Production rate of runaway electrons in dynamic scenarios: a probabilistic backward Monte-Carlo method

The computation of the production rate of runaway electrons (RE) is important because, if not avoided or mitigated, RE can severely damage the plasma facing components. Recently, we proposed a novel approach to solve this problem using the backward Monte-Carlo (BMC) method [1]. The BMC is based on the Feynman-Kac formula that establishes a link between the adjoint Fokker-Planck problem (which gives the probability of runaway) and the stochastic differential equations describing the trajectories of RE in the presence of collisions. Computationally, the BMC is a deterministic algorithm that reduces the problem to the evaluation of Gaussian integrals 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 [1] we use a more detailed collision operator and extend 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.

[1] Zhang and del-Castillo-Negrete, Phys. of Plasmas 24, 092511 (2017).