Disruption Mitigation in Tokamaks with Shattered Pellet Injection

Abstract

The sudden loss of confinement of the energy content of fusion plasmas in off-normal events, called disruptions, are among the most severe threats to the future of fusion energy based on the tokamak design. An efficient disruption mitigation system will therefore be of utmost importance for future large, high-current devices such as ITER. The potentially greatest threat to be mitigated is posed by currents carried by highly energetic electrons, called runaway electrons, which may cause severe damage upon wall impact. The disruption mitigation system must also ensure a sufficiently homogeneous deposition of the thermal energy on the plasma-facing components, and avoid excessive forces on the machine due to currents flowing in the surrounding structures. The currently envisaged mitigation method is to make a massive material injection when an emerging disruption is detected, attempting to better control the plasma cooling and energy dissipation. In this work, we perform numerical simulations assessing the performance of the most up to date mitigation schemes based on shattered pellet injections in an ITER-like setting, with a particular focus on the generation of runaway electrons. The main mitigation scheme investigated is a two-stage shattered pellet injection, with a diluting deuterium injection followed by a neon injection aiming to radiatively dissipate the plasma energy content. Our studies indicate that the diluting deuterium injection can efficiently reduce the runaway generation due to the hot-tail mechanism, by allowing for an intermediate equilibration of the superthermal electron population between the injections. The fraction of the initial thermal energy content conducted to the plasma-facing components is also reduced compared to a single-stage injection with the same composition, reducing the localised heat loads. During non-nuclear operation, the maximum runaway current was found to be reduced to acceptable levels with realistic two-stage injection parameters. On the other hand, during nuclear operation, the unavoidable runaway seed from tritium decay and compton scattering was found to be amplified to several mega-amperes by the avalanche mechanism for all investigated injection parameters. The reason is that the intense cooling from the injected material leads to a high induced electric field and a substantial recombination, resulting in an enhanced avalanche multiplication.