Modelling of shattered pellet injection experiments on the ASDEX Upgrade tokamak

TL;DR

This study models the impact of fragment size, speed, and composition on the effectiveness of shattered pellet injection in tokamak disruption mitigation, aligning simulation results with experimental observations.

Contribution

It introduces a 1.5D modeling approach to analyze how SPI parameters influence material assimilation during disruptions in AUG.

Findings

Smaller, faster fragments assimilate quicker for mixed deuterium-neon pellets.

Larger, faster fragments assimilate more material at the global reconnection event.

Neon content influences the amount of neon assimilated, with a self-regulating limit at higher levels.

Abstract

In a shattered pellet injection (SPI) system the penetration and assimilation of the injected material depends on the speed and size distribution of the SPI fragments. ASDEX Upgrade (AUG) was recently equipped with a flexible SPI to study the effect of these parameters on disruption mitigation efficiency. In this paper we study the impact of different parameters on SPI assimilation with the 1.5D INDEX code. Scans of fragment sizes, speeds and different pellet compositions are carried out for single SPI into AUG H-mode plasmas. We use a semi-empirical thermal quench (TQ) onset condition to study the material assimilation trends. For mixed deuterium-neon pellets, smaller/faster fragments start to assimilate quicker. However, at the expected onset of the global reconnection event (GRE),larger/faster fragments end up assimilating more material. Variations in the injected neon content lead to a large difference in the assimilated neon for neon content below $< 10^{21}$ atoms. For larger injected neon content, a self-regulating mechanism limits the variation in the amount of assimilated neon. We use a back-averaging model to simulate the plasmoid drift during pure deuterium injections with the back-averaging parameter determined by a interpretative simulation of an experimental pure deuterium injection discharge. Again, larger and faster fragments are found to lead to higher assimilation with the material assimilation limited to the plasma edge in general, due to the plasmoid drift. The trends of assimilation for varying fragment sizes, speeds and pellet composition qualitatively agree with the previously reported experimental observations.