Publications (11)4.09 Total impact
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Article: Fast‐Ion Response to Energetic‐Particle‐Driven MHD Activity in Heliotron J
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ABSTRACT: In Heliotron J, low magnetic shear configuration, instabilities with frequency chirping in the frequency range of Alfvén eigenmodes have been observed in tangentially injected neutral beam plasmas. These modes are induced by energetic-particle driven magnetohydrodynamic (MHD) instabilities such as global Alfvén eigenmode or energetic particle mode. A hybrid directional Langmuir probe system has been installed into Heliotron J to investigate the response of fast-ion fluxes to the MHD modes. A high coherent response of the ion flux to the bursting modes has been observed not only by the co-directed probe but also by the counter-directed one. A linear correlation between the response of the co-directed ion flux and the mode amplitude has been found. The radial profile of the response of the co-directed ions has decreased with the minor radius and has not been obtained significantly outside last closed flux surface. These results indicate that the fast-ion response is due to a resonant convective oscillation. The ion flux response of the counter-directed probe has appeared in the growth phase of the mode burst. Its phase relation is different from that of co-directed one and magnetic probe located at the Heliotron J vacuum vessel. Two candidates of the detected ion flux of the counter-directed probe have been discussed (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)Beiträge aus der Plasmaphysik 07/2010; 50(6‐7):534 - 539. -
Article: Effect of magnetic field ripple on electron cyclotron current drive in Heliotron J
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ABSTRACT: Electron cyclotron current drive (ECCD) experiments have been conducted in the helical heliotron device, Heliotron J. A wide configuration scan shows that the electron cyclotron (EC) driven current is strongly dependent on the magnetic ripple structure where the EC power is deposited. As the EC power is deposited on the deeper ripple bottom, the EC current flowing in the Fisch–Boozer direction decreases, and the reversal of directly measured EC driven current is observed. Measurement results using electron cyclotron emission and soft-x ray spectrum diagnostics imply that high-energy electrons are generated for ripple top heating while they are suppressed for ripple bottom heating, indicating that the generation and confinement of trapped electrons have an important role on ECCD. For ripple top heating, the typical ECCD efficiency is estimated as γ = neIECR/PEC = 0.8 × 1017 A W−1 m−2 and , where ne is in 1020 m−3, IEC in A, R in m, PEC in W and Te in keV. The normalized ECCD efficiency is found to be independent of the absorbed EC power for both ripple top and bottom heating cases.Nuclear Fusion 01/2010; 50(2):025003. · 4.09 Impact Factor -
Article: Configuration Control Studies in Heliotron J
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ABSTRACT: Heliotron J is a flexible concept exploration facility for the helical‐axis heliotron concept. One of the major objectives of the Heliotron J study is to experimentally confirm the effects of the new ideas introduced into this concept to improve the plasma performance. As a part of such experiments, the bumpiness (εb) and rotational transform (ι/2π) control studies have been performed. The εb‐control experiments have revealed the εb‐dependence of the fast ion confinement is qualitatively consistent with the drift optimization viewpoint. However, the bulk plasma confinement studies suggest that the low effective helical ripple configuration seems to be preferable for the confinement improvement for ECH‐only plasma. The ι/2π‐control experiments for ECH‐only and∕or ECH+NBI plasmas have revealed the existence of windows in the vacuum ι(a)/2π for the high quality H‐mode. In NBI‐only plasmas, it was found out that the transition in NBI‐only plasma occurs at a certain toroidal current, which depends on the vacuum ι(a)/2π and the bumpiness but is independent of Pinj. This suggests the relation of the onset of the transition to the modification of the rotational transform caused by the plasma current.AIP Conference Proceedings. 04/2008; 996(1):259-268. -
Article: Configuration Control Experiment in Heliotron J
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ABSTRACT: Heliotron J is a flexible configuration device to explore a new concept optimization for the helical-axis heliotron configuration. The field ripple along the toroidal direction, which is called 'bumpiness' is a key component for the improvement of the particle confinement. From the fast ions generated by the ICRF minority heating, the high bumpy configuration is most favorable and the fast ions of about 30 keV are observed. The toroidal current control is also important for the Heliotron J field configuration. Using the combination of the change of the bumpiness and the deposition position, the toroidal current can be controlled by about 7 kA. The bumpy field dependence on the global confinement is investigated for ECH and NBI plasmas. The confinement in the high and medium bumpy configurations is better than that in the low bumpy configuration. The difference of the bumpy dependence for the ECH and NBI plasmas is possibly caused by the improved fast-ion confinement for the NBI plasmas in the high bumpy configuration. -
Article: Development of a microwave AM reflectometer for electron density profile measurement in Heliotron J
