DSMC modeling of the differentially pumped Magnum-PSI vacuum system

The FOM Institute for Plasma Physics Rijnhuizen (FOM = Foundation for Fundamental Research on Matter) has started a new line of research to study the interaction of intense plasma fluxes with a material surface. An important experimental tool for this programme will be the Magnum-PSI high-flux linear plasma generator operating in the ITER- relevant regime of plasma surface interaction (PSI). ITER is the next step in fusion reactor research (ITER is Latin for 'the way'). In Magnum-PSI a plasma beam is guided from the source to a target by a strong axial magnetic field. Besides ionized particles, the plasma source produces hot neutral gas which has to be prevented from reaching the target region. In this paper it is investigated to what extent a differential pumping scheme can prevent the influx of neutral gas from the source in the target region. The results of neutral gas simulations using the Direct Simulation Monte Carlo (DSMC) method will be discussed. We will focus on the supersonic expansion as a function of background pressure and determine the influence of the skimmers on the expansion. We will demonstrate that differential pumping can be used in Magnum-PSI to reach low enough pressures in the target region. Finally, we give the optimum position of the skimmer.