Numerical investigation of the DTT cryopump performance via 3D Direct Simulation Monte Carlo modeling

DTT (Divertor Tokamak Test Facility) is a new facility, currently under construction in Frascati, Italy. The goal is to provide a basis to perform various scaled experiments for testing different magnetic configurations and alternative solutions for the power exhaust system of DEMO. The DTT pumping system design uses cryopumps as the primary pumping solution and up to 10 pumping openings. The cryopump system was developed at the Karlsruhe Institute of Technology (KIT). In the present work, the pumping capabilities of the DTT cryopump are estimated by performing a 3D numerical investigation of the neutral gas dynamics in the pumping duct of DTT including the entire complex cryopump geometry. The investigation is based on the Direct Simulation Monte Carlo (DSMC) method, which allows for a precise description of the neutral gas dynamics over the entire range of the gas collisionality. The values of the pumping probability for deuterium and neon were determined for two scenarios: open and closed divertor toroidal gaps. For open gaps, the probabilities were obtained as 0.4 for deuterium and 0.62 for neon, with a slight increase in both values observed when the gaps were closed. The results suggest that achieving the target of ten ports for deuterium pumping seems feasible, while for neon, partial elimination of toroidal leakages may be required. The importance of these simulations lies in the fact that, given the imposed simplifications, they delineate the existing pumping capabilities of the DTT particle exhaust and can act as a guide regarding the pumping capabilities among various plasma configurations. Moreover, this work demonstrates the level of geometric complexity that can be adopted in numerical modeling and highlights the effort needed to determine the values of the pumping probability, which are dependent on the chosen pumping technology.