Preliminary experimental tests on the cryogenic pellet injection technology for the DTT machine in collaboration with ORNL

In a tokamak, cryogenic pellets are employed both for fueling and disruption mitigation. The pellet injection systems are under definition for the Italian Divertor Tokamak Test (DTT), for both purposes. For fueling, DTT is evaluating High-Field Side injection, utilizing a curved guide tube from the upper section of the first wall. For the full-power discharges planned for DTT, Shattered-Pellet Injector (SPI) is being considered as an alternative disruption mitigation system. A Two-Stage Gas Gun (TSG) is under evaluation as one of the pellet acceleration techniques for this purpose. In June 2024, two experimental campaigns were organized at Oak Ridge National Laboratories (ORNL) to provide a preliminary characterization of both cryogenic pellet injection systems for DTT. The first set of experiments focused on evaluating the injection strategy for fueling pellets. A single stage gas gun was employed as the driving configuration. Multiple dedicated pellet shots were conducted to assess the maximum flight speed of 3 mm deuterium pellets before they were subjected to damage within a curved guide tube designed for HFS injection. The second set of experiments was dedicated to characterizing the acceleration of 8.1 mm deuterium pellets, a size compatible with the pellets intended for the SPI in the DTT machine, with the TSG test facility developed at ENEA-Frascati and operating in ORNL. Deuterium pellets were fired to evaluate the maximum speed achievable using the TSG propulsion system. The preliminary tests performed achieved speeds around 1.2 km/s.