Reducing the L-H transition power threshold in ITER-similar-shape DIII-D hydrogen plasmas. (1st December 2022)

• Record Type: Journal Article
• Authors: Schmitz, L.; Wilcox, R.S.; Shiraki, D.; Rhodes, T.L.; Yan, Z.; McKee, G.R.; Callahan, K.J.; Chrystal, C.; Haskey, S.R.; Liu, Y.Q.; Laggner, F.; Zeng, L.; Osborne, T.H.; Grierson, B.A.; Paz-Soldan, C.; Leuthold, N.; Lyons, B.C.; Gohil, P.; Petty, C.C.
• Is Part Of: Nuclear fusion. Volume 62:Number 12(2022)
• Publication Date: 2022-12-01
• Subjects: L-H transition -- ITER -- hydrogen -- helium; Nuclear fusion -- Periodicals; 621.48405
• DOI: 10.1088/1741-4326/ac94e1
• Languages: English
• ISSNs: 0029-5515

Abstract

Abstract: 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).