Simulating the nonlinear interaction of relativistic electrons and tokamak plasma instabilities: Implementation and validation of a fluid model

Abstract

For the simulation of disruptions in tokamak fusion plasmas, a fluid model describing the evolution of relativistic runaway electrons and their interaction with the background plasma is presented. The overall aim of the model is to self-consistently describe the non-linear coupled evolution of runaway electrons (REs) and plasma instabilities during disruptions. In this model, the runaway electrons are considered as a separate fluid species in which the initial seed is generated through the Dreicer source, that eventually grows by the avalanche mechanism (further relevant source mechanisms can easily be added). Advection of the runaway electrons is considered primarily along field lines, but also taking into account the E × B drift. The model is implemented in the non-linear magnetohydrodynamic (MHD) code JOREK based on Bezier finite-elements, with current-coupling to the thermal plasma. Benchmarking of the code with the one-dimensional runaway electron code GO is done using an artificial thermal quench on a circular plasma. As a first demonstration, the code is applied to the problem of an axisymmetric cold vertical displacement event in an ITER plasma, revealing significantly different dynamics between cases computed with and without runaway electrons. Though it is not yet feasible to achieve fully realistic runaway electron velocities close to the speed of light in complete simulations of slowly evolving plasma instabilities, the code is demostrated to be suitable to study various kinds of MHD-RE interaction in MHD-active and disruption relevant plasmas.

First predictive simulations for deuterium shattered pellet injection in ASDEX Upgrade

“…Using the free boundary JOREK-STARWALL code [32], also vertical stability and control will be taken into account and runaway electron dynamics will be studied using the models described in Refs. [13,14]. Numerical improvements which are presently ongoing, are expected to reduce the computational costs and enhance numerical stability at fully realistic parameters.…”

“…Previous studies for massive gas and shattered pellet injection [7][8][9][10][11][12] demonstrate some of the capabilities. The code allows also to study the dynamics of runaway electrons [13,14] and vertical displacement events [15,16] which are closely related to (mitigated) disruptions. In the present article, only a small subset of the code features is invoked to perform first predictive simulations for deuterium SPI into an ASDEX Upgrade H-mode plasma.…”

Evaluation of the Dreicer runaway generation rate in the presence of high- impurities using a neural network

“…In this scenario, we observed that the plateau runaway current was significantly reduced when using the neural network instead of the Connor-Hastie formula, which demonstrates the need to account for partially ionized atoms for realistic modelling of Dreicer generation. The neural network presented here can therefore be useful to improve runaway-electron modelling in tokamak simulation codes such as astra (Fable et al 2016), jorek (Bandaru et al 2019), and ets (Pokol et al 2019…”

Propagation characteristics of parallel propagating waves in a relativistic magnetized electron plasma

“…Magnetized relativistic environments are known to exist in different situations like in active galactic nuclei (jets produced by the rotation of heavy black holes) (Petropoulou et al 2019), other astrophysical objects (such as neutron stars, pulsars, micro-quasars, etc.) (Beskin et al 1988;Chabrier et al 2002;Melrose 2017) and Van Allen radiation belts (Horne et al 2005), and also have primary importance in tokamak plasmas (Bandaru et al 2019) and laser-plasma interactions (Banerjee et al 2002).…”