A 3D Monte-Carlo study of the W7-X island divertor transport for different magnetic configurations

With a magnetic structure depending on the rotational transform, the control coil current and the plasma beta, the low shear stellarator W7-X provides a high flexibility in modifying the edge magnetic islands to optimize the island divertor transport. The magnetic field around the islands is usually weakly stochastic and the degree of the stochasticity is affected by the shear, the control-coil field and the plasma current including the beta effect. In this paper, we analyse and compare the plasma and neutral transport for different divertor configurations using the 3D Monte Carlo code EMC3-EIRENE. Three typical divertor configurations covering a wide topology range have been studied. The effect of change in magnetic topology on the recycling-related island divertor operation is studied and the transport characteristics are obtained. An optimization is done with respect to the density control and particle balance in steady-state operations and the effects of stochasticity on the energy transport are characterized