Feasibility study for cryogenic pellets production with pure gaseous helium cooling for ITER Disruption Mitigation System

CEA/DSBT designed and operated a test bench for pellet injection cooled by liquid helium (LHe) to study the production and the acceleration of large cryogenic pellets for the ITER Disruption Mitigation System (DMS). Large (ø28.5 mm) protium (¹H) pellets of ∼3 g were successfully formed in 20 mins and then accelerated above 500 m/s. However, LHe cooling is not distributed by ITER cryoplant and shall be replaced by supercritical helium (SHe) cooling.

To evaluate the feasibility of using SHe as a coolant, the CEA/DSBT test bench is modified to operate with pure gaseous helium cooling (GHe at 1.25 bara), which behave more like SHe than LHe used up to now. The main difference between GHe and LHe cooling is the temperature variation induced by the thermal heat removal. In LHe cooling, the heat exchange occurs mainly at a constant temperature due to the liquid to gas phase change (latent heat). In the opposite, in GHe cooling, only the specific heat is available for heat removal, resulting in a cooling temperature increase. Excessive temperature rise is a drawback to the goal of reduced pellet formation times, where the cell temperature must be kept as low as possible.

This paper presents a comparison of LHe and GHe cooling using the current ∅28.5 mm in-situ condensation cell and highlights some pellet formation key parameters such as the species (pure protium and pure neon or protium/neon mix) and the cold cell temperature. The protium pellets were the most extensively compared, as they are the fastest to produce and the most characterised with LHe cooling. All other types of DMS pellets were also briefly studied. The GHe cooling results show similar pellet formation durations, pellet aspects and speeds as those with LHe cooling. This proof of principle using gaseous cooling is a significant step in the design study of the ITER-DMS cold cell using SHe cooling.