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

Abstract

The L–H transition power threshold P LH is observed to increase with applied n = 3 resonant magnetic perturbations (RMP) in ITER-similar-shape plasmas with balanced neutral beam torque injection in DIII-D. The increase is most pronounced with added electron–cyclotron heating: P LH increases with decreasing edge plasma collisionality as P LH/P LH-08 ~ (ν *)-0.5, where P 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 RMP may have to be applied before the L–H transition to safely suppress the first edge-localized mode. 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). 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.more » 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 P 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. This motivates further exploration of the RMP spectrum dependence of P LH for possible mitigation of the observed threshold increase.« less

Authors:

Schmitz, L.  ^[1];

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

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Paz-Soldan, C.  ^[4];

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Gohil, P. ^[4]; Lyons, B.  ^[4];

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Petty, C. C.  ^[4];

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Orlov, D.  ^[5];

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Marinoni, A. ^[6]

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+ Show Author Affiliations

1. Univ. of California, Los Angeles, CA (United States)
2. Univ. of Wisconsin, Madison, WI (United States)
3. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
4. General Atomics, San Diego, CA (United States)
5. Univ. of California San Diego, La Jolla, CA (United States)
6. Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)

Publication Date:

2019-09-24

Research Org.:

General Atomics, San Diego, CA (United States); Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Fusion Energy Sciences (FES)

OSTI Identifier:

1569024

Alternate Identifier(s):

OSTI ID: 1761760

Grant/Contract Number:  

FC02-04ER54698; FG02-08ER54984; FG02-89ER53296; FG02-08ER54999; AC05-00OR22725

Resource Type:

Accepted Manuscript

Journal Name:

Nuclear Fusion

Additional Journal Information:

Journal Volume: 59; Journal Issue: 12; Journal ID: ISSN 0029-5515

Publisher:

IOP Science

Country of Publication:

United States

Language:

English

Subject:

70 PLASMA PHYSICS AND FUSION TECHNOLOGY; L-H transition; Resonant magnetic perturbations; Turbulence