Off-axis neutral beam current drive for advanced scenario development in DIII-D

DIII-Dにおける先進シナリオ開発のための周辺部中性粒子ビーム電流駆動

村上 和功*; Park, J. M.*; Petty, C. C.*; Luce, T. C.*; Heidbrink, W. W.*; Osborne, T. H.*; Prater, R.*; Wade, M. R.*; Anderson, P. M.*; 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.*; Holcomb, C. T.*; Hollmann, E. M.*; Hong, R. M.*; Hyatt, A. W.*; Lohr, J.*; Lanctot, M. J.*; Makowski, M. A.*; McCune, D. C.*; Politzer, P. A.*; Scoville, J. T.*; St.John, H. E.*; 鈴木 隆博; Taylor, T.*; West, W. P.*; Unterberg, E. A.*; Van Zeeland, M. A.*; Yu, J. H.*

Murakami, Masanori*; Park, J. M.*; Petty, C. C.*; Luce, T. C.*; Heidbrink, W. W.*; Osborne, T. H.*; Prater, R.*; Wade, M. R.*; Anderson, P. M.*; 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.*; Holcomb, C. T.*; Hollmann, E. M.*; Hong, R. M.*; Hyatt, A. W.*; Lohr, J.*; Lanctot, M. J.*; Makowski, M. A.*; McCune, D. C.*; Politzer, P. A.*; Scoville, J. T.*; St.John, H. E.*; Suzuki, Takahiro; Taylor, T.*; West, W. P.*; Unterberg, E. A.*; Van Zeeland, M. A.*; Yu, J. H.*

DIII-Dでは、小半径の50%という周辺部で電流駆動できるようにするために、2つの中性粒子ビーム(NB)の垂直方向角度を可変とする改造を計画している。計算によると、NBを下側に向けた場合には、プラズマ電流とトロイダル磁場が同方向の場合に電流駆動効率が大きくなる。この効果は大きく、ITERでの計算では現状のトロイダル磁場の方向を反転させると周辺部NB駆動電流は20%以上増加する。この計算結果を検証するためにDIII-Dでプラズマを上下方向に寄せることでNBに対する磁力線の角度を変化させ、そのときの周辺部NB駆動電流を調べた。MSE計測によるとNB駆動電流は計算通り4045%変化した。この結果から、DIII-DのNBに垂直方向角度を可変とする改造を行うことで、ITERや将来のトカマクのためのシナリオ開発を強力に推進できるだけでなく、輸送・高エネルギー粒子・加熱電流駆動の物理を自在に調べられるようになることが期待される。

Modification of the two existing DIII-D neutral beam lines is planned to allow vertical steering to provide off-axis neutral beam current drive (NBCD) peaked as far off-axis as half the plasma minor radius. New calculations for a downward-steered beam indicate strong current drive with good localization off-axis so long as the toroidal magnetic field, BT, and the plasma current, Ip, point in the same direction. This is due to good alignment of neutral beam injection (NBI) with the local pitch of the magnetic field lines. This model has been tested experimentally on DIII-D by an injecting equatorially-mounted NBs into reduced size plasmas that are vertically displaced with respect to the vessel midplane. The existence of off-axis NBCD is evident in the changes seen in sawtooth behavior in the internal inductance. By shifting the plasma upward or downward, or by changing the sign of the toroidal field, measured off-axis NBCD profiles measured with motional Stark effect data and internal loop voltage show a difference in amplitude (40%-45%) consistent with predicted differences predicted by the changed NBI alignment with respect to the helicity of the magnetic field lines. The effects of NB injection direction relative to field line helicity can be large even in ITER: off-axis NBCD can be increased by more than 20% if the BT direction is reversed. Modification of the DIII-D NB system will strongly support scenario development for ITER and future tokamaks as well as providing flexible scientific tools for understanding transport, energetic particles and heating and current drive.