of the Physical Society of Japan, 10.1143/JPSJ.81.115001, 81, 11, 2012.11, [URL].
44. J. Kohagura, M. Yoshikawa, Y. Shima, Y. Morikawa, T. Yamada, D. Akita, K. Tatebe, M. Ichimura, M. Hirata, Ryuya Ikezoe, T. Yokoyama, T. Imai, A new frequency-multiplied interferometer system in the GAMMA 10 tandem mirror, Review of Scientific Instruments, 10.1063/1.4731006, 83, 10, 2012.10, [URL], A new interferometer is installed on the west anchor cell of the GAMMA 10 tandem mirror. In GAMMA 10, we have used a heterodyne-type interferometer with a 70-GHz IMPATT oscillator and a 150-MHz oscillator for frequency modulation. The new interferometer consists of a 17.5-GHz phase locked dielectric resonator oscillator and a 37.5-MHz temperature-compensated crystal oscillator, as well as frequency multipliers. The main motivation for the new interferometer using frequency multipliers is to achieve a stable and cost effective interferometer. Direct anchor heating experiments with new anchor ion cyclotron range of frequency antennas in both the west and the east anchor cells are carried out. Density increases in both anchor cells are clearly observed using the new interferometer..
45. Akio Sanpei, Kensuke Oki, Mitsuru Nakamura, Akio Higashi, Hidehiko Motoi, Daisuke Fukahori, Haruhiko Himura, Sadao Masamune, Satoshi Ohdachi, Nobuhiro Nishino, Takumi Onchi, Ryuya Ikezoe, Initial result of successive SXR imaging measurement in low-A RFP, Plasma and Fusion Research, 10.1585/pfr.6.2406096, 6, 1 SPECIAL ISSUE, 2011.12, [URL], A fast successive soft-X ray (SXR) imaging system where SXR camera and high-speed camera for the study of time evolution of magnetic islands has been constructed. A preliminary experimental result in which we observed tangential SXR images from quasi-periodic quasi-single helicity (QSH) state in low-aspect-ratio (low-A) reversed field pinch (RFP) is presented. We successfully obtained time evolution of SXR images from tangential port. By comparison obtained images with simulated images, we may conclude that the evolution of experimental SXR image suggests the rotating QSH RFP configuration. The filamentous configuration tends to be observed in QSH state rather than in multi-helicity (MH) state..
46. Ryuya Ikezoe, Makoto Ichimura, Tatsunori Sato, Yasutaka Imai, Tatsuya Murakami, Toru Iwai, Yumeto Ugajin, Takuro Yokoyama, Yoriko Shima, Masayuki Yoshikawa, Tsuyoshi Imai, Measurement of spatiotemporal behavior of Alfvén-ion-cyclotron waves in the GAMMA10 tandem mirror, Plasma and Fusion Research, 10.1585/pfr.6.2402047, 6, 1 SPECIAL ISSUE, 2011.12, [URL], Spatiotemporal behavior of Alfvén-ion-cyclotron (AIC) waves excited in GAMMA10 is investigated using a microwave reflectometer and magnetic probes located at the edge. The frequency spectrum of the AIC waves has several discrete peaks. Simultaneous measurement of the internal density fluctuation and edge magnetic fluctuation of AIC waves shows that AIC waves have different radial structures in the initial excitation phase, but in the later steady state, each of them has the same structure in the radial direction. The results indicate that several AIC waves are excited as the same eigenmode in the radial direction..
47. Yasutaka Imai, Makoto Ichimura, Ryuya Ikezoe, Tatsuya Murakami, Toru Iwai, Takuro Yokoyama, Yumeto Ugajin, Tatsunori Sato, Tsuyoshi Imai, Observation of the pitch angle distribution of high-energy ions and their radial transport in the central cell of GAMMA10, Plasma and Fusion Research, 10.1585/pfr.6.2402152, 6, 1 SPECIAL ISSUE, 2011.12, [URL], Suppression of the radial transport of the particles caused by low-frequency density fluctuations is one of the important subjects in the GAMMA10 tandem mirror. In GAMMA10, plasmas are mainly produced and heated with Ion Cyclotron Range of Frequency (ICRF) waves. The plasmas heated with ICRF waves have strong temperature anisotropy, because the ions are accelerated perpendicularly to the magnetic field line. In the GAMMA10 experiments, two types of low-frequency density fluctuations are observed; one is a drift-type fluctuation and the other is a flute-type fluctuation. In this paper, we study the radial transport of ICRF-produced high-energy ions which is caused by the low-frequency density fluctuations..
