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) similar to 1.5 MA and ITER-similar edge safety factor q (95) similar to 3.6) show that the L-H transition power threshold P (LH) can be reduced substantially (similar to 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)(rho = 0.95) similar to 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) similar to 1 MA (increased edge safety factor q (95) similar to 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).