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Title: Phase-space dynamics of runaway electrons in magnetic fields

Abstract

Dynamics of runaway electrons in magnetic fields are governed by the competition of three dominant physics: parallel electric field acceleration, Coulomb collision, and synchrotron radiation. Examination of the energy and pitch-angle flows reveals that the presence of local vortex structure and global circulation is crucial to the saturation of primary runaway electrons. Models for the vortex structure, which has an O-point to X-point connection, and the bump of runaway electron distribution in energy space have been developed and compared against the simulation data. Lastly, identification of these velocity-space structures opens a new venue to re-examine the conventional understanding of runaway electron dynamics in magnetic fields.

Authors:
ORCiD logo [1]; ORCiD logo [1]; ORCiD logo [1]
  1. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Publication Date:
Research Org.:
Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
Sponsoring Org.:
USDOE Laboratory Directed Research and Development (LDRD) Program; USDOE Office of Science (SC), Fusion Energy Sciences (FES) (SC-24)
OSTI Identifier:
1364543
Report Number(s):
LA-UR-16-26162
Journal ID: ISSN 0741-3335
Grant/Contract Number:
AC52-06NA25396
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Plasma Physics and Controlled Fusion
Additional Journal Information:
Journal Volume: 59; Journal Issue: 4; Journal ID: ISSN 0741-3335
Publisher:
IOP Science
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; Magnetic Fusion Energy; runaway electron; synchrotron radiation; phase space

Citation Formats

Guo, Zehua, McDevitt, Christopher Joseph, and Tang, Xian-Zhu. Phase-space dynamics of runaway electrons in magnetic fields. United States: N. p., 2017. Web. doi:10.1088/1361-6587/aa5952.
Guo, Zehua, McDevitt, Christopher Joseph, & Tang, Xian-Zhu. Phase-space dynamics of runaway electrons in magnetic fields. United States. doi:10.1088/1361-6587/aa5952.
Guo, Zehua, McDevitt, Christopher Joseph, and Tang, Xian-Zhu. Thu . "Phase-space dynamics of runaway electrons in magnetic fields". United States. doi:10.1088/1361-6587/aa5952. https://www.osti.gov/servlets/purl/1364543.
@article{osti_1364543,
title = {Phase-space dynamics of runaway electrons in magnetic fields},
author = {Guo, Zehua and McDevitt, Christopher Joseph and Tang, Xian-Zhu},
abstractNote = {Dynamics of runaway electrons in magnetic fields are governed by the competition of three dominant physics: parallel electric field acceleration, Coulomb collision, and synchrotron radiation. Examination of the energy and pitch-angle flows reveals that the presence of local vortex structure and global circulation is crucial to the saturation of primary runaway electrons. Models for the vortex structure, which has an O-point to X-point connection, and the bump of runaway electron distribution in energy space have been developed and compared against the simulation data. Lastly, identification of these velocity-space structures opens a new venue to re-examine the conventional understanding of runaway electron dynamics in magnetic fields.},
doi = {10.1088/1361-6587/aa5952},
journal = {Plasma Physics and Controlled Fusion},
number = 4,
volume = 59,
place = {United States},
year = {Thu Feb 16 00:00:00 EST 2017},
month = {Thu Feb 16 00:00:00 EST 2017}
}

Journal Article:
Free Publicly Available Full Text
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  • In this paper, the secular full-orbit simulations of runaway electrons with synchrotron radiation in tokamak fields are carried out using a relativistic volume-preserving algorithm. Detailed phase-space behaviors of runaway electrons are investigated in different dynamical timescales spanning 11 orders. In the small timescale, i.e., the characteristic timescale imposed by Lorentz force, the severely deformed helical trajectory of energetic runaway electron is witnessed. A qualitative analysis of the neoclassical scattering, a kind of collisionless pitch-angle scattering phenomena, is provided when considering the coupling between the rotation of momentum vector and the background magnetic field. In large timescale up to 1 s,more » it is found that the initial condition of runaway electrons in phase space globally influences the pitch-angle scattering, the momentum evolution, and the loss-gain ratio of runaway energy evidently. However, the initial value has little impact on the synchrotron energy limit. It is also discovered that the parameters of tokamak device, such as the toroidal magnetic field, the loop voltage, the safety factor profile, and the major radius, can modify the synchrotron energy limit and the strength of neoclassical scattering. The maximum runaway energy is also proved to be lower than the synchrotron limit when the magnetic field ripple is considered.« less
  • The phase-space dynamics of runaway electrons is studied, including the influence of loop voltage, radiation damping, and collisions. A theoretical model and a numerical algorithm for the runaway dynamics in phase space are developed. Instead of standard integrators, such as the Runge-Kutta method, a variational symplectic integrator is applied to simulate the long-term dynamics of a runaway electron. The variational symplectic integrator is able to globally bound the numerical error for arbitrary number of time-steps, and thus accurately track the runaway trajectory in phase space. Simulation results show that the circulating orbits of runaway electrons drift outward toward the wall,more » which is consistent with experimental observations. The physics of the outward drift is analyzed. It is found that the outward drift is caused by the imbalance between the increase of mechanical angular momentum and the input of toroidal angular momentum due to the parallel acceleration. An analytical expression of the outward drift velocity is derived. The knowledge of trajectory of runaway electrons in configuration space sheds light on how the electrons hit the first wall, and thus provides clues for possible remedies.« less
  • In a recent work [J. R. Martin-Solis and R. Sanchez, Phys. Plasmas 13, 012508 (2006)], the increase that the presence of stochastic magnetic fields causes on the synchrotron radiation losses of relativistic runaway electrons was quantified using a guiding-center approximation. Here, we complete those studies by considering instead the mechanism which dominates the interaction at the gyromotion level. It is shown that, under typical tokamak conditions, the resonant cyclotron interaction with high enough parallel (to the magnetic field) wave numbers (k{parallel}) modes can create, even for moderate magnetic fluctuation levels, an upper bound on the runaway energy. Implications for disruption-generatedmore » runaway electrons will be also discussed.« less