First EMC3-Eirene simulations of the TCV snowflake divertor

Abstract

Abstract. One of the approaches to solve the heat load problem in a divertor tokamak is the so called 'snowflake' configuration, a magnetic equilibrium with two nearby x-points and two additional divertor legs. Here we report on the first EMC3-Eirene simulations of plasma-and neutral particle transport in the scrape-off layer of a series of TCV snowflake equilibria with different values of σ, the distance between the x-points normalized to the minor radius of the plasma. The constant cross-field transport coefficients were chosen such that the power-and particle deposition profiles at the primary inner strike point match the Langmuir probe measurements for the σ = 0.1 case. At the secondary strike point on the floor, however, a significantly larger power flux than that predicted by the simulation was measured by the probes, indicating an enhanced transport across the primary separatrix. As the ideal snowflake configuration (σ = 0) is approached, the density as well as the radiation maximum are predicted to move from the target plates upwards to the x-point by the simulation.

Numerical study of potential heat flux mitigation effects in the TCV snowflake divertor

“…For the simulations of the SF + equilibria presented in Ref. [1] we created grids, where the radial surfaces of the confinement-and SOL regions were aligned with the flux surfaces for convenience, while the private flux region covering the structures around the secondary separatrix was a single zone constructed by simple linear interpolation between the separatrix and the wall. Our initial approach to addressing the SF − was to apply the same technique for the entire plasma edge covering this region with such a non-flux surface aligned grid.…”

“…[1] and the newly constructed series of SF − grids (ρ x2 > 1.0), we can now compare nine configurations with ρ x2 ranging from 0.988 to 1.020. For ρ x2 = 0.988 the secondary X-point is so far away from the separatrix that we also refer to this configuration as the SN.…”

“…5.3 we focus on pure deuterium simulations here. Two sets of radially, poloidally and toroidally constant transport coefficients were used (A) D ⊥ = 0.5 m 2 /s and χ ⊥ = 1.5 m 2 /s and (B) D ⊥ = χ ⊥ = 0.6 m 2 /s, where set (B) is the one also used in [1]. 2D profiles of the poloidal cross section of several quantities computed by EMC3-Eirene for (A) are shown in Figs.…”

“…1). Motivated by the assumption that the exact SF would constitute the optimum situation we recently reported on a series of EMC3-Eirene simulations of SF + equilibria that approach the exact SF up to a spatial distance between the X-points of 0.01×a, where a is the minor radius of the plasma [1]. Assuming a purely diffusive transport with spatially constant coefficients, it was found in the simulations that the power fluxes to the secondary SPs are about 1 % of the total input power P in only, while experimentally 10 % was measured.…”

“…In continuation of recent EMC3-Eirene simulations of a series of 'snowflake plus' (SF + ) configurations in TCV [1] we report on simulations with the same code applied to 'snowflake minus' (SF − ) configurations, where the secondary X-point (x 2 ) is located in the common flux region of the primary separatrix on the low-field side of the primary one. While for the SF + the power flux to the secondary strike points (SPs) was only of the order of 1 % in the simulation and ∼ 10 % experimentally, a much higher flux was found for the SF − simulations, consistent with recent measurements [2].…”