Power exhaust and core-divertor compatibility of the baffled snowflake divertor in TCV

Abstract

A baffled Snowflake Minus Low-Field Side (SF-LFS) is geometrically-optimised in TCV, increasing divertor neutral pressure, to evaluate the roles of divertor closure (comparing with an unbaffled SF-LFS) and magnetic geometry (comparing with a baffled Single Null, SN) in power exhaust and core-divertor compatibility. Ohmically-heated L-mode discharges in deuterium, with a line-averaged core density of approximately 4 x 10^19 m^-3, are seeded with nitrogen to approach detached conditions. Baffles in the SF-LFS configuration are found to reduce the peak outer target heat flux by up to 23%, without significantly affecting the location of the inter-null radiation region or the core-divertor compatibility. When compared to the baffled SN, the baffled SF-LFS exhibits a reduction in outer target heat flux by up to 66% and the ability to balance the strike-point distribution of heat flux. These benefits are less significant with N_2 seeding, with similar peak target quantities (such as heat flux, electron temperature and ion flux) and divertor radiated power. Despite a radiating region located farther from the confined plasma for the SF-LFS than the baffled SN, no change in core confinement is observed. Core effective charge even indicates an increase in core impurity penetration for the SF-LFS. These experiments constitute a good reference for detailed model validations and extrapolations, exploring important physics such as core impurity shielding and the dependence of divertor cross-field transport on magnetic geometry.

Overview of the TCV tokamak experimental programme

“…Operating the snowflake minus in the baffled TCV divertor revealed significant particle fluxes to the top of the baffle, indicative of divertor over-closure. This hypothesis is supported by experiments on a variant of the snowflake minus, where only the outer divertor is baffled, which achieved a higher divertor neutral pressure than the fully baffled snowflake [35]. This will be further tested with the next combination of baffles foreseen for 2021.…”

Comparison of detachment in Ohmic plasmas with positive and negative triangularity

“…Because of its strong dependence upon the local electron temperature, the CIII emission location was found to be a good indicator of the position of a low temperature region along the divertor leg. Figure 14 plots tomographic inversions of CIII divertor images taken from the MANTIS remapped to the upstream distance to the separatrix, dRus, using LP and the procedure described in [41] to account for non-ambipolar conditions. (c) Parallel heat flux profiles inferred from Infra-red thermography, remapped to the upstream distance to the separatrix, dRus.…”

“…We start by investigating the role of δ top , figure 15(a). The outer strikepoint parallel heat-flux is determined from both Langmuir Probes (LP) using the methodology developed in [41], figure 15(b), and Infra-Red thermography (IR), figure 15(c). For both diagnostics, NT and PT show a similar peak parallel heat flux, whilst the absolute peak value differs by approximately 20% between the diagnostics.…”

“…Shape of the NT (blue, #74571) and PT (red, #74413) equilibria considered in section 4.4. (b) Parallel heat flux profiles,remapped to the upstream distance to the separatrix, dRus, using LP and the procedure described in[41] to account for non-ambipolar conditions. (c) Parallel heat flux profiles inferred from Infra-red thermography, remapped to the upstream distance to the separatrix, dRus.…”