L – H transition trigger physics in ITER-similar plasmas with applied n = 3 magnetic perturbations

Schmitz, L.; Kriete, D. M.; Wilcox, R. S.; Rhodes, T. L.; Zeng, L.; Yan, Z.; McKee, G. R.; Evans, T. E.; Paz-Soldan, C.; Gohil, P.; Lyons, B.; Petty, C. C.; Orlov, D.; Marinoni, A.

doi:10.1088/1741-4326/ab36bf

Title: L – H transition trigger physics in ITER-similar plasmas with applied n = 3 magnetic perturbations

Journal Article · 24 September 2019 · Nuclear Fusion

DOI: https://doi.org/10.1088/1741-4326/ab36bf · OSTI ID:1569024

^[1]; Kriete, D. M. ^[2]; Wilcox, R. S. ^[3]; Rhodes, T. L. ^[4]; Zeng, L. ^[4]; Yan, Z. ^[2]; McKee, G. R. ^[2];

^[5];

^[5]; Gohil, P. ^[5];

^[5];

^[6]; Marinoni, A. ^[7]

Univ. of California, Los Angeles, CA (United States); General Atomics, Energy & Advanced Concepts, DIII-D
Univ. of Wisconsin, Madison, WI (United States)
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Univ. of California, Los Angeles, CA (United States)
General Atomics, San Diego, CA (United States)
Univ. of California San Diego, La Jolla, CA (United States)
Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)

In this work, the L – H transition power threshold Ρ_LH is observed to increase with applied n = 3 Resonant Magnetic Perturbations (RMP) in ITER-similar-shape (ISS) plasmas with balanced neutral beam torque injection in DIII-D. The increase is most pronounced with added ECH: Ρ_LH increases with decreasing edge plasma collisionality as Ρ_LH/Ρ_LH-08 ~ (v*)^-0.5, where Ρ_LH-08 is the 2008 ITPA multi-machine power threshold scaling. This result raises concerns for H-mode access at low edge collisionality in ITER, where resonant magnetic perturbations (RMP) may have to be applied before the L – H transition to safely suppress the first ELM. Non-axisymmetric modifications with RMP include a simultaneous reduction of the radial electric field (E_r) well depth and E × B shear. This can be attributed to increasing edge toroidal co-current rotation, and is consistent with substantially increased local long-wavelength turbulence [measured via Beam Emission Spectroscopy (BES)]. At high RMP perturbation strength the edge electric field E_r reverses sign locally (becomes positive), with changes in dominant turbulence modes. Edge magnetic stochasticity provides an attractive explanation of the observed modifications, and the observed changes in toroidal rotation and E_r are consistent with a simple fluid model describing radial electron current flow along stochastic fieldlines. The observed collisionality dependence of the L-mode edge electric field with applied RMP is also qualitatively consistent with this model. Reflectometry data indicate a significant reduction of the normalized L-mode radial density gradient a/L_n at high RMP field with simultaneous increase in radial particle flux and electron thermal flux from power balance analysis. We conjecture that the increase of Ρ_LH with RMP results from the combined effects of reduced E × B flow shear (increasing turbulent transport levels) and toroidal/poloidal flow modulation due to edge stochasticity. Initial experiments indicate that non-resonant n = 3 magnetic perturbations lead only to relatively small changes in E_r, E × B shear and fluctuation characteristics, and have less impact on the L – H transition power threshold. In conclusion, this motivates further exploration of the RMP spectrum dependence of Ρ_LH for possible mitigation of the observed threshold increase.

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Research Organization:: General Atomics, San Diego, CA (United States)

Sponsoring Organization:: USDOE Office of Science (SC), Fusion Energy Sciences (FES) (SC-24)

Grant/Contract Number:: FC02-04ER54698; FG02-08ER54984; FG02-89ER53296; FG02-08ER54999; AC05-00OR22725

OSTI ID:: 1569024

Journal Information:: Nuclear Fusion, Journal Name: Nuclear Fusion Journal Issue: 12 Vol. 59; ISSN 0029-5515

Publisher:: IOP ScienceCopyright Statement

Country of Publication:: United States

Language:: English

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