Speaker
Description
MAST Upgrade is designed and built to have a tightly fitting baffle to allow the use of either a conventional or long legged closed divertor configuration. This design allows the main chamber plasma to be decoupled from the divertor plasma, giving greater power dissipation before the divertor target in the closed divertor chamber and better access to detachment, whilst maintaining high performance core plasmas. This contribution will compare the effect of optimally closed conventional divertor geometries on MAST-U on the SOL widths and target profile broadening, with MAST-like open divertor geometries where the strike point is at a radius smaller than that of the baffle nose. A sudden rise in midplane Dα is observed as the strike-point decreases below Rsp < 0.75 m, which indicates a transition from baffled (closed) to unbaffled (open) divertor geometry. The amount of flux going through the gap between the strike line and the baffle nose also plays an important role in trapping the neutrals in the divertor – so called plasma plugging. It has been seen that in an optimally closed double null divertor in beam heated H-mode the λq is smaller than a comparable plasma with a more open divertor, i.e. Rsp < 0.75m. Furthermore, in an LSN geometry with a closed divertor, the λq is further reduced. Previous studies (Thornton et al [1]) of the SOL width in an open divertor configuration noted there was no correlation of the SOL width over a range of drsep between 2 and 7 mm. The S factor in the Eich fit [2], representing the SOL broadening due to cross-field diffusion, is larger in closed conventional divertor configurations compared to (MAST-like) open conventional divertor. This is likely due to the closed nature of the divertor increasing recycling and neutral interaction which can spread the power over a larger space. This is notable, given that on MAST it was postulated that the broadening in the divertor did not show any notable variance with various core and divertor parameters and was concluded to be due to the open divertor geometry.
This work has been funded by the EPSRC Energy Programme [grant number EP/W006839/1].
[1] A J Thornton et al 2014 Plasma Phys. Control. Fusion 56 055008
[2] T Eich et al 2011Phys. Rev. Lett. 107, 215001