48. Takumi Onchi, Ryuya Ikezoe, Kensuke Oki, Akio Sanpei, Haruhiko Himura, Sadao Masamune, Nobuhiro Nishino, Haruhisa Koguchi, Observation of helical structure by imaging diagnostics in a low-aspect-ratio reversed field pinch, Journal of the Physical Society of Japan, 10.1143/JPSJ.80.114501, 80, 11, 2011.11, [URL], Study of helical structure has been in progress in shallow reversal reversed field pinch (RFP) plasmas in a lowaspect- ratio machine RELAX (R=a = 0:5 m/0.25 m, A = 2) by using visible light and soft-X-ray (SXR) imaging diagnostics. Simple helix and its toroidal rotation have been observed in tangential visible light images taken by a highspeed camera. Similar simple filament structures have also been observed in tangential images obtained by a pin-hole SXR camera assembled with Image-intensified charged coupled device (ICCD). The SXR experimental images have been compared with calculated tangential images using model profiles for emissivity to identify the plausible SXR emissivity profile. It has been found that a model profile of helical core having SXR emissivity 2-4 times higher than the background provides 2D SXR images in reasonable agreement with the experimental results. Time evolution of the SXR emissivity profile measured with a photodiode array has shown that the emissivity is the highest in the core region with a quasi-periodic oscillation of the peak position..
49. Akio Sanpei, Kensuke Oki, Mitsuru Nakamura, Akio Higashi, Hidehiko Motoi, Daisuke Fukahori, Haruhiko Himura, Sadao Masamune, Satoshi Ohdachi, Nobuhiro Nishino, Takumi Onchi, Ryuya Ikezoe, Tangential image of helical SXR emissivity structure in low-aspect-ratio RFP, IEEE Transactions on Plasma Science, 10.1109/TPS.2011.2160369, 39, 11 PART 1, 2410-2411, 2011.11, [URL], A hot helical structure in low-aspect-ratio (A) reversed field pinch (RFP) is obtained with the soft X-ray (SXR) imaging technique. Tangential SXR imaging camera and high-speed camera system is applied to low-A RFP for studying quasi-single-helicity RFP state. The helical structure correlated with m = 1/n = 4 tearing mode is identified by means of a subtraction technique..
50. Ryuya Ikezoe, K. Oki, Takumi Onchi, Y. Konishi, M. Sugihara, S. Fujita, A. Sanpei, H. Himura, S. Masamune, Extended operational regimes and MHD behavior in a low-aspect-ratio reversed field pinch in RELAX, Plasma Physics and Controlled Fusion, 10.1088/0741-3335/53/2/025003, 53, 2, 2011.02, [URL], Operational regimes have been investigated over a wide range of discharge parameters in a low-aspect-ratio (low-A) reversed field pinch (RFP) RELAX. Two distinctive regimes have been identified, possibly characteristic to low-A RFP. One is a very shallow-reversal regime, and the other is an extremely deep-reversal regime where a field-reversal parameter lower than-1 could be sustained. Innewly attained extremely deep-reversal plasmas, the amplitudes of the resonant modes were suppressed to a lower level with enhanced soft-x-ray emission intensity. The extremely deep-reversal regime in low-A RFP may have a potential to become a new operational regime with improved plasma performance..
