Modeling the spatial dependence of runaway electron generation during disruptions

This work investigates the spatial dependence of runaway electron (RE) generation during modeled shattered pellet injection (SPI) induced disruptions. We employ the Kinetic Orbit Runaway electrons Code (KORC) to evolve the RE distribution function by simulating tracer particles under prescribed fields from the extended-magnetohydrodynamic code NIMROD. While REs can potentially damage plasma-facing components in reactor-scale tokamaks, SPI technology is being developed as the mainline disruption mitigation strategy for ITER. However, results from SPI testing in the DIII-D, JET, KSTAR, and ASDEX-U tokamaks need predictive modeling to scale up to ITER. In this study, we consider the two dominant mechanisms of RE generation: the Dreicer source, where thermal electrons are accelerated by the large electric field induced during a disruption, and the avalanche source comprised of large-angle, knock-on collisions by seed REs on the thermal population. A detailed analysis considering the interplay between collisional and particle trapping effects will be carried out to understand RE generation mechanisms. RE generation will first be explored in validated simulations of DIII-D SPI experiments as a stepping-stone to eventual JET simulations of the ongoing experimental campaign.