Modelling of shattered pellet injection experiments on the ASDEX Upgrade tokamak

TL;DR

This study uses simulations and experiments on the ASDEX Upgrade tokamak to analyze how pellet fragment size, speed, and composition affect disruption mitigation effectiveness, providing insights for ITER's DMS design.

Contribution

It introduces a detailed simulation approach to study SPI parameters and compares results with experimental data, highlighting key factors influencing material assimilation during disruptions.

Findings

Larger and faster fragments increase material assimilation.

Neon fraction saturation limits neon assimilation.

Simulation results align qualitatively with experiments.

Abstract

A disruption mitigation system (DMS) is necessary for fusion-grade tokamaks like ITER in order to ensure the preservation of machine components throughout their designated operational lifespan. To address the intense heat and electromagnetic loads that occur during a disruption, a shattered pellet injection (SPI) system will be employed. The penetration and assimilation (ionized material that stays inside the plasma volume) of the injected material is influenced by various SPI parameters, including the fragment sizes, speeds, and composition of the shattered fragments. An SPI system was installed on the ASDEX Upgrade tokamak to study the effect of the aforementioned parameters. In this thesis, 1.5D simulations with the INDEX code have been utilised to conduct parametric scans, thus examining the influence of fragment sizes, velocities, and pellet composition on the efficacy of disruption mitigation. When injecting only deuterium, I found material assimilation to be limited to the edge of the plasma with larger and faster fragments leading to higher assimilation. For mixed deuterium/neon injections, again, larger and faster fragments enabled higher assimilation. The amount of assimilated neon increased with increasing injected neon amounts but saturated for larger neon fraction pellets. I also carried out comparisons with previous experimental results of penetration, material assimilation and pre-TQ duration. Previous experimental results for pure deuterium injections indicated that larger and faster fragments exhibit greater penetration, aligning with findings from the simulations. Simulated material assimilation trends for pure deuterium injections were also found to be qualitatively similar to the experiments. Nonetheless, a major difference in the quantitative assimilation values was identified, likely associated with the experimental assimilation criterion.