The role of B T-dependent flows on W accumulation at the edge of the confined plasma

Abstract

Near-separatrix impurity accumulation between the crown and the outer midplane of tokamaks is a common feature in results from codes such as SOLPS-ITER and DIVIMP; however, experimental evidence of accumulation has only recently been obtained and is reported here. The codes find that the poloidal distribution of impurity ions in the scrape-off layer (SOL) depends primarily on toroidal field (B T)-dependent parallel flow patterns of the background plasma and the parallel ion temperature gradient (∇‖ T ion) force. Experimentally, Mach probes used in L-mode plasmas with favorable (for H-mode access) B T measure fast (M ∼ 0.3-0.5) inner-target-directed (ITD) background plasma flows at the crown of single-null discharges. This study reports a set of DIVIMP simulations for two similar H-mode discharges from the DIII-D W metal rings campaign differing primarily in B T-direction to assess the effect that fast ITD flows have on the distribution of W ions in the SOL. It is found that for imposed ITD flows of M = 0.3, W ions that otherwise accumulate due to the ∇‖ T ion-force are largely flushed out. It is also found that doubling the radial diffusion coefficient from 0.3 to 0.6 m2 s-1 prevents accumulation due to rapid cross-field transport into the far-SOL, where background plasma flows drain W ions to the divertors. Far-SOL W distributions from DIVIMP are then used to specify input to the impurity transport code 3DLIM, which is used to interpretively model collector probe (CP) deposition patterns measured in the 'wall-SOL'. It is demonstrated that the deposition patterns are consistent with the DIVIMP predictions of near-SOL accumulation for the unfavorable-B T direction, and little/no accumulation for the favorable-B T direction. The wall-SOL CPs have thus provided the first experimental evidence, albeit indirect, of near-SOL W accumulation - finding it occurs for the unfavorable-B T direction only. For the favorable-B T direction, fast flows can largely prevent accumulation from occurring.