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

## TL;DR

This paper presents a new fluid model integrated into the JOREK MHD code to simulate the nonlinear interaction between relativistic runaway electrons and plasma instabilities during tokamak disruptions, validated through benchmarking and applied to ITER scenarios.

## Contribution

The paper introduces a self-consistent fluid model for runaway electrons coupled with MHD simulations, implemented in JOREK, and demonstrates its application to disruption scenarios in tokamaks.

## Key findings

- The model successfully simulates runaway electron dynamics during disruptions.
- Benchmarking shows good agreement with existing 1D runaway electron code GO.
- Application to ITER-like plasma reveals different behaviors with and without runaway electrons.

## 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 nonlinear 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, which 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 ExB drift. The model is implemented in the nonlinear 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 demonstrated to be suitable to study various kinds of MHD-RE interactions in MHD-active and disruption relevant plasmas.