Shattered Pellet Injection Simulations With NIMROD*

Using the newly developed Particle-in-Cell (PiC) based Shattered Pellet Injection (SPI) model in the NIMROD code, nonlinear 3D extended MHD initial value SPI driven thermal quench simulations show that MHD activity triggered by injection of impurity fragments strongly impacts the thermal quench dynamics and alters the mixing and radiation efficiency and toroidal peaking factor.  Given the critical nature of the Disruption Mitigation System (DMS) in the operation of ITER, accurate simulations are essential to developing Disruption Mitigation Strategies for ITER.  SPI is the primary candidate for ITER's DMS.  Building upon prior work on Massive Gas Injection (MGI) simulations by V. A. Izzo (Nuclear Fusion 55, 073032 (2015)), we have developed a PiC based model to mimic the fragment plume of the injected shattered pellet.  An analytic expression has been developed to model the ablation of the frozen impurity mixture assuming each fragment is a isolated uniform sphere.  This discrete PiC based SPI plume model captures the essential features of the plume front shielding the fragments that follow and offers the flexibility to conveniently vary trajectory, size, and distribution of the fragments that make up the shattered plume.  D3D and ITER SPI simulations indicate that the thermal quench proceeds in two phases. Initially, the outer plasma is shed via interchange-like filaments while preserving the core temperature.  This results in a steep gradient and  triggers the second phase of the thermal quench, an external kink-like event that collapses the core. Based on these simulations we will comment on advanced mitigation scenarios such as multiple SPI injectors and the prospects for the ITER DMS.