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We report on the development of the X divertor configuration [1] on MAST Upgrade and the reduction in heat and particle fluxes at the divertor targets this configuration achieves over a conventional divertor configuration. Using the Toksys [2] and TED [3] frameworks, we design magnetic equilibria with increased poloidal flux expansion at the target compared with a conventional divertor configuration and apply feed-forward changes to poloidal field coil currents to achieve the designed magnetic configurations in Ohmic, beam-heated L mode and beam-heated H mode plasmas, in double null and lower single null.
A strategy of zeroing the radial and vertical magnetic fields Br and Bz just below the strike point produces high target poloidal flux expansion but is very sensitive to plasma evolution and cannot be sustained with only 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 without imposing a field null constraint at all produces similarly-robust scenarios with lower peak flux expansion. Adding strike point position feedback after forming the X divertor maintains acceptable flux expansion for much 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 but only in the far SOL, with modest flux expansion at the strike point. These scenarios also showed increased interaction of the SOL with the divertor entrance and a corresponding increase in upstream neutral influx and radiated power. While peak heat and particle fluxes were reduced by 1/3 and 1/2 respectively, it was not possible to isolate 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 while sacrificing the degree of peak flux expansion was more effective at avoiding 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, with the effect being more pronounced in lower single null than in double null.
[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.