Sherwood 2015

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approvedrunaway_liu.pdf2015-04-07 15:02:20Chang Liu
approvedsherwood_abstract.pdf2015-01-19 13:39:20Chang Liu

Runaway electron distribution functions in momentum space with the synchrotron radiation effect

Author: Chang Liu
Requested Type: Poster Only
Submitted: 2015-01-19 13:37:36

Co-authors: Dylan Brennan, Allen Boozer, Amitava Bhattacharjee

Contact Info:
Princeton Plasma Physics Laboratory
100 Stellarator Rd
Princeton, NJ   08540

Abstract Text:
Runaway electron (RE) physics is an important aspect of the disruption studies and a critical area for current research. Recently there have been several experiments dedicated to RE generation, and the results seem to indicate that the critical electric field exceeds the Connor-Hastie critical electric field E_c by at least a factor of three. [1] The important physics effects, in addition to the Coulomb drag force, are synchrotron radiation, pitch-angle scattering, and the avalanche. In this study, we focus at first on the effect of synchrotron radiation. In the classical RE theory when E is less than E_c, RE will lose energy until they join the Maxwellian distribution. The synchrotron radiation of the electrons can produce a back-reaction force on RE and cause energy loss. For E/E_c on the order of but larger than unity, the back-reaction force is comparable to the electric field and the collisional drag. Therefore RE will be subject to an energy limit and the distribution will finally reach a steady state. Here we present a calculation of the steady state RE distribution including the synchrotron radiation effects and quantitatively compare to related work on this subject. [2, 3] The numerical methods follow [4], with the addition of a radiation force into the kinetic equation. At present we only consider the Dreicer RE generation and don’t include the secondary RE generation. The results show that there exists a higher critical electric field E_s. When E<E_s, the steady state RE tail decays exponentially and the total number of RE is very small. When E>E_s, the RE distribution will form a bump-on-tail before exhibiting exponential decay. The steady state distribution shows energy balance between the electric field, the drag force, and the radiation back-reaction force. This result is also quantitatively consistent with a test-particle model in which the various forces will form attractors in the momentum space [2]. We also found that the RE growth rate is strongly decreased by the radiation force when E/E_c is not much greater than 1. These results can help explain the experimental results [1] that the critical electric field found at which the RE signal starts to decay is several times larger than E_c.

[1] R.S. Granetz et al., Phys. Plasmas 21, 072506 (2014).
[2] J.R. Martín-Solís et al., Phys. Plasmas 5, 2370 (1998).
[3] F. Andersson et al., Phys. Plasmas 8, 5221 (2001).
[4] M. Landreman et al., Comp. Phys. Comm. 185, 847 (2014).


March 16-18, 2015
The Courant Institute, New York University