Recent parameter scans of Shattered Pellet Injection (SPI) thermal quench simulations with the 3D extended MHD code NIMROD have revealed that the perpendicular thermal conduction is the key parameter that controls the level of tearing mode activity. Large amplitude tearing modes during the thermal quench simulations are often the cause of convergence errors; typically the anisotropic thermal conduction struggles through growing stochasticity. Reduced values of the perpendicular thermal conduction preserves the core temperature longer, maintaining a lower resistivity, reducing the tearing drive and demands on the anisotropic thermal conduction solvers. This improvement along with others have allowed for dual injector SPI simulations that demonstrate the benefits of multiple injectors, significantly reducing the toroidal n=1 mode dominance activity displayed in single-injector NIMROD simulations, and resulting in a more benign thermal quench.

The 2019 SciDAC CTTS (Center for Tokamak Transient Simulations) Coding Camp was held at General Atomics from June 18-20. The attendees were: Roman Samulyak and Nicolas Bosviel from Stonybrook University, Lang Lao, Paul Parks, Brendan Lyons, Joseph McClenaghan, Yueqiang Liu, and Wen Wu from General Atomics, Charlson Kim from SLS2 Consulting, and Jie Zhang from USTC. Bob Harvey of CompX and Nizar Naitlho and James Yuan from Stonybrook University attended remotely. The several days were spent in lively discussion of pellet ablation models and how best to couple the mesoscale Lagrangian particle ablation code with the global extended MHD codes NIMROD and M3D-C1. Most significantly, the Coding Camp was able to resolve an outstanding discrepancy in the determination of which specific Lagrangian particles are passed to the global MHD codes. Previously, particles were chosen using an arbitrary temperature criterion. The new method uses a physically intuitive criterion; particles that drift off the ablation channel are passed to the global MHD codes. This eliminates issues of double counting and reduces the potential for inconsistencies growing between the two codes. Coupling the global MHD codes to the mesoscale Lagrangian particle ablation code will provide more accurate ablation physics that would be otherwise be computationally intractable with a global code alone.

Gary Staebler visited MAST-U at the Culham Centre for Fusion Energy May 13-24 to work on a study of the saturated fluctuation spectrum of electron turbulence simulations in order to understand the role of zonal flows. These simulations were made for MAST conditions by Greg Colyer and Colin Roach. They have a complex mix of equilibrium ExB shear, fluctuating broadband zonal flows and static zonal potential corrugations. The relative strength of the fluctuating and static zonal flows changes with electron collisions. At low collision rate the static zonal mode dominates but at high enough collision rate the fluctuating zonal flows are dominant and the turbulence intensity is high. The goal of this analysis is to generalize the model of the saturated potential fluctuation spectrum that is used in the TGLF quasilinear transport code.