Recent dedicated DIII-D experiments in low-torque, ITER-similar-shape (ISS) hydrogen plasmas (at a plasma current Ip ∼ 1.5 MA and ITER-similar edge safety factor q95 ∼ 3.6) show that the L-H transition power threshold PLH can be reduced substantially (∼30%) with L-mode helium admixtures nHe/ne ⩽ 25%. In the ensuing H-mode, helium ion fractions nHe/nH remain below 25%. H-mode normalized pressure and confinement quality are only slightly affected by helium seeding, and Zeff ⩽ 2.15 (including helium and carbon content). The plasmas investigated here are electron-heat dominated, with temperatures Te(0)/Ti(0) ⩾ 1 and edge heat flux ratio Qe/Qi(ρ = 0.95) ∼ 1.2–1.5. Without mitigation, PLH is higher by a factor of 2–3 in comparison to similar ISS deuterium plasmas. ISS hydrogen plasmas with lower plasma current Ip ∼ 1 MA (increased edge safety factor q95 ∼ 5.1) exhibit a substantially lower power threshold. This plasma current dependence, also observed previously on ASDEX-U and in JET, is not accounted for by the commonly used 2008 ITPA multi-machine threshold scaling, but could potentially allow H-mode access at marginal heating power during the initial plasma current ramp-up. Attempts to reduce PLH with low-field- and high-field-side hydrogen pellet injection, using 1.7 mm diameter pellets, have not demonstrated a robust threshold reduction, in contrast to successful earlier experiments with larger 2.7 mm pellets. Techniques for reducing PLH are very important for ITER, in particular for accessing H-mode in hydrogen plasmas during the Pre-Fusion Power Operation-1 (PFPO-1) campaign with marginal auxiliary heating power (20–30 MW of ECH).
Determining the access to H–mode in the ITER pre–fusion and fusion power operation phases at low plasma current with full–radius TGLF–SAT2 simulations of L–mode plasmas
H-mode plasmas in the pre-fusion power operation 1 phase of the ITER research plan