Divertor turbulent transport in the single null and snowflake in the TCV tokamak

Abstract

The relative importance of divertor radial turbulent particle fluxes is considered by comparing it against the fluxes in the main-chamber outer midplane (OMP) in a variety of conditions and divertor geometries in the tokamak à configuration variable. Within the first power falloff length, the radial turbulent fluxes in the leg of the outer divertor are consistently found to be small, and about 1/5th the magnitude measured at the OMP. In a low-density single null divertor, magnetic shear is found to play a strong role in isolating the main-chamber turbulence from the divertor. The snowflake divertor is purported to have turbulence-enhancing properties in the volume between the two X-points but was instead found to further reduce the divertor turbulent fluxes compared to the single null. Depending on the collisionality, the electric field fluctuations and radial turbulent fluxes were higher near the X-point than at the outer midplane, which is likely due to the binormal compression of the flux bundles consistent with analytical models of the resistive X-point mode. Density and potential fluctuation amplitudes decrease monotonically with distance from the OMP with a slope that depends on collisionality.