Lifetime and universal distribution of seed runaway electrons

Journal Article · 20 November 2017 · Physics of Plasmas

DOI: https://doi.org/10.1063/1.5001931 · OSTI ID:1540126

Fontanilla, Adrian K. ^[1]; Breizman, Boris N. ^[2]

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1. Univ. of Texas, Austin, TX (United States). Inst. for Fusion Studies; DOE/OSTI
2. Univ. of Texas, Austin, TX (United States). Inst. for Fusion Studies

The lifetime of pre-existing runaway electrons determines how likely the runaways will undergo avalanche multiplication. We estimate the lifetime of runaway electrons via kinetic analysis. We show that the rate of runaway decay depends on the combination of parameters α≡(Z+1)/$$\sqrt{{\bar{τ}}_{rad}}$$ (where $$τ_{rad}$$ is the synchrotron timescale normalized to the collisional timescale and Z is the ion charge) compared to the electric field. We identify two cases where the decay rate is slow enough to enable a quasi-steady shape of the runaway distribution function. This distribution and its lifetime represent the eigenfunction and the lowest eigenvalue of the kinetic equation. In one case, α$$\ll$$1: the field required to sustain the pre-existing runaways is barely larger than the Connor-Hastie critical value. In the same manner as by Aleynikov and Breizman [Phys. Rev. Lett. 114, 155001 (2015)], we solve the kinetic equation perturbatively but extend the work to demonstrate that the lifetime grows exponentially with the field at a rate that depends on α. Finally, in the second case, α$$\gg$$1: the sustainment field is much greater than the Connor-Hastie value, and the largeness of the field in this case enables us to universalize the kinetic equation via the re-scaling procedure.

• Research Organization: Univ. of Texas, Austin, TX (United States)
• Sponsoring Organization: USDOE Office of Science (SC)
• Grant/Contract Number: FG02-04ER54742; SC0016283
• OSTI ID: 1540126
• Alternate ID(s): OSTI ID: 1420640
• Journal Information: Physics of Plasmas, Journal Name: Physics of Plasmas Journal Issue: 11 Vol. 24; ISSN 1070-664X
• Publisher: American Institute of Physics (AIP)Copyright Statement
• Country of Publication: United States
• Language: English