Modelling and validation of cryogenic pellet formation for the ITER DMS Shattered Pellet Injector

The ITER Disruption Mitigation System (DMS) plans to use large, 28.5x57 mm (d x L) Hydrogen, Neon and mixture pellets, which have not been used before at such large diameters. To study the process of the pellet fragmentation and to allow testing of key components for the ITER DMS, a support laboratory has been established as part of the ITER DMS Task Force activities at the Centre for Energy Research, Budapest, Hungary.

The pellets are produced by desublimating the gas in a cryogenically cooled barrel, and launched by a gas pressure pulse. A model has been developed for the desublimation process to predict the required cooling power and pellet formation times. It calculates conduction of heat, resulting from desublimation and gas heating at the pellet surface, to the cold head. The surface temperature and desublimation rate are calculated self-consistently, resulting in nearly two orders of magnitude drop in the desublimation rate as the pellet layer thickens. The modelled pellet formation times are close to the measured values. In the experiments often a light-scattering snow-like layer forms on the pellet surface, allowing launch at much lower propellant gas pressures than expected. The contribution discusses the above processes and prospect of pellet formation in the ITER DMS.