51. Y. Yamaguchi, M. Ichimura, T. Yokoyama, A. Fukuyama, Ryuya Ikezoe, Y. Imai, T. Murakami, T. Iwai, T. Sato, Y. Ugajin, T. Imai, Ion-cyclotron resonance heating experiment in the quadruple minimum-b field on GAMMA 10, Fusion Science and Technology, 10.13182/FST11-A11625, 59, 1 T, 250-252, 2011.01, [URL], In the GAMMA 10 tandem mirror, Magneto Hydro Dynamic (MHD) stabilization is kept with quadruple minimum-B anchor configuration. In the previous heating experiments, Ion-Cyclotron Range of Frequency (ICRF) antenna installed in the central cell was used for the anchor heating. Fast Alfvén wave excited in the central cell is partly converted to the slow wave in the nonaxisymmetric transition region between the central and the anchor cells, and heats ions in the minimum-B well. In order to produce higher performance plasmas in the central cell, the ion heating should be enhanced in the anchor cell. In this study, an experiment is carried out in the anchor cell to heat ions by ICRF waves without mode conversion. A bar-type antenna is installed in the anchor cell. Applied frequency is adjusted to ion-cyclotron resonance frequency in the minimum-B well. By the additional ion heating with the bar-type antenna, remarkable increase in the diamagnetic signal has been observed in the anchor cell. It is confirmed that the additional heating by the bar-type antenna can also keep MHD stabilization..
52. M. Ichimura, Y. Yamaguchi, Ryuya Ikezoe, Y. Imai, T. Murakami, T. Iwai, T. Yokoyama, T. Sato, Y. Ugajin, T. Imai, Radial transport of high-energy ions caused by low-frequency fluctuations in the gamma10 tandem mirror, Fusion Science and Technology, 10.13182/FST11-A11583, 59, 1 T, 98-103, 2011.01, [URL], Plasmas with high ion-temperature of several keV have been produced by using ion-cyclotron range of frequency (ICRF) heating in the GAMMA 10 tandem mirror. In such high performance plasmas, high and lowfrequency fluctuations are excited and ions trapped in the magnetic field interact with such fluctuations. Three types of wave-particle interactions have been observed in the GAMMA 10 tandem mirror. The turning point diffusion near the ion cyclotron resonance layer has been observed in minimum-B configuration on the anchor cell. Pitch angle scattering of high-energy ions due to the AIC modes and low-frequency waves which have differential frequencies between discrete peaks of the AIC modes are clearly detected. The drift-type fluctuations are clearly observed in the central cell. By using a semiconductor detector, high-energy ions are detected at the radial location far from the plasma edge. The fluctuation, of which frequency is the same as that of drift-type fluctuation, is observed in the signal of high-energy ions. From the pitch angle distribution of the phase differences between both fluctuations, radial transport of high-energy ions caused by drift-type fluctuations near their turning points in the confining mirror field is suggested in the experiments..
53. T. Imai, M. Ichimura, Y. Nakashima, I. Katanuma, M. Yoshikawa, T. Kariya, R. Minami, Y. Miyata, Y. Yamaguchi, Ryuya Ikezoe, T. Shimozuma, S. Kubo, Y. Yoshimura, H. Takahashi, T. Mutoh, K. Sakamoto, M. Mizuguchi, M. Ota, H. Ozawa, K. Hosoi, F. Yaguchi, R. Yonenaga, Y. Imai, T. Murakani, K. Yagi, T. Nakamura, H. Aoki, H. Iizumi, T. Ishii, H. Kondou, H. Takeda, N. Ichioka, S. Masaki, T. Yokoyama, Status and plan of GAMMA 10 tandem mirror program, Fusion Science and Technology, 10.13182/FST11-A11563, 59, 1 T, 1-8, 2011.01, [URL], Recent progress and near future plan of GAMMA 10 efforts are presented. With high power plug electron cyclotron heating (ECH) up to ~ 400 kW, the ion confining potential of more than 2 kV was confirmed. The drift type low frequency fluctuations were suppressed by the positive radial electric field produced by plug ECH. It is found that the efficient EC heating on mirror devices from a strong B field side requires the minimization of the stray microwave in addition to the 100% X-mode excitation to avoid the enhancement of the ion loss. The development of a gyrotron, the key tool of these ECH experiments, has been made in collaboration with NIFS (National Institute for Fusion Science), More than 1.5 MWfor more than Is has been demonstrated at 77GHz. The plan of the boundary plasma research program with modification of GAMMA 10 is in progress. The new program includes the physics and technology studies of the divertor and SOL plasmas and PWI relevant to torus plasmas like ITER. The high heat flux experiments using the open end mirror throat has been started and we have obtained successful preliminary data, which include the heat flux of 8 MW/m2..
