Fast-ion transport in qmin > 2, high-β steady-state scenarios on DIII-D

Results from experiments on DIII-D [J. L. Luxon, Fusion Sci. Technol. 48, 828 (2005)] aimed at developing high beta steady-state operating scenarios with high-q(min) confirm that fast-ion transport is a critical issue for advanced tokamak development using neutral beam injection current drive. In DIII-D, greater than 11 MW of neutral beam heating power is applied with the intent of maximizing beta(N) and the noninductive current drive. However, in scenarios with q(min) > 2 that target the typical range of q(95) = 5-7 used in next-step steady-state reactor models, Alfven eigenmodes cause greater fast-ion transport than classical models predict. This enhanced transport reduces the absorbed neutral beam heating power and current drive and limits the achievable beta(N). In contrast, similar plasmas except with q(min) just above 1 have approximately classical fast-ion transport. Experiments that take q(min) > 3 plasmas to higher beta(P) with q(95) = 11-12 for testing long pulse operation exhibit regimes of better than expected thermal confinement. Compared to the standard high-q(min) scenario, the high beta(P) cases have shorter slowing-down time and lower del beta(fast), and this reduces the drive for Alfvenic modes, yielding nearly classical fast-ion transport, high values of normalized confinement, beta(N), and noninductive current fraction. These results suggest DIII-D might obtain better performance in lower-q(95), high-q(min) plasmas using broader neutral beam heating profiles and increased direct electron heating power to lower the drive for Alfven eigenmodes. (C) 2015 AIP Publishing LLC.