Experimental and Numerical Investigation of Expanding Mercury Vapor Jets

Mercury-driven diffusion pumps are currently actively researched as a potential pumping solution for the exhaust gases of fusion reactors. Compared with the typically used cryopumps, they show the advantage that they operate continuously, which reduces the overall tritium inventory of the power plant. Mercury is considered as the operating fluid because it is compatible with tritium. While mercury-driven vapor diffusion pumps have been used extensively in the past, there is a lack of a modern experimental setup that can be used to validate numerical simulations of such pumps. This is rectified by the nozzle experiment for mercury expansion investigations (NEMESIS) experiment which has been set up at KIT in the past years. In this experiment, mercury is boiled and the vapor expanded through a nozzle into a vacuum vessel. The static pressure distribution in the expanding jet is measured by inserting a pressure probe at different axial positions. In addition, the jet’s fore-vacuum stability is determined with respect to the boiler power. In the present work, we describe the experimental setup and the obtained results. In addition, they are compared with direct simulation Monte Carlo (DSMC) calculations in order to validate the modeling assumptions and boundary conditions. Good qualitative agreement was found and the observed quantitative discrepancies are explained on a physical basis.