Author: Diego del-Castillo-Negrete
Requested Type: Pre-Selected Invited
Submitted: 2018-03-01 19:18:36
Co-authors: G. Zhang
Oak Ridge National Laboratory
P.O.Box 2008 MS 6169
Oak Ridge , TN 37831-2
The understanding and control of runaway electrons (RE) is a top priority of the nuclear fusion program because, if not avoided or mitigated, RE can severely damage the plasma facing components. A problem of particular interest is the computation of the RE production rate for given conditions, e.g. the plasma temperature and the electric field. Recently  we proposed a novel approach to solve this problem using the backward Monte Carlo (BMC) method. The BMC is based on the direct numerical solution of the Feynman-Kac formula that establishes a link between the solution of the adjoint Fokker-Planck problem (which gives the probability of runaway) and the stochastic differential equations describing the trajectories of RE in phase space in the presence of collisions. Computationally, the BMC is a deterministic algorithm that reduces the problem to the evaluation of Gaussian integrals that can be efficiently computed with high accuracy using Gauss-Hermite quadrature rules. Following a description of the method, we present results on the computation of the time evolution of the probability of runaway, the expected runaway time, the expected loss time, and the production rate. Going beyond the results presented in , we use a more detailed collision operator and, most importantly, we extend the application of the BMC to dynamic scenarios where the electric field and the plasma temperature exhibit time dependence. In particular, we compute the RE production rate due to hot-tail generation during a rapid drop in plasma temperature.
 G. Zhang and D. del-Castillo-Negrete, “A backward Monte-Carlo method for time-dependent runaway electron simulations” Phys. of Plasmas 24, 092511 (2017).