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Discussion on experimental investigation and validation of neutral beam current drive for ITER through ITPA joint experiments

ITPA装置間比較実験によるITERのための中性粒子ビーム電流駆動実験研究と検証についての議論

鈴木 隆博; Akers, R.*; Gates, D. A.*; G$"u$nter, S.*; Heidbrink, W. W.*; Hobirk, J.*; Luce, T. C.*; 村上 和功*; Park, J. M.*; Turnyanskiy, M.*; ITPA Integrated Operation Scenarios Topical Group*

Suzuki, Takahiro; Akers, R.*; Gates, D. A.*; G$"u$nter, S.*; Heidbrink, W. W.*; Hobirk, J.*; Luce, T. C.*; Murakami, Masanori*; Park, J. M.*; Turnyanskiy, M.*; ITPA Integrated Operation Scenarios Topical Group*

国際トカマク物理活動(ITPA)の下で、世界の主要な5つのトカマク装置(AUG(独国), DIII-D(米国), JT-60U(日本), MAST(英国)及びNSTX(米国))においてNBCD駆動電流分布を測定し理論を検証するための国際装置間比較実験を実施した。その実験で得られた成果について議論を行う。さまざまな装置での実験を行い、プラズマ電流0.6-1.2MA,トロイダル磁場0.3$$sim$$3.7T, NBの加速エネルギー67$$sim$$350keVと広いダイナミックレンジの実験条件にわたってNB駆動電流分布(あるいは全電流分布のNBCDによる変化)を測定し、NB駆動電流とその空間分布の測定結果は、比較的小さい高速イオンの拡散係数(0$$sim$$0.5m$$^2$$/s)を仮定した理論計算で説明できることがわかった。また、AUG及びDIII-Dの結果によると、低い加熱パワーのときにはNB駆動電流分布は高速イオンの拡散係数として0m$$^2$$/sを仮定した理論計算で説明できる一方で、高い加熱パワー(AUGでは5MW、DIII-Dでは7.2MW)のときには0.3$$sim$$0.5m$$^2$$/s程度の拡散係数を仮定した理論計算が最もよく実験と合致するとの結果を得た。背景プラズマ乱流による高速イオンの散乱とそれによる再分配と考えられる。

Joint experiments investigating the off-axis neutral beam current drive (NBCD) capability to be utilized for advanced operation scenario development in ITER was conducted in 5 tokamaks (AUG, DIII-D, JT-60U, MAST and NSTX) through the ITPA. We discuss results obtained in the joint experiments, where the toroidal field, Bt, covered 0.3-3.7 T, the plasma current, Ip, 0.6-1.2 MA, and the beam energy, Eb, 67-350 keV. A current profile broadened by off-axis NBCD was observed in MAST. In DIII-D, good agreement between the measured and calculated NB driven current profile was observed. In JT-60U, agreement between measured and calculated NBCD location was obtained, when the NBCD location (0.3-0.6 in r/a), heating power (6-13 MW), triangularity d (0.25-0.45), and Eb (85 and 350 keV) were widely scanned. In AUG (at low d 0.2) and DIII-D, introduction of a fast ion diffusion coefficient of Db 0.3-0.5 m$$^2$$/s in the calculation gave better agreement at high heating power (5 and 7.2 MW), suggesting anomalous transport of fast ions by turbulence. It was found through these ITPA joint experiments that NBCD related physics quantities reasonably agree with calculations (with Db=0-0.5 m$$^2$$/s) in all devices when there is no MHD activity except ELMs.

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