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Drift kinetic theory of alpha transport by tokamak perturbations

Part of: Invited Contributions from the 18th European Fusion Theory Conference Focus on Fusion

Published online by Cambridge University Press:  08 March 2021

Elizabeth A. Tolman Open the ORCID record for Elizabeth A. Tolman [Opens in a new window]  and
Peter J. Catto Open the ORCID record for Peter J. Catto [Opens in a new window]
Elizabeth A. Tolman*
Affiliation:
Plasma Science and Fusion Center, Massachusetts Institute of Technology, Cambridge, MA02139, USA Institute for Advanced Study, Princeton, NJ08540, USA
Peter J. Catto
Affiliation:
Plasma Science and Fusion Center, Massachusetts Institute of Technology, Cambridge, MA02139, USA
*
†Email address for correspondence: tolman@ias.edu
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Abstract

Upcoming tokamak experiments fuelled with deuterium and tritium are expected to have large alpha particle populations. Such experiments motivate new attention to the theory of alpha particle confinement and transport. A key topic is the interaction of alpha particles with perturbations to the tokamak fields, including those from ripple and magnetohydrodynamic modes like Alfvén eigenmodes. These perturbations can transport alphas, leading to changed localization of alpha heating, loss of alpha power and damage to device walls. Alpha interaction with these perturbations is often studied with single-particle theory. In contrast, we derive a drift kinetic theory to calculate the alpha heat flux resulting from arbitrary perturbation frequency and periodicity (provided these can be studied drift kinetically). Novel features of the theory include the retention of a large effective collision frequency resulting from the resonant alpha collisional boundary layer, correlated interactions over many poloidal transits and finite orbit effects. Heat fluxes are considered for the example cases of ripple and the toroidal Alfvén eigenmode (TAE). The ripple heat flux is small. The TAE heat flux is significant and scales with the square of the perturbation amplitude, allowing the derivation of constraints on mode amplitude for avoidance of significant alpha depletion. A simple saturation condition suggests that TAEs in one upcoming experiment will not cause significant alpha transport via the mechanisms in this theory. However, saturation above the level suggested by the simple condition, but within numerical and experimental experience, which could be accompanied by the onset of stochasticity, could cause significant transport.

Keywords

fusion plasmaplasma wavesplasma instabilities
Type
Research Article
Information
Journal of Plasma Physics , Volume 87 , Issue 2 , April 2021 , 855870201
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Copyright
Copyright © The Author(s), 2021. Published by Cambridge University Press

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