Effect of Two-Stage Shattered Pellet Injection on Tokamak Disruptions

TL;DR

This study evaluates a two-stage deuterium-neon shattered pellet injection scheme in tokamak disruptions, demonstrating its potential to reduce runaway electrons and heat loads, with findings sensitive to plasma opacity and impurity levels.

Contribution

It introduces a novel two-stage injection approach and analyzes its effectiveness in mitigating runaway electrons and heat loads in ITER-like plasmas.

Findings

Optimal deuterium quantities depend on plasma opacity.

Two-stage injection reduces hot-tail seed and heat load.

Runaway seed sources remain challenging during nuclear operation.

Abstract

An effective disruption mitigation system in a tokamak reactor should limit the exposure of the wall to localized heat losses and to the impact of high current runaway electron beams, and avoid excessive forces on the structure. We evaluate with respect to these aspects a two-stage deuterium-neon shattered pellet injection in an ITER-like plasma, using simulations with the DREAM framework [M. Hoppe et al (2021) Comp. Phys. Commun. 268, 108098]. To minimize the obtained runaway currents an optimal range of injected deuterium quantities is found. This range is sensitive to the opacity of the plasma to Lyman radiation, which affects the ionization degree of deuterium, and thus avalanche runaway generation. The two-stage injection scheme, where dilution cooling is produced by deuterium before a radiative thermal quench caused by neon, reduces both the hot-tail seed and the localized transported heat load on the wall. However, during nuclear operation, additional runaway seed sources from the activated wall and tritium make it difficult to reach tolerably low runaway currents.