Description
Plasma scenarios with an X divertor configuration [1] developed on MAST Upgrade exhibit a reduction in heat and particle fluxes at the divertor targets compared with conventional divertor configurations. We use the Toksys [2] and TED [3] frameworks to modify a conventional divertor magnetic equilibrium to increase poloidal flux expansion at the target and apply feed-forward changes to poloidal field coil currents to realise these configurations in Ohmic and beam-heated L-mode and H-mode plasmas.
The highest target poloidal flux expansion is achieved by zeroing the radial and vertical magnetic field components just below the strike point; this strategy is however very sensitive to plasma evolution and cannot be sustained by feed forward control. Moving the field null further from the target reduces the peak flux expansion but gives a more robust scenario. Designing solely for target flux expansion instead of imposing a field null constraint also achieves a robust scenario at the cost of with lower peak flux expansion. Adding strike point position feedback control after forming the X divertor maintains acceptable flux expansion for longer durations, but only for lower poloidal flux expansion scenarios.
In Ohmic and NBI-heated L mode plasmas using the field-null strategy, we achieved high peak flux expansion only in the far SOL, with modest flux expansion at the strike point. Increased interaction of the SOL with the divertor entrance and a corresponding increase in upstream neutral influx and radiated power precluded isolating the impact of the divertor geometry from the impact of the increased wall interaction at the divertor entrance. Moving the location of the peak flux expansion closer to the strike point and sacrificing peak flux expansion avoided excessive wall interaction. This scenario achieved a 10%-30% reduction in peak parallel heat flux and earlier detachment onset in N2-seeded H mode plasmas compared with the conventional divertor.
[1] M. Kotschenreuther et al Phys. Plasmas 20, 102507 (2013)
[2] H. Anand et al 2024 Nucl. Fusion 64 086051
[3] O P Bardsley et al 2024 Plasma Phys. Control. Fusion 66 055006
Work supported by EPSRC EP/W006839/1 and US-DOE DE-AC05-00OR22725.