Reducing the L-H transition power threshold in ITER-similar-shape DIII-D hydrogen plasmas

Recent dedicated DIII-D experiments in low-torque, ITER-similar-shape (ISS) hydrogen plasmas (at a plasma current I _p ∼ 1.5 MA and ITER-similar edge safety factor q ₉₅ ∼ 3.6) show that the L-H transition power threshold P _LH can be reduced substantially (∼30%) with L-mode helium admixtures n _He/n _e ⩽ 25%. In the ensuing H-mode, helium ion fractions n _He/n _H remain below 25%. H-mode normalized pressure and confinement quality are only slightly affected by helium seeding, and Z _eff ⩽ 2.15 (including helium and carbon content). The plasmas investigated here are electron-heat dominated, with temperatures T _e(0)/T _i(0) ⩾ 1 and edge heat flux ratio Q _e/Q _i(ρ = 0.95) ∼ 1.2-1.5. Without mitigation, P _LH is higher by a factor of 2-3 in comparison to similar ISS deuterium plasmas. ISS hydrogen plasmas with lower plasma current I _p ∼ 1 MA (increased edge safety factor q ₉₅ ∼ 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 P _LH 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 P _LH 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).