Title: W7-X and the sawtooth instability: towards realistic simulations of current-driven magnetic reconnection

Abstract

Magnetic reconnection in W7-X is studied by means of global numerical simulations in a series of models of increasing complexity. The magnetic geometry ranges from that of a cylinder to the full three-dimensional field of W7-X, and the equations solved range from ideal magnetohydrodynamics (MHD) to gyrokinetics. We simulate plasmas from the first operation phase with electron cyclotron current drive (ECCD). These are characterized by an equilibrium magnetic field featuring an ECCD-distorted 'humped' profile of the rotational transform ι, with ι = 1 in two radial locations. Such plasmas generally show sawtooth activity, hence motivating the present study. We pay particular attention to the role of equilibrium current density gradients in the destabilization of reconnecting modes. When the equilibrium temperature and density gradients are artificially suppressed (to eliminate the pressure gradient drive), the perturbed electrostatic potential is radially localized between the locations at which ι = 1. This is shown with a purely collisionless gyrokinetic model, in cylindrical geometry. In the real toroidal geometry of W7-X, for a non-ideal MHD model including a uniform resistivity, electron inertia and (numerical) viscosity, the same qualitative behaviour is observed. In particular, even if a resonant (m, n) = (1, -1) perturbation is initialized,more » the most unstable mode is the (m, n) = (-4, 4), where m and n are the poloidal and toroidal mode numbers, respectively. Other modes are destabilized due to geometric coupling. The growth rate of this instability scales as η^1/3, where η is the plasma resistivity, thus suggesting that ECCD drives ideal MHD stable W7-X plasmas towards non-ideal marginality. An ideal magnetohydrodynamic analysis confirms the result. A fluid-kinetic hybrid version of the EUTERPE code shows that gyrokinetic ions have a stabilizing effect on these modes. For W7-X relevant collisionalities, the growth rate scales linearly with the electron skin depth, d_e. Implications of our results for sawtoothing W7-X operation are discussed.« less

Authors:

Zocco, Alessandro  ^[1];

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• Search DOE PAGES for ORCID "0000-0003-2617-3658"

Mishchenko, Alexey  ^[1];

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Nührenberg, Carolin  ^[1];

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Könies, Axel  ^[1];

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Kleiber, Ralf  ^[1];

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• Search DOE PAGES for ORCID "0000-0002-2261-2855"

Borchardt, Matthias  ^[1];

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Slaby, Christoph ^[1]; Zanini, Marco  ^[1];

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Stange, Torsten  ^[1];

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Laqua, Heinrich Peter ^[1]; Rahbarnia, Kian  ^[1];

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Thomsen, Henning  ^[1];

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Wolf, R. C.  ^[1];

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Helander, Per  ^[1];

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• Search DOE PAGES for ORCID "0000-0002-0460-590X"

Hatzky, Roman  ^[2];

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Cole, Michael J.  ^[3]

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• Search DOE PAGES for ORCID "0000-0001-9682-989X"

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+ Show Author Affiliations

1. Max Planck Institute for Plasma Physics, Greifswald (Germany)
2. Max Planck Society, Garching (Germany). Max Planck Institute for Plasma Physics
3. Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)

Publication Date:

Thu Jun 24 00:00:00 EDT 2021

Research Org.:

Princeton Plasma Physics Laboratory (PPPL), Princeton, NJ (United States)

Sponsoring Org.:

EUROFusion; USDOE

OSTI Identifier:

1810626

Grant/Contract Number:  

AC02-09CH11466; 633053

Resource Type:

Accepted Manuscript

Journal Name:

Nuclear Fusion

Additional Journal Information:

Journal Volume: 61; Journal Issue: 8; Journal ID: ISSN 0029-5515

Publisher:

IOP Science

Country of Publication:

United States

Language:

English

Subject:

70 PLASMA PHYSICS AND FUSION TECHNOLOGY