Results from recent detachment experiments in alternative divertor geometries on TCV

An overview of initial experiments performed on TCV to explore detachment in alternative divertor magnetic geometries will be presented. These experiments expand on previous TCV X-Divertor and Snowflake-plus detachment studies [1-2] and the results are compared to detachment experiments in more standard TCV shapes. The long-term goal of this work is to determine whether variations in magnetic geometry lead to improvements in divertor power losses and detachment control, both needed for DEMO. The variations in magnetic geometry studied include increased connection length, poloidal flux expansion, and flaring both near the x-point (snowflake) and near the target (X-Divertor and X-Point target configurations), and total flux expansion (Super-X Divertors), obtained by increasing the outer strike point major radius, Rt. Snowflake, X-, and X-point target divertor configurations were achieved in Ohmic L-mode plasmas and unfavorable ion gradB drift direction. Ramps of density and nitrogen level were performed up to a maximum determined by disruption limits. The planned Super-X target was more difficult to realize. The plasma disrupted once Rt was increased by more than ~45% compared to the Rt in more standard TCV shapes, well below the targeted ~70% increase. Furthermore, plasmas near the maximum achieved Rt disrupted relatively early during density ramps, at n/nG of ~0.4 compared to ~0.6 in the other configurations. First insights from these experiments on the geometry dependence of the detachment behavior will be presented. The primary focus will be the assessment of the divertor regimes based on wall-embedded Langmuir probe measurements; ion flux to the different strikepoints and degree of detachment, target electron temperatures, and parallel pressure variations in different conditions and geometries. Consistency checks with other diagnostics, in particular upstream measurements from the fast reciprocating probe, target heat flux measurements from infrared cameras, and information from a new divertor spectroscopy system will be performed to develop a robust picture of the detachment process in the different geometries.