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 (BT)-dependent parallel flow patterns of the background plasma and the parallel ion temperature gradient (∇||Tion) force. Experimentally, Mach probes used in L-mode plasmas with favorable (for H-mode access) BT 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 BT-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 ∇||Tion-force are largely flushed out. It is also found that doubling the radial diffusion coefficient from 0.3 to 0.6 m2/s 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 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-BT direction, and little/no accumulation for the favorable-BT direction. The wall-SOL collector probes have thus provided the first experimental evidence, albeit indirect, of near-SOL W accumulation – finding it occurs for the unfavorable-BT direction only. For the favorable-BT direction, fast flows can largely prevent accumulation from occurring.

Modeling turbulent impurity transport in the SOL of DIII-D with a reduced model

“…The agreement with the RCP T e , n e and Mach number data is also reasonable, figures 2(d)-(f). It must be noted that in OSM-EIRENE the Mach probe data is input directly to OSM-EIRENE; that is, the correct flow pattern at the midplane is imposed adhoc and is not self-consistently solved from the 1D fluid equations (as has been done previously [26,27]). At the time of this experiment the LLAMA diagnostic [28], which infers neutral density via Lyman-alpha emission, had not yet been installed.…”

“…Qualitatively speaking, the unfavorable curvature at the outboard midplane would result in more turbulent MHD activity, which may translate to larger fluctuations in E θ . This is compounded by the observation that impurities may accumulate near the outboard midplane in certain magnetic configurations, providing a localized source of impurities into the core [14,27]. Therefore, while core contamination from the inboard side is fully deserving of dedicated research, it is reasonable to focus on transport at the outboard midplane as it is likely where the largest number of impurities cross the separatrix.…”

Simulation study of the influence of E× B drift on tungsten impurity transport in the scrape-off layer

“…Therefore, it is of great importance to understand the transport of W impurities in the scrape-off layer (SOL), which determines the source of the tungsten contamination inside the confined plasma. Many efforts have been conducted on this subject for various existing tokamaks, such as EAST [19][20][21][22], DIII-D [23,24], JET [25][26][27], and ASDEX-Upgrade [28], and for future fusion reactors, such as ITER [29,30], CFETR [31][32][33], and DEMO [34].…”