Disruption Mitigation in Tokamaks with Massive Material Injection

Doctoral thesis, 2025

The sudden loss of confinement of the energy content of fusion plasmas in off-normal events, called disruptions, is 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 initiate a massive material injection, primarily in the form of a shattered cryogenic pellet, when an emerging disruption is detected, and so attempt to better control the plasma cooling.

In this thesis, we develop modeling tools for the various physical phenomena present during a tokamak disruption mitigated by a massive material injection. This includes extending existing numerical tools with the capability to handle effects of partial ionization in the cooling plasma on the generation of runaway electrons, and the material assimilation in the plasma following a shattered pellet injection. It also includes the implementation of a method for estimating the losses of runaway electrons due to the vertical motion of the plasma, and the resulting scrape-off against the wall. These tools are then used to simulate a wide range of disruption mitigation scenarios in reactor-scale tokamak devices. Finally, we also develop an analytical model for the radial transport of the relatively cold and dense material recently ablated from a shattered pellet upon exposure to the hot plasma, and the acceleration of the pellet shards due to asymmetries in the ablation.

We find that the severity of a disruption in a reactor-scale device can be significantly reduced by a carefully chosen injection scheme. In particular, a two-stage injection might efficiently reduce the localised heat loads and the runaway generation due to the hot-tail mechanism, by allowing for an intermediate equilibration of the superthermal electron population between the injections. However, the strong runaway avalanche associated with a high plasma current was found to be able to amplify even a very small runaway seed, such as those produced by tritium decay and Compton scattering during nuclear operation, to several mega-amperes. With scrape-off losses included, plausible scenarios with a successful runaway mitigation exist, but the results are sensitive to the details of the evolution of the current density profile. This mitigation scheme might also be further complicated by a relatively large shard deceleration and outward drift of the recently ablated pellet material.