Validation of on- and off-axis neutral beam current drive against experiment in DIII-D

DIII-D実験による中心部及び周辺部中性粒子ビーム電流駆動の検証

Park, J. M.*; 村上 和功*; Petty, C. C.*; Heidbrink, W. W.*; Osborne, T. H.*; Holcomb, C. T.*; Van Zeeland, M. A.*; Prater, R.*; Luce, T. C.*; Wade, M. R.*; Austin, M. E.*; Brooks, N. H.*; Budny, R. V.*; Challis, C. D.*; DeBoo, J. C.*; de Grassie, J. S.*; Ferron, J. R.*; Gohil, P.*; Hobirk, J.*; Hollmann, E. M.*; Hong, R. M.*; Hyatt, A. W.*; Lohr, J.*; Lanctot, M. J.*; Makowski, M. A.*; McCune, D. C.*; Politzer, P. A.*; St.John, H. E.*; 鈴木 隆博; West, W. P.*; Unterberg, E. A.*; Yu, J. H.*

Park, J. M.*; Murakami, Masanori*; Petty, C. C.*; Heidbrink, W. W.*; Osborne, T. H.*; Holcomb, C. T.*; Van Zeeland, M. A.*; Prater, R.*; Luce, T. C.*; Wade, M. R.*; Austin, M. E.*; Brooks, N. H.*; Budny, R. V.*; Challis, C. D.*; DeBoo, J. C.*; de Grassie, J. S.*; Ferron, J. R.*; Gohil, P.*; Hobirk, J.*; Hollmann, E. M.*; Hong, R. M.*; Hyatt, A. W.*; Lohr, J.*; Lanctot, M. J.*; Makowski, M. A.*; McCune, D. C.*; Politzer, P. A.*; St.John, H. E.*; Suzuki, Takahiro; West, W. P.*; Unterberg, E. A.*; Yu, J. H.*

DIII-Dにおいてプラズマを垂直方向に変位させることで、プラズマ周辺部(off-axis)での中性粒子ビーム電流駆動(NBCD)を実証した。モーショナルシュタルク効果計測により測定した磁力線ピッチ角の時間変化からポロイダル磁束の時間変化を評価し、小半径の半分程度にピークを持ち空間的に広がったoff-axis NBCD分布を示す結果を得た。多くの場合、測定したoff-axis NBCD分布は有限軌道効果を考慮した軌道追跡モンテカルロコードによる高速イオンの減速計算によるものと一致する。中性化した高速イオンが発するD光の2次元イメージング計測によるとビームイオン密度分布は凹状になっており古典的な減速の描像と一致する。NB駆動電流の大きさはビームと磁力線のなす角と関係しており、両者のなす角が小さくなるように選ぶことでoff-axis NBCDの効率は中心部(on-axis)NBCDと同程度によくすることができる。測定したoff-axis NBCDは入射パワーとともに増加するが、高パワーにおいて測定と計算が合うためには計算に適度な高速イオン拡散を入れる必要がある。

Neutral beam current drive (NBCD) experiments in DIII-D using vertically shifted plasmas to move the current drive away from the axis have clearly demonstrated robust off-axis NBCD. Time-dependent measurements of magnetic pitch angles by the motional Stark effect diagnostic are used to obtain the evolution of the poloidal magnetic flux, which indicates a broad off-axis NBCD profile with a peak at about half the plasma radius. In most cases, the measured off-axis NBCD profile is consistent with calculations using an orbit-following Monte-Carlo code for the beam ion slowing down including finite-orbit effects, provided there is no large-scale MHD activity such as Alfvn eigenmodes modes or sawteeth. Good agreement is found between the measured pitch angles and those from simulations using transport-equilibrium codes. Two-dimensional image of Doppler-shifted fast ion D light emitted by neutralized energetic ions shows clear evidence for a hollow profile of beam ion density, consistent with classical beam ion slowing down. The magnitude of off-axis NBCD is sensitive to the alignment of the beam injection relative to the helical pitch of the magnetic field lines. If the signs of B and I yield the proper helicity, both measurement and calculation indicate that the efficiency is as good as on-axis NBCD because the increased fraction of trapped electrons reduces the electron shielding of the injected ion current, in contrast with electron current drive schemes where the trapping of electrons degrades the efficiency. The measured off-axis NBCD increases approximately linearly with the injection power, although a modest amount of fast ion diffusion is needed to explain an observed difference in the NBCD profile between the measurement and the calculation at high injection power.