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Kinetic-ballooning-mode turbulence in low-average-magnetic-shear equilibria

Part of: Focus on Fusion

Published online by Cambridge University Press:  15 June 2021

I.J. McKinney Open the ORCID record for I.J. McKinney [Opens in a new window] ,
M.J. Pueschel ,
B.J. Faber Open the ORCID record for B.J. Faber [Opens in a new window] ,
C.C. Hegna ,
A. Ishizawa  and
P.W. Terry
I.J. McKinney*
Affiliation:
Department of Engineering Physics, University of Wisconsin-Madison, Madison, WI53706, USA
M.J. Pueschel
Affiliation:
Eindhoven University of Technology, 5600 MBEindhoven, The Netherlands Dutch Institute for Fundamental Energy Research, 5612 AJEindhoven, The Netherlands Institute for Fusion Studies, University of Texas at Austin, Austin, TX78712, USA
B.J. Faber
Affiliation:
Department of Engineering Physics, University of Wisconsin-Madison, Madison, WI53706, USA
C.C. Hegna
Affiliation:
Department of Engineering Physics, University of Wisconsin-Madison, Madison, WI53706, USA
A. Ishizawa
Affiliation:
Graduate School of Energy Science, Kyoto University, Uji, Kyoto611-0011, Japan
P.W. Terry
Affiliation:
Department of Physics, University of Wisconsin-Madison, Madison, WI53706, USA
*
†Email address for correspondence: [email protected]
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Abstract

Kinetic-ballooning-mode (KBM) turbulence is studied via gyrokinetic flux-tube simulations in three magnetic equilibria that exhibit small average magnetic shear: the Helically Symmetric eXperiment (HSX), the helical-axis Heliotron-J and a circular tokamak geometry. For HSX, the onset of KBM being the dominant instability at low wavenumber occurs at a critical value of normalized plasma pressure $\beta ^{\rm KBM}_{\rm crit}$ that is an order of magnitude smaller than the magnetohydrodynamic (MHD) ballooning limit $\beta ^{\rm MHD}_{\rm crit}$ when a strong ion temperature gradient (ITG) is present. However, $\beta ^{\rm KBM}_{\rm crit}$ increases and approaches the MHD ballooning limit as the ITG tends to zero. For these configurations, $\beta ^{\rm KBM}_{\rm crit}$ also increases as the magnitude of the average magnetic shear increases, regardless of the sign of the normalized magnetic shear. Simulations of Heliotron-J and a circular axisymmetric geometry display behaviour similar to HSX with respect to $\beta ^{\rm KBM}_{\rm crit}$. Despite large KBM growth rates at long wavelengths in HSX, saturation of KBM turbulence with $\beta > \beta _{\rm crit}^{\rm KBM}$ is achievable in HSX and results in lower heat transport relative to the electrostatic limit by a factor of roughly five. Nonlinear simulations also show that KBM transport dominates the dynamics when KBMs are destabilized linearly, even if KBM growth rates are subdominant to ITG growth rates.

Keywords

plasma instabilitiesplasma nonlinear phenomenafusion plasma
Type
Research Article
Information
Journal of Plasma Physics , Volume 87 , Issue 3 , June 2021 , 905870311
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Copyright
Copyright © The Author(s), 2021. Published by Cambridge University Press

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