Theory and modelling activities in support of the ITER disruption mitigation system

In 2018, an international Task Force (TF) dedicated to the ITER Disruption Mitigation System (DMS) has been formed. The present paper summarizes Theory and Modelling (T&M) activities performed within this TF. The ITER DMS is planned to rely on Shattered Pellet Injection (SPI). The most critical issue, at present, is that of Runaway Electrons (REs). Indeed, recent T&M work suggests that previously envisaged means of RE avoidance or mitigation may prove inefficient. However, RE simulations have not yet taken into account the possibly beneficial role of magnetohydrodynamic (MHD) instabilities. Furthermore, new promising schemes for RE avoidance or mitigation are being investigated. These comprise, among others: 1) a prompt plasma dilution before the disruption by means of pure H2 SPI in order to suppress hot tail RE generation, and 2) should a RE beam form, a "cleanout" of the beam's companion plasma, via (again) pure H2 SPI into the beam, which may lead to a benign beam termination. The paper also discusses the status of 3D MHD modelling. The JOREK, M3D-C 1 and NIMROD codes have been extended in recent years to be able to simulate SPI. Good cooperation exists between