The effects of alternative divertor configurations on upstream SOL turbulence in TCV

In attached and detached lower single-null L-mode plasma discharges at TCV, we observed [1] turbulent SOL filaments to mostly populate the resistive X-point regime. In the far and, in certain conditions, near SOL, upstream filaments extend beyond the X-point and into the diverter region. This raised the question if, and to what extent, the divertor magnetic geometry influences upstream filament dynamics, and how this could be used in the design of a fusion reactor. We therefore extend this study to X-point Target (XPT) and SnowFlake (SF) divertor configurations. In SF-diverted discharges, we find that the position of the secondary X-point affects filament cross-field size and radial velocity, in the measure of its effects on the parallel connection length. When the latter is increased by ~50-100% filament size and velocity also increase by up to 30%. Filaments in XPT-diverted plasmas appear to be less sensitive to the geometry, consistent with the parallel distance from the midplane to the secondary X-point, which is 2x larger in XPT-diverted discharges than in SF. The implications of these findings for the prospects of alternative divertor solutions will be discussed.

[1] N. Offeddu et al., Cross-field and parallel dynamics of SOL filaments in TCV, Nucl. Fus. (accepted)