Effect of neutral leaks on pumping efficiency in 3D DEMO divertor configuration

Abstract In this paper we analyze the pumping efficiency of a generic 3D DEMO divertor configuration based on the size of inter-cassette gaps. Due to symmetry conditions we consider only one segment, which is composed by 3 divertor cassettes with the pumping port located at the bottom of the middle cassette and with four inter-cassette gaps. The width of inter-cassette gaps or the number of pumping ports is defined by the requirement to minimize the outflow of particles from the divertor to the plasma and hence, the highest possible pumping efficiency for the particle throughput, expected in DEMO, to be achieved. The DIVGAS code is used to perform a sensitivity analysis of the pumping efficiency, defined as a ratio of pumped particle flux to the particle throughput for different inter-cassette widths. The imposed neutral boundary conditions of pressure and temperature represent a moderate divertor pressure operating point. The analysis shows that for the reference case of 20 mm a reduction on the pumped flux of the order of 10% is observed when compared with the case of a completely sealed divertor. For smaller gaps the pumped flux reduction is found to be negligible, while for large values of the gap width the pumped flux reduction may reach the value of 20%. Furthermore, almost 80% of the incoming particles are moving towards the x-point, independent of the gap width. Therefore, the gap width does not influence the outflux towards the x-point, but only strongly affects the pumping efficiency. This analysis is meant to contribute to the ongoing DEMO divertor design efforts.