Speaker
Description
The snowflake (SF) divertor is studied in the MAST-U tokamak as an alternative concept for next-step compact fusion devices and is planned for high-power NSTX-U tokamak experiments. The studies focus on the SF plasma transport mechanisms and their scaling with plasma current in the range 0.4-1.0 MA. The SF divertor features divertor geometry, radiation and transport enhancements (cf. standard X-point divertor) that may improve divertor power handling.
Recent MAST-U experiments have utilized 1.5-3.2 MW NBI-heated, 0.6-0.75 MA H-mode discharges to study heat and particle exhaust over eight strike points, a unique aspect of MAST-U up-down divertor symmetry. Feed-forward plasma control algorithms based on TokSys and TED