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Author: Christopher J. McDevitt
Requested Type: Pre-Selected Invited
Submitted: 2017-03-17 16:22:57

Co-authors: Zehua Guo, Xian-Zhu Tang

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

Abstract Text:
A detailed study of the physics responsible for the formation of an avalanche instability of runaway electrons is carried out. Such an instability arises due to the generation of secondary electrons via large-angle collisions with an existing `seed' population of energetic electrons, and is thought to be the most efficient means through which a significant fraction of the thermal current can be converted into runaway electrons during a disruption. As a means of accurately modeling this process, a set of large-angle collision operators of varying complexity, ranging from a simple source term to a novel energy-momentum conserving form, are developed and implemented. The use of a conservative form allows for the back reaction of the secondary electrons onto the primary electrons to be self-consistently accounted for. The incorporation of such a feedback process is shown to require the modification of the Coulomb logarithm in order to avoid double counting [1]. Utilizing this framework, it is found that the avalanche threshold is tightly linked to the merger of an O and X point in momentum space. Such a close correlation is shown to be largely independent of the details of the large-angle collision operator employed, and thus provides a robust indicator of an avalanche instability. Surprisingly, while the momentum space structure is strongly modified in the presence of a large-angle collision operator compared to the primary electron distribution, an O-X merger model derived from considerations of the primary electron distribution provides an excellent approximation to the avalanche threshold. This paradox is resolved by noting that near marginality the momentum space structure of the runaway population approaches a form characteristic of the primary electrons, allowing insight gained from the primary distribution to be used to characterize the avalanche threshold. This work was supported by DOE OFES.
[1] T. Fulop et al. IAEA, (2016)

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