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ABSTRACT: A microwave reflectometer is developed for electron density profile measurement in Heliotron J. An amplitude modulation (AM) type system is adopted to reduce density fluctuation effects. The carrier frequency ranges from 33 to 56 GHz, and the modulation frequency is 100 MHz. The X-mode is selected as the propagation mode in order to measure a hollow density profile typically which is observed in ECH plasmas. A test-bench examination using an aluminum reflection plate shows that the measured phase shift agrees well with that expected from the change in the plate position if the dependence on the RF signal power is taken into account. A numerical program to reconstruct density profile from the measured phase shift data has been also developed. The result confirms that the program works well for modeled flat or hollow density profiles. -
Article: Directional Probe Experiments in the Heliotron J
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Article: Development of Long-distance Correlation Diagnostic for Turbulence Structure Analysis and its Application to HJ-LHD Comparative Studies
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Article: Improvement of Local Profile and Long-distance Correlation Diagnostic for the Turbulence Structure Analysis and its Application to the HJ-LHD Comparative Studies
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Article: Velocity Distribution of Fast Ions Generated by ICRF Heating in Heliotron J
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ABSTRACT: Fast ion velocity distribution is investigated using ICRF minority heating in Heliotron J with special emphasis on the effect of the toroidal ripple of magnetic field strength. The effect of the magnetic configuration on the fast ion confinement is one of the most important issues in helical devices. Here, the pitch angle dependences of energy spectra for three bumpy cases are measured for the first time, then, the fast ions up to 34 keV are observed in the high bumpy case during ICRF heating in Heliotron J. The configurations used in this study are as follows; the bumpiness (B_04/B_00, where B_04 is the bumpy component and B_00 is the averaged magnetic field strength) are 0.15 (high), 0.06 (medium) and 0.01 (low) at the normalized radius of 0.67. The configuration of B_04/B_00= 0.06 corresponds to the standard configuration in Heliotron J. In high bumpy cases, the higher energy flux is measured near 120 deg in pitch angle although the ions are considered to be accelerated in the perpendicular direction by ICRF heating. To understand experimental results, Monte Carlo calculation is performed. The numerical model consists of orbit tracing, Coulomb collisions and acceleration by the ICRF heating. Minority protons are regarded as test particles and the heating is simulated by the velocity kick in the perpendicular direction in velocity space when ions cross the cyclotron layer. The calculation results using Monte Carlo method represents that the accelerated ion distribution has its peak in the range between 20 deg and 30 deg from the perpendicular direction. This result is considered to be caused mainly due to the existence of the loss region around the perpendicular direction. -
Article: Effect of Magnetic Field Ripple on ECCD in Heliotron J
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ABSTRACT: Electron cyclotron current drive (ECCD) experiment has been made in the helical-heliotron device, Heliotron J. A wide configuration scan shows that the EC driven current is strongly dependent on the magnetic ripple structure where the EC power is deposited. As the EC power is deposited on the deeper ripple bottom, the EC driven current flowing in the Fisch-Boozer direction decreases, and the reversal of directly measured EC driven current is observed. High energy electrons are suppressed for ripple bottom heating, indicating that generation and confinement of trapped electrons have an important role on ECCD. For ripple top heating, the typical ECCD efficiency is estimated gamma=n_eI_EC R/P_EC=0,8x10^17 A/Wm^2 and zeta=e^3n_eI_EC R/epsilon_0^2P_ECT_e,=0,05. The normalized ECCD efficiency is found to be independent on the absorbed EC power for both ripple top and bottom heating cases. -
Article: Effect of Bumpy Magnetic Field on Energy Confinement in NBI Plasmas of Heliotron J
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ABSTRACT: The energy confinement in the NBI plasmas has been investigated in the helical-axis heliotron device, Heliotron J. The configuration control experiments for bumpiness ( 竜 _b), being one of the side-bands of the helical component, show that the plasma performance in NBI plasmas depends on not only the effective helical ripple (竜_eff) but also the bumpiness. The preferable enhancement factor of the experimental energy confinement time to the international stellarator scaling law ISS95 has been obtained in the high- 竜_b configuration and in the low-竜_eff (i.e. medium- 竜_b ) condition. The improvement in the ion and electron temperature contributes to the enhancement of the plasma performance in the high- 竜_b configurations since the fast ion confinement is found to be better in this configuration. The control of bumpiness is effective not only in the energetic particle transport but also in the global energy confinement in NBI plasmas of Heliotron J.
Top co-authors
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Institutions
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2008–2010
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Kyoto University
- Graduate School of Energy Science
Kyoto, Kyoto-fu, Japan
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