Investigation of conventional and Super-X divertor configurations of MAST Upgrade using scrape-off layer plasma simulation

Abstract

One of the first studies of MAST Upgrade divertor configurations with SOLPS5.0 are presented. We focus on understanding main prospects associated with the novel geometry of the Super-X divertor (SXD). This includes a discussion of the effect of magnetic flux expansion and volumetric power losses on the reduction of target power loads, the effect of divertor geometry on the divertor closure and distribution of neutral species and radiation in the divertor, the role of the connection length in broadening the target wetted area. A comparison in conditions typical for MAST inter-ELM H-mode plasmas confirms improved performance of the Super-X topology resulting in significantly better divertor closure with respect to neutrals (the atomic flux from the target increased by a factor of 6, but the atomic flux from the divertor to the upper SOL reduced by a factor of 2), increased radiation volume and increased total power loss (a factor of 2) and a reduction of target power loads through both magnetic flux expansion and larger volumetric power loss in the divertor (a factor of 5-10 in attached plasmas). The reduction of the target power load by SXD further increases with collisionality (high density or detached regimes) thanks to larger importance of volumetric power losses. It is found that a cold divertor plasma leads to stronger parallel temperature gradients in the SOL which drive more parallel heat flux, meaning that the effectiveness of perpendicular transport in spreading the power at the target can be reduced, and this needs to be taken into account in any optimisation.

The effect of the Super-X divertor of MAST Upgrade on impurity radiation as modelled by SOLPS

“…The distribution of impurity species (not discussed here) shows more neutral and ion impurity particles at low ionization stages in the inner divertor of the CD, and more impurity ions at higher ionization stages in the outer part of the SOL around the X-point in the SXD configurations. Concerning carbon radiation, the total level (7.8% in the CD vs 9.7% in the SXD1) as well as the power radiated above the X-point are comparable in the CD and SXD1 (note that in attached plasmas without seeded impurity, the differences in the power radiated by carbon are larger between the CD and SXD than reported here, and the SXD1 achieves better carbon impurity screening [4]). …”

“…For the CD, this corresponds to a puffing rate of 3 Â 10 22 N=s (nitrogen atoms per second), and for the SXD1 and SXD2 to a puffing rate of 4 Â 10 21 N=s. In these cases, plasma parameters in the divertor are more similar in all configurations than without impurity puffing (the case analyzed in detail in [4]). Fig.…”

“…Pumping occurs on cryopumps where R ¼ 0:9 and which are located behind the outer targets. For more details see [4].…”

“…2). The two SXD configurations need much smaller puffing rates for the particle flux to roll over (the temperatures are reduced already before the impurity puffing due to larger target radius and larger power losses [4]) and also the particle flux at the transition to detachment is smaller. In addition, the SXD configurations achieve a larger degree of detachment with a larger pressure drop along the field line as well as a sharper reduction of the target power load compared to the CD where the detachment is more gradual.…”

“…Such flexibility enables the effects of different parameters to be separated when studying the divertor performance and power exhaust issues. The MAST-U SXD has been extensively studied by simulation in [4,5], from which this paper follows. The previous work discussed the potential of the SXD to increase power losses in the divertor, the divertor closure with respect to neutrals, and its potential to reduce divertor temperatures, target power loads and the density required for the transition to detachment.…”