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

Journal Article · · Nuclear Fusion
 [1];  [2];  [2];  [1];  [3];  [3];  [1];  [4];  [5];  [4];  [5];  [1];  [4];  [4];  [6];  [6];  [4];  [4];  [4]
  1. Univ. of California, Los Angeles, CA (United States)
  2. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
  3. Univ. of Wisconsin, Madison, WI (United States)
  4. General Atomics, San Diego, CA (United States)
  5. Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)
  6. Columbia Univ., New York, NY (United States)
+ Show Author Affiliations
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 $$\leqslant$$ 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 $$\leqslant$$ 2.15 (including helium and carbon content). The plasmas investigated here are electron-heat dominated, with temperatures Te(0)/Ti(0) $$\geqslant$$ 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. Finally, 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).
Research Organization:
General Atomics, San Diego, CA (United States); Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)
Sponsoring Organization:
USDOE Office of Science (SC)
Grant/Contract Number:
AC02-09CH11466; AC05-00OR22725; FC02-04ER54698; FG02-08ER54999; SC0019352; SC0020287; SC0022270
OSTI ID:
1898976
Journal Information:
Nuclear Fusion, Journal Name: Nuclear Fusion Journal Issue: 12 Vol. 62; ISSN 0029-5515
Publisher:
IOP ScienceCopyright Statement
Country of Publication:
United States
Language:
English

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Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY
ITER
L-H transition
helium
hydrogen