Evaluating electron cyclotron current drive stabilization of neoclassical tearing modes in ITER; Implications of experiments in ASDEX upgrade, DIII-D, JET, and JT-60U

ITERにおける電子サイクロトロン電流駆動による新古典テアリングモードの安定化の評価; ASDEX-upgrade, DIII-D, JET, JT-60Uにおける実験結果の反映

La Haye, R. J.*; Prater, R.*; Buttery, R. J.*; 林 伸彦; 諌山 明彦; Maraschek, M. E.*; Urso, L.*; Zohm, H.*

La Haye, R. J.*; Prater, R.*; Buttery, R. J.*; Hayashi, Nobuhiko; Isayama, Akihiko; Maraschek, M. E.*; Urso, L.*; Zohm, H.*

新古典テアリングモード(NTM)は理想MHD限界より低いベータ領域で発生してプラズマの性能を制限する。本論文では、ASDEX-Upgrade, DIII-D, JET, JT-60UにおけるNTM実験の結果を反映したNTM安定化のモデルを構築し、ITERにおける電子サイクロトロン電流駆動(ECCD)によるNTM安定化の予測計算を行った結果を記述している。NTMに起因する磁気島幅は、ある値まで小さくなったときに自発的に消滅するという性質がある。ASDEX-Upgrade, DIII-D, JETにおいてベータ値を徐々に下げて磁気島幅を小さくした実験、及び、ASDEX-Upgrage, DIII-D, JT-60UにおけるECCDによるNTM安定化実験の結果から、この幅はイオンバナナ幅の2倍に比例することが実験的に明らかになった。この結果を反映し、ITERの前側ミラー駆動方式、及び後側ミラー駆動方式のECCDシステムを用いたNTM安定化のシミュレーションを行った。その結果、前側ミラー駆動方式ではECCD幅が狭くできるために、安定化に必要なパワーが小さくできる反面、その場合はECCD位置のずれの許容範囲は小さくなることが明らかになった。

Resistive neoclassical tearing mode (NTM) islands will be the principal limit on stability and performance in ITER as beta is well below the ideal kink limit. NTM island control in ITER is predicted to be challenging both because of the relatively narrower marginal island widths and the relatively broader electron cyclotron current drive (ECCD). Measurements from ASDEX Upgrade, DIII-D, and JET in beta rampdown experiments are used to determine the marginal island size for m/n=3/2 NTM removal. This is compared to data from ASDEX Upgrade, DIII-D and JT-60U with elimination of the m/n=3/2 island by continuous ECCD at near constant beta. The empirical marginal island size is consistent in both sets of removal experiments and found to be about twice the ion banana width. A common methodology is developed for fitting the saturated m/n=3/2 island before (or without) ECCD in all four experimental devices. To this is added (and model tested to experiments) the effect of unmodulated co-ECCD on island stabilization including both replacing the missing bootstrap current and making the classical tearing stability index more negative. The experimentally benchmarked model is then used to evaluate ITER. The ITER ECCD upper launcher with up to 20 MW of injected power is appraised with or without modulation for both the m/n=3/2 mode and the m/n=2/1 NTM (which can lock to the resistive wall and induce disruption). An m/n=2/1 rotating island model with drag from eddy current induced in the resistive wall is used to predict the necessary ECCD to keep the island from locking as a function of the rotation in ITER. The planned relatively wide ECCD should be capable of regulating the island width to avoid mode locking with the anticipated rotation in ITER but there is little margin available for inevitable misalignment. Narrower ECCD of more power and/or more rotation in ITER would increase confidence in island control and successful operation.