The effect of the secondary x-point on the Scrape-Off Layer transport in the TCV Snowflake Minus divertor

TCV experiments show evidence that the cross-field transport in the low-field side (LFS) Scrape-Off Layer (SOL) of the conventional Single-Null (SN) configuration is strongly enhanced when placing a secondary x-point in the common flux region of the primary separatrix. Such a magnetic configuration is known as the Snowflake Minus (SF-) [1]. The exhaust properties of the SF- configuration were investigated on TCV in Ohmically heated, L-mode attached plasmas with a range of x-point separations, magnetic field directions and locations of the secondary x-point (low-field side, LFS, or high-field side, HFS, SOL). The target heat flux profiles at all strike points are simultaneously measured with an Infrared Thermography (IR) system [2] and the main SOL plasma kinetic profiles with a fast reciprocating probe (RCP) [3]. The measured power repartition between the two branches of the SOL diverted by the secondary x-point yields an effective heat flux width ?q,u eff for the SOL. It is found that the LFS SOL cross-field transport is strongly enhanced by the presence of the secondary x-point, for both magnetic field directions, with ?q,u eff being a factor three larger than the value measured by the RCP at the outer mid-plane or by IR at the outer target of a comparable SN. The RCP inspection of the low poloidal field region reveals that the profiles of SOL density and ion saturation current develop steep gradients in proximity of the secondary separatrix, while the profiles flatten in between the two separatrixes, compared to the SN divertor. In contrast, the cross-field transport in the HFS SOL is not significantly affected by the presence of the secondary x-point. However, similar to comparable SN discharges, it is sensitive to the magnetic field direction, with a broader SOL width for reversed field. Placing the secondary x-point on one side of the SOL also affects the power balance between inner and outer divertor. This is interpreted as the effect of changed parallel transport in the SOL due to the changes in the geometry of the flux tubes caused by the presence of the secondary x-point, and was already seen in fluid calculations assuming constant cross-field transport coefficients.