The effect of the secondary x-point on the scrape-off layer transport in the TCV snowflake minus divertor

Abstract

The snowflake (SF) magnetic configuration is investigated as a potential power exhaust solution for a fusion reactor, but also to improve our understanding of divertor physics in general. Unlike a conventional single null (SN), it features an additional nearby x-point, which deeply modifies the magnetic field in the scrape-off layer (SOL) and can thereby affect parallel and cross-field transport of heat and particles. This paper investigates the power exhaust properties of the snowflake minus (SF-) configuration on the TCV tokamak, for Ohmically heated, L-mode, low-density, attached plasmas for 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)). Due to the relatively large x-point separation in physical space, this study probes x-point transport features in general, rather than reactor relevant aspects of the SF configuration. The target heat fluxes at all strike points are simultaneously monitored with an infrared (IR) thermography system and the kinetic profiles of the SOL at the outer mid-plane with a reciprocating probe (RCP). The placement of the additional x-point is seen to affect the inner-outer divertor power balance. An effective heat flux width for the SOL in the low poloidal field region is inferred from the measured power repartition between the two SOL manifolds created by the secondary x-point. For the LFS SF- configuration (secondary null in the LFS SOL), the is two times larger than that measured by the RCP at the outer mid-plane and by IR at the outer target of a comparable SN, while the outer mid-plane SOL profiles are similar to the SN. This is interpreted as the effect of increased effective cross-field diffusivity in the intra-null region relative to the rest of the SOL. For the HFS SF- configuration (secondary null in the HFS SOL), no such increased transport is observed. The pressure-driven plasma convection expected near the primary null cannot explain the increased , which is instead consistent with interchange ballooning-like turbulence enhanced by the low poloidal field.