# Test particles dynamics in the JOREK 3D non-linear MHD code and application to electron transport in a disruption simulation

## TL;DR

This paper introduces a test particle tracker in the JOREK 3D non-linear MHD code to study electron confinement during tokamak disruptions, revealing that some electrons survive due to magnetic surface reformation.

## Contribution

A novel test particle module in JOREK enables detailed electron orbit analysis during disruptions, highlighting mechanisms of electron survival.

## Key findings

- Electrons are less deconfined at higher energies, especially at 10MeV.
- Magnetic surface reformation at the core aids electron confinement.
- Electron survival rates decrease with increasing energy.

## Abstract

In order to contribute to the understanding of runaway electron generation mechanisms during tokamak disruptions, a test particle tracker is introduced in the JOREK 3D non-linear MHD code, able to compute both full and guiding center relativistic orbits. Tests of the module show good conservation of the invariants of motion and consistency between full orbit and guiding center solutions. A first application is presented where test electron confinement properties in a JET massive gas injection-triggered disruption simulation are investigated. It is found that electron populations initialised before the thermal quench (TQ) are typically not fully deconfined in spite of the global stochasticity of the magnetic field during the TQ. The fraction of "survivors" decreases from few tens down to a few tenths of percent as the electron energy varies from 1keV to 10MeV. The underlying mechanism for electron "survival" is the prompt reformation of closed magnetic surfaces at the plasma core and, to a smaller extent, the subsequent reappearance of a magnetic surface at the edge. It is also found that electrons are less deconfined at 10MeV than at 1MeV which appears consistent with a phase averaging effect due to orbit shifts at high energy.