Helium experiments on Alcator C-Mod in support of ITER early operations

摘要

Helium majority experiments on Alcator C-Mod were performed to compare with deuterium discharges, and inform ITER early operations. ELMy H-modes were produced with a special plasma shape at B_T = 5.3 T, I_P = 0.9 MA, at q_(95) ~ 3.8. The He fraction ranged over, n_(He,L)_L = 0.2-0.4, with n_(D,L)_L = 0.15-0.26, compared to D plasmas with n_(D,L)_L = 0.85-0.97. The power to enter the H-mode in He was found to be greater than ~2 times that for D discharges, in the low density region <1.4 x 10~(20)/m~3. However, it appears to follow the D threshold for higher densities. The stored energies in the He discharges were about 80% of those in D, and about 40% higher net power was required to sustain them compared to D. Global particle confinement times for tungsten of T_w~*/T_E - 4 were obtained with ELMy H-modes in He, however accumulation occurred when the ELMs were irregular and infrequent. The electron temperatures and densities in the pedestal were similar between D and He discharges, and the ∆T_e/T_e and ∆n_e_e values were similar or larger in He than D. The higher net power required to access the H-mode, and sustain it in flattop, for He discharges in C-Mod, imply some limitations for He operation in ITER.