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

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/n _L = 0.2-0.4, with n _D,L/n _L = 0.15-0.26, compared to D plasmas with n _D,L/n _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 × 10²⁰/m³. 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 /τ _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 ΔT _e/T _e and Δn _e/n _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.