Reproduction of collector probe deposition profiles using the far-SOL impurity transport code 3DLIM

Abstract

• DIII-D collector probe deposition patterns are successfully reproduced in 3DLIM. • 3DLIM requires tungsten to enter far-SOL from inner target direction when Bx∇B↑. • Radial transport of tungsten in the far-SOL is best modelled using convection. • Poloidal deposition patterns imply a convection-dominated SOL is most appropriate. First results are reported from the 3D Monte Carlo far-SOL impurity transport code 3DLIM. Tungsten deposition profiles measured on a Collector Probe (CP) located in the far-SOL near the outer midplane, OMP, during W tracer experiments in DIII-D are reproduced by 3DLIM. Radial deposition profiles are replicated showing the effect that a decrease in connection length from the CP to the nearest wall contact point has on impurity transport to the probe, as well as the effect of assuming purely diffusive vs convective radial transport. For purely diffusive radial transport, a diffusion coefficient of 10 m 2 /s best reproduces deposition patterns on both sides of the CP, but for purely convective radial transport a speed of 125 m/s is shown to have better agreement with the ITF deposition profile. Deposition profiles show peaking in W content along the length of the CP edges that is also reproduced in 3DLIM, but only when assuming a convection-dominated SOL plasma parallel transport prescription for the background plasma. The degree of the peaking is shown to be a secondary indicator of the effective location of the W source in the near-SOL OMP relative to the far-SOL (near/far from the separatrix). Identifying the location of the effective source provides insight into near-SOL impurity dynamics, including the existence and location of impurity accumulation near the OMP separatrix. Such accumulation typically occurs in SOLPS and other edge code modeling, but has hitherto been difficult to confirm experimentally. The impurity density at the edge is the boundary condition for impurity levels in the confined plasma.