Sticking Coefficients from Molecular Dynamics Simulations for the Design of Fusion Reactor Cryopumps*

In deuterium-tritium (DT) fusion reactors, only about 1% of the fuel undergoes fusion. The remaining fuel must be pumped out and processed through a tritium plant. A system called cryogenic direct internal recycling (DIR) has been developed that will bypass the tritium plant and reduce the required size of the tritium plant and tritium inventory. Cryogenic DIR uses a system of cryopumps to remove impurities and return processed material to the fueling system. In order to separate the helium from the DT fuel, a continuous cryogenic snail pump has been designed to cryotrap deuterium and tritium while allowing helium to pass through. The efficiency of the snail pump depends on the sticking coefficient, which is the probability that a particle adheres to the cryopump surface upon collision. Molecular dynamics simulations were used to calculate the sticking coefficients of D₂ on Cu, He on Cu, and He on D₂ for a range of surface and gas temperatures. At the snail pumps operating conditions, the calculated sticking coefficients are 0.910 for D₂ on Cu, 0.672 for He on Cu, and 0.620 for He on D₂. While the sticking coefficients D₂ on Cu were as expected, the He coefficients were higher than anticipated. These high values could be due to potential parameters that become inaccurate at cryogenic temperatures. Reliable sticking coefficient predictions enable more accurate modeling of cryopumps for DIR systems, supporting more efficient fuel recovery and improving the safety, cost, and sustainability of DT fusion reactors.