Gas flow and dust in Shattered Pellet Injection

The ITER Disruption Mitigation System (DMS) utilizes Shattered Pellet Injection (SPI) technology for the injection of a high amount of cryogenic material into the plasma. The ITER DMS Support Laboratory at the HUN-REN Centre for Energy Research studies pellet formation, launch and shattering using ITER-size pellet. The aim of this paper is to study the propagation of cryogenic dust and propellant gas around the pellet. Any propellant gas or debris getting into the plasma ahead of the shattered pellet might compromise the mitigation efficiency due to reduced material assimilation caused by premature cooling of the plasma. With a schlieren diagnostic, a solid hydrogen dust cloud and debris was observed to arrive before the pellet, in addition to shock waves, which is a direct evidence of propellant gas flow. A supersonic Mach-number of the gas flow was be calculated from the shockwaves. Dimension reduction on the motion picture data can depict the entire launch process in one panorama image, which can be further reduced to a single time series for each pellet launch, allowing statistical analysis on two types of pellets, produced with different freezing recipes. The time evolution of pellet launches shows a general behavior regardless of pellet velocity, material, and freezing recipe. Less debris arrives before pellets prepared with lower initial barrel pressures. The experimental proof of the gas flow, dust and debris ahead of the pellet delivered critical information for the ITER DMS, highlighting the capabilities of schlieren diagnostic.