April 15-17

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Author: Chris J McDevitt
Requested Type: Poster
Submitted: 2019-02-22 18:59:16

Co-authors: X.-Z. Tang

Contact Info:
Los Alamos National Laboratory
Los Alamos National Laboratory
Los Alamos, NM   87545
United States

Abstract Text:
The impact of tokamak geometry on runaway generation has been the subject of extensive study. The primary tool used to date to examine this problem has been bounce-averaged formulations, whereby the rapid transit or bounce motion of an electron is removed in the low collisionality limit. While the low-collisionality limit is generally well satisfied for relativistic electrons, it becomes either marginal, or even drastically, violated for electrons near the critical energy to run away under conditions characteristic of a disrupting plasma. Specifically, the large inductive electric fields possible during a tokamak disruption can result in the critical energy for an electron to run away to be reduced to hundreds of eVs. At such modest energies, electrons are instead in the high collisionality regime, thus invalidating the underlying assumptions of bounce-averaged formulations.

In this work, a drift kinetic formulation is used to describe runaway generation under such challenging scenarios. This formulation does not require the system to be in the asymptotically low collisionality limit, and thus provides a higher physics fidelity description of runaway generation. The impact of toroidal geometry on runaway generation processes is found to differ qualitatively compared to bounce-averaged predictions. In particular, for the large electric fields which may be present during a disruption, the efficiency of Dreicer production is found to increase as a function of minor radius, rather than undergo a sharp decrease as predicted by bounce-averaged formulations. In addition, the rate of avalanche amplification of a runaway population is found to be largely insensitive to the minor radius for the large electric fields characteristic of a disrupting tokamak plasma. Ongoing work is devoted toward applying these results to the self-consistent description of a disrupting plasma.

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