Evolution of radiation profiles in in strongly baffled divertor on MAST Upgrade
Plasma detachment in tokamaks is useful for reducing heat flux to the target. It involves interactions of the plasma with impurities and neutral particles, leading to significant losses of plasma power, momentum, and particles. Accurate mapping of plasma emissivity in the divertor and X-point region is essential for assessing the relationship between particle flux and radiative detachment. The recently validated InfraRed Video Bolometer (IRVB) diagnostic, in MAST-U1 enables this mapping with higher spatial resolution than more established methods like resistive bolometers. In previous preliminary work2 , the evolution of radiative detachment was characterised in L-mode (power entering the scrape-off layer, PSOL 0.4MW). With a conventional divertor the inner leg consistently detached ahead of the outer leg (upstream density, nup = 0.3 and 0.4 · 1019 #/m3 , respectively), and radiative detachment preceded particle flux detachment (nup = 0.4 · 1019 #/m3 ). This work presents results also from the third MAST-U experimental campaign, fuelled from the low field side instead of the high field one, including Ohmic and beam heated L-mode shots (with a power exiting the core PSOL ∼ 1-1.5MW). We will show that by increasing PSOL both inner and outer target radiative detachment density increases, but much more on the outer leg then the inner one, suggesting an uneven power distribution between the legs. At the same time particle flux roll over and the negative effect on confinement happen at similar densities, indicating that for increasing PSOL detachment could be compatible with high performing plasmas. This dataset also supports predictions from the analytical detachment location sensitivity (DLS) model3–5 that the inner leg radiation front moves unstably from the target to the X-point, while this is gradual on the outer one.