Heat and particle exhaust in double-null configuration in WEST: Experimental study and modeling with SOLEDGE-EIRENE

Abstract

The WEST tokamak allows for high power steady-state plasma discharges in Lower Single-Null (LSN), Double-Null (DN) and Upper Single-Null (USN) configurations. In this contribution, first results from experimental L-mode DN discharges with 3.1 MW of additional heating in the WEST tokamak and associated modeling with the SOLEDGE-EIRENE code are presented. The goal is to investigate if the magnetic configuration could help to reduce the peak heat load on the targets, and how sensitive is the power sharing between the strike points to a change in magnetic equilibrium. This study, previously performed on other devices such as DIII-D, Alcator C-mod and MAST-U, provide new data for machines with a compact divertor configuration and tungsten walls. During the experiment, the distance between the two separatrices at the outer midplane dR_sep was scanned from –32 mm (USN) to 42 mm (LSN), with the ion Bx∇B drift directed towards the lower divertor. The measured power fall off length mapped at the outer midplane can vary between configurations but is centered around λ_q = 15.6 ± 2.4 mm. In the DN configuration, Langmuir probes measurements have shown that 86 % of the power is directed towards the outer strike points. This inner/outer imbalance is consistent with the ones measured on DIII-D, Alcator C-mod and MAST-U. The influence of ballooned radial transport, that drives most of the power towards the low field side, has been reproduced in transport simulation with imposed poloidally ballooned cross-field transport. The peak heat flux measured on all strike points is reduced by half in DN compared to LSN configuration. In certain conditions of high radiations (Ohmic discharges or with ICRH), the DN configuration can trigger a MARFE on the High-Field Side (HFS) that moves in opposition to the oscillations of the plasma vertical position. This MARFE has been measured to account for 0.3 % of the particles in the plasma. Temperature and density of the core plasma are unaffected by its presence.