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Stabilisation of short-wavelength instabilities by parallel-to-the-field shear in long-wavelength E × B flows

Part of: Focus on Fusion

Published online by Cambridge University Press:  09 November 2020

M. R. Hardman Open the ORCID record for M. R. Hardman [Opens in a new window] ,
M. Barnes Open the ORCID record for M. Barnes [Opens in a new window]  and
C. M. Roach Open the ORCID record for C. M. Roach [Opens in a new window]
M. R. Hardman*
Affiliation:
Rudolf Peierls Centre for Theoretical Physics, University of Oxford, OxfordOX1 3PU, UK Culham Centre for Fusion Energy, UKAEA, AbingdonOX14 3DB, UK
M. Barnes
Affiliation:
Rudolf Peierls Centre for Theoretical Physics, University of Oxford, OxfordOX1 3PU, UK
C. M. Roach
Affiliation:
Culham Centre for Fusion Energy, UKAEA, AbingdonOX14 3DB, UK
*
†Email address for correspondence: [email protected]
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Abstract

Magnetised plasma turbulence can have a multiscale character: instabilities driven by mean temperature gradients drive turbulence at the disparate scales of the ion and the electron gyroradii. Simulations of multiscale turbulence, using equations valid in the limit of infinite scale separation, reveal novel cross-scale interaction mechanisms in these plasmas. In the case that both long-wavelength (ion-gyroradius-scale) and short-wavelength (electron-gyroradius-scale) linear instabilities are driven far from marginal stability, we show that the short-wavelength instabilities are suppressed by interactions with long-wavelength turbulence. Two novel effects contributed to the suppression: parallel-to-the-field-line shearing by the long-wavelength ${{\boldsymbol {E}} \times \boldsymbol {B}}$ flows, and the modification of the background density gradient by the piece of the long-wavelength electron adiabatic response with parallel-to-the-field-line variation. In contrast, simulations of multiscale turbulence where instabilities at both scales are driven near marginal stability demonstrate that when the long-wavelength turbulence is sufficiently collisional and zonally dominated the effect of cross-scale interaction can be parameterised solely in terms of the local modifications to the mean density and temperature gradients. We discuss physical arguments that qualitatively explain how a change in equilibrium drive leads to the observed transition in the impact of the cross-scale interactions.

Keywords

plasma nonlinear phenomenaplasma simulationfusion plasma
Type
Research Article
Information
Journal of Plasma Physics , Volume 86 , Issue 6 , December 2020 , 905860601
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Copyright
Copyright © The Author(s), 2020. Published by Cambridge University Press

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