Abstract Details
Abstracts
Author: Sid J Leigh
Requested Type: Consider for Invited
Submitted: 2023-03-09 04:11:53
Co-authors: K. Imada, A. V. Dudkovskaia, J. W. Connor, H. R. Wilson
Contact Info:
York Plasma Institute, University of York
Genesis 1/2, Church Lane
Heslington, York, England YO105DD
United Kingdom
Abstract Text:
Neoclassical tearing modes (NTMs) are resistive MHD instabilities where magnetic islands form on toroidal flux surfaces, removing pressure gradient there. Islands amplify when their width w and the poloidal beta both exceed thresholds, thus endangering high-gain confinement. NTMs can been suppressed by reducing w below the threshold width w_c. Experimentally, w_c is comparable to the trapped ion orbit width ρ_bi~√ε ρ_θi, where ε is the inverse aspect ratio and ρ_θi is the ion poloidal gyroradius [1]. To reliably predict w_c at finite ρ_bi, we must account for the drift-kinetic effects of ions, and the differing electron response. We achieve this by simplifying the drift-kinetic equation in the limit of w/r~ρ_θi≪1 for minor radius r, and self-consistently recalculating the electrostatic potential from quasineutrality.
We present and compare results from two drift-kinetic models of threshold scale islands (w~ρ_bi). Our 4D model, DK-NTM, operates in the ε≪1 limit at arbitrary collisionality [2,3,4]. We revise our earlier w_c:ρ_bi scaling result from [4] and extend to finite island propagation frequency to identify solutions that satisfy torque balance, and quantify the ion polarization current contribution to w_c. Our 3D model, RDK-NTM, operates in general geometry in the limit of low collisionality. In the ε≪1 limit, it predicted w_c=2.85ρ_bi [4], consistent with the experimental data of [1]. Recent studies of plasma shaping parameters in finite ε find that higher triangularity plasmas are more NTM-prone, consistent with DIII-D observations [5], and that the scaling of w_c with poloidal beta is in agreement with EAST observations [5,6].
[1] La Haye et al (2012) Phys. Plasmas 19 062506
[2] Imada et al (2018) Phys. Rev. Lett. 121 175001
[3] Imada et al (2019) Nucl. Fusion 59 046016
[4] Dudkovskaia et al (2021) PPCF 63 054001
[5] Dudkovskaia et al (2023) Nucl. Fusion 63 016020
[6] Shi et al (2021) arXiv:2103.15506 [physics.plasm-ph]
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