54. Takumi Onchi, Ryuya Ikezoe, K. Oki, A. Sanpei, H. Himura, S. Masamune, Tangential soft-x ray imaging for three-dimensional structural studies in a reversed field pinch, Review of Scientific Instruments, 10.1063/1.3455216, 81, 7, 2010.07, [URL], Tangential soft-x ray (SXR) imaging diagnostic has been developed and three-dimensional (3D) structure of the internal magnetic surface has been deduced by comparing the experimental and calculated two-dimensional SXR images in a reversed field pinch. The SXR imaging system, consisting of a MCP, a fluorescent plate, and an intensified charge coupled device camera, has been installed in REversed field pinch of Low-Aspect-ratio eXperiment (RELAX) machine. Major characteristics of an experimental SXR image could be reproduced by numerical calculations of the image using a single island model, suggesting a helical hot core in RELAX. The SXR imaging system could be useful for 3D structural studies when tangential and vertical simultaneous imaging systems would be installed, with appropriate numerical modeling of 3D structure of the magnetic surfaces..
55. S. Masamune, A. Sanpei, Ryuya Ikezoe, Takumi Onchi, K. Oki, T. Yamashita, H. Shimazu, H. Himura, R. Paccagnella, MHD properties of low-aspect ratio RFP in RELAX, Journal of Fusion Energy, 10.1007/s10894-008-9172-z, 28, 2 SPEC. ISS., 187-190, 2009.06, [URL], The low-aspect-ratio (A) reversed field pinch (RFP) offers attractive properties such as enhanced bootstrap current and simpler MHD mode dynamics. The RELAX (REversed field pinch of Low-Aspect ratio eXperiment) machine with the world's lowest A of 2 (R/a = 0.5 m/0.25 m) has been constructed to explore the RFP properties in low-A regime. In flat-topped low-A RFP discharges in RELAX, plasma current of ~50 kA has been attained with discharge duration of ~2 ms. In round-topped discharges with plasma current of ~70 kA, quasi-periodic growth of a single helical mode has been observed. When the dominant m = 1/n = 4 mode grows, the toroidal mode spectrum looks like that of the quasi-single helicity (QSH) RFP state with higher amplitude. MHD equilibrium analyses using a reconstruction code have shown that the bootstrap current fraction is lower than ~5% in the present RELAX plasmas, and it will be ~25% if we could achieve the plasma density of 4 × 10 19 m -3 and electron temperature of 300 eV at plasma current of ~100 kA..
56. Akio Sanpei, O. K.I. Kensuke, Ryuya Ikezoe, Takumi Onchi, Ken Ichi Murata, Hiroyuki Shimazu, Tetsuo Yamashita, Shinichi Fujita, Haruhiko Himura, Sadao Masamune, Jay K. Anderson, Equilibrium reconstruction and estimation of neoclassical effect in low-aspect-ratio reversed field pinch experiments on RELAX, journal of the physical society of japan, 10.1143/JPSJ.78.013501, 78, 1, 2009.01, [URL], A neoclassical equilibrium for low-aspect-ratio reversed field pinch (RFP) plasma is reconstructed from initial experimental results of the standard discharge in a low-aspect-ratio RFP device (RELAX). We estimate the magnitude of the bootstrap current, which is a self-induced plasma parallel current, from the reconstructed equilibrium, and illustrate how the bootstrap current depends on plasma parameters..
57. Kensuke Oki, Ryuya Ikezoe, Takumi Onchi, Akio Sanpei, Haruhiko Himura, Sadao Masamune, Roberto Paccagnella, Observation of large-scale profile change of magnetic field in a low-aspect ratio reversed field pinch, Journal of the Physical Society of Japan, 10.1143/JPSJ.77.075005, 77, 7, 2008.07, [URL].
58. Ryuya Ikezoe, Takumi Onchi, K. Murata, K. Oki, H. Shimazu, T. Yamashita, A. Sanpei, H. Himura, S. Masamune, Effects of lowering aspect ratio on magnetic fluctuations in RFP, 35th European Physical Society Conference on Plasma Physics, EPS 2008 Combined with the 10th International Workshop on Fast Ignition of Fusion Targets
35th EPS Conference on Plasma Physics 2008, EPS 2008 - Europhysics Conference Abstracts
, P4.067, 2008.06.
59. A. Sanpei, S. Masamune, Ryuya Ikezoe, Takumi Onchi, K. Murata, K. Oki, H. Shimazu, T. Yamashita, H. Himura, Neoclassical equilibrium in a low-aspect ratio RFP machine RELAX, 35th European Physical Society Conference on Plasma Physics, EPS 2008 Combined with the 10th International Workshop on Fast Ignition of Fusion Targets
35th EPS Conference on Plasma Physics 2008, EPS 2008 - Europhysics Conference Abstracts
, 32, 1518-1521, 2008.
60. Sadao Masamune, Akio Sanpei, Ryuya Ikezoe, Takumi Onchi, Ken Ichi Murata, Kensuke Oki, Hiroyuki Shimazu, Tetsuo Yamashita, Haruhiko Himura, Characterization of initial low-aspect ratio RFP plasmas in "Relax", Journal of the Physical Society of Japan, 10.1143/JPSJ.76.123501, 76, 12, 2007.12, [URL], A reversed field pinch (RFP) machine with aspect ratio of as low as 2 (R/a = 0.51 m/0.25 m) has been constructed for the experimental study of new RFP regime. Low-aspect ratio RFP plasmas have been produced in a resistive wall vacuum vessel for the first time. Initial results on discharge characteristics of the low-A RFP plasmas are presented. This experiment serves our study on new RFP regime with expected characteristics of simpler MHD dynamic arising from equilibrium profiles with less densely spaced rational surfaces of the dominant modes, which have been revealed by the recent progress in RFP research..
61. S. Masamune, A. Sanpei, H. Himura, Ryuya Ikezoe, Takumi Onchi, M. Nagata, H. Hiramatsu, D. Ishijima, Research plans for low-aspect ratio reversed field pinch, Fusion Science and Technology, 51, 2 T., 197-199, 2007.02, In recent years, reversed field pinch (RFP) research has demonstrated great progress which alters conventional RFP concepts and introduced new RFP regime such as current profile control and new MHD mode dynamics. The low-aspect ratio (low-A) RFP configuration may have some advantages to explore these new regimes. After a brief description on the new RFP regimes, we will discuss our research plans in a low-A RFP which is under construction..
62. S. Masamune, A. Sanpei, Ryuya Ikezoe, Takumi Onchi, H. Himura, MHD mode control studies in medium- and low-aspect ratio RFPs, 33rd European Physical Society Conference on Plasma Physics 2006, EPS 2006
33rd EPS Conference on Plasma Physics 2006, EPS 2006 - Europhysics Conference Abstracts
, 3, 1648-1651, 2006.
63. R. Ikezoe, T. Onchi, K. Oki, A. Sanpei, H. Himura, S. Masamune, Characterization of MHD behavior in a low-aspect ratio RFP, J. Plasma Fusion Res. SERIES , 8, 1066, 2009.09.
64. R. Ikezoe, T. Onchi, K. Oki, A. Sanpei, H. Himura, S. Masamune, Quasi-Periodic Growth of a Single Helical Instability in a Low-Aspect Ratio RFP, Plasma Fusion Res., 3, 029, 2008.05.
65. R. Ikezoe, A. Sanpei, T. Onchi, K. Murata, K. Oki, H. Shimazu, T. Yamashita, H. Himura, S. Masamune, MHD equilibrium and stability of a low-aspect ratio RFP, Joint conference of 17th international Toki conference on physics of flows and turbulence in plasmas and 16th international stellarator/heliotron workshop 2007, 2007.10.

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