Neutral particle modelling and particle exhaust in the Wendelstein 7-X stellarator

The divertor and the pumping system of Wendelstein 7-X is designed for stationary operation at various modes of operation. Maximum fluxes up to 1024 electron–ion-pairs/s are expected at the targets. Various cases of plasma–wall interaction have to be considered: at low densities convective power losses and proceeding to higher densities a high recycling mode, including detachment dominated by radiation. External fluxes (NBI, gas puffing and pellet injection) up to 1022 particles/s have to be pumped. The geometry of the target plates, the installation of additional baffle plates and the position of the pumping gap were optimised on the basis of numerical studies with the 3D neutral particle code EIRENE. For particle exhaust turbomolecular pumps (TMPs) and cryo-panels will be installed. Taking in account the capabilities of the TMP in the magnetic stray field an effective pumping speed for H2 of 4200 l/s at one divertor box will be reached. Including the cryo-panels integrated inside the divertor boxes a total pumping speed of up to 200 000 l/s will be provided.