Introduction of a 3D global non-linear full-f particle-in-cell model for runaway electrons in JOREK

Abstract Disruptions present one of the leading concerns for reliable tokamak operation. The acceleration of electrons from the thermal bulk to relativistic energies, so-called runaway electron (RE) generation, is in particular a problem for future high current machines such as ITER. Accurately predicting the generation and impact of REs is therefore essential for making informed decisions concerning machine design and the use of disruption mitigation systems. This requires high-fidelity modeling also accounting for the large MHD activity observed throughout disruptions, which is made especially difficult by the mutual coupling between REs and the companion plasma. The non-linear 3D extended MHD code JOREK is a powerful tool for studying disruption and RE physics. This work details recent developments in JOREK, introducing a hybrid fluid-kinetic model where the REs are modeled kinetically and coupled to the non-linear MHD equations using a full-f particle-in-cell approach. The model goes beyond the state of the art and can accurately capture phase space distributions and dynamics of REs, drift orbits, and transport and losses caused by stochastic fields. Benchmarks are presented for both 2D and 3D configurations, concerning the impact of REs on the force balance and linear tearing mode growth rates, where a good agreement with analytically derived results is found. In addition, a demonstration of a particularly complicated non-linear application with high relevance to large machines is made, namely a RE benign termination linked to a violent burst of MHD activity.