Detachment in alternative divertor geometries on TCV

The dependence of divertor detachment on magnetic geometry is explored on the TCV tokamak, with the goal of improving our basic understanding of this process and optimizing the divertor geometry for a power plant. In the experiments, connection length from the midplane to the outer target is varied by up to a factor two by either modifying the poloidal flux expansion or the divertor leg length. The outer target major radius, and hence total flux expansion, is scanned continuously with a variation of 70% to explore the properties of a Super-X divertor. The effect of an additional X-point close to the primary one (Snowflake divertor) and close to the outer target (X-point target divertor) is also assessed [1-2]. To study detachment across this large parameter space, experiments to date have mainly focused on Ohmic density ramps and on private-flux-region nitrogen seeding [3-4]. We find that although increasing the connection length through poloidal flux expansion has little effect on the density threshold for detachment of the outer leg, it allows access to a higher degree of detachment and provides improved control of the peak radiatio location. If, instead, the connection length is increased by increasing the divertor leg length, the detachment threshold is reduced by as much as 25%. Contrary to expectations from analytical and advanced numerical calculations, total flux expansion is found to have very little effect on the detachment behavior [1]. Additional X-points in the scrape-off layer can result in a zone of enhanced radiation around the secondary X-point and fully detach the part of the outer leg that sits inside the secondary X-point. This enhanced radiation region is particularly pronounced in the Snowflake geometry with sufficient nitrogen seeding [2], consistent with predictions from EMC3-Eirene calculations [5].