Infrared measurements of the heat flux spreading under variable divertor geometries in TCV

The safe and stable operation of a future fusion reactor depends critically on the ability to control the heat loads on the material surfaces facing the plasma. The heat fluxes are particularly high at the strike points in the divertor, where the plasma interacts directly with the wall. By varying the divertor geometry it is possible to increase the power radiated or transferred to neutrals and to spatially extend the scrape-off layer (SOL), with the common goal of distributing the total power over a greater surface. In a diverted plasma, the heat flux profile at the divertor strike points is largely determined by three competing mechanisms: (I) transport of heat along the field lines, (II) cross-field transport in the SOL region (with the LCFS as a source of heat), (III) cross-field transport both in the SOL and in the private flux region (without source). Mechanisms II and III spread the heat flux profile at the divertor and the experimental profiles are well parametrised by the convolution of an exponential decay and a Gaussian, representing mechanisms II and III respectively [1]. Infrared (IR) thermography is an invaluable tool with which to measure the heat flux distribution independently of the plasma parameters. Langmuir probes and thermocouples in the graphite protection tiles provide independent measurements to cross-check IR estimates. The IR system of TCV was recently upgraded to provide coverage of a wider range of divertor configurations and simultaneous measurements at both strike points of a conventional divertor geometry. Using the magnetic shaping flexibility of TCV, multiple divertor configurations ranging from modifications of the classical single null to alternative ones have been tested under attached divertor leg conditions and are presented in this paper. While the outer strike point is generally well fitted with the decay length and an additional spreading in the divertor itself, the inner divertor view displays a double-peak heat flux profile in forward B field, which may be caused by drifts in the SOL [2] and has been previously detected in other tokamaks. In order to take into account the effect of such drifts on the target profile shape, an extension of the parametrisation from [1] representing a radial redistribution of heat is proposed. [1] Eich T et al. 2011 Physical Review Letters 107 215001 [2] Canal G et al. 2015 Nuclear Fusion 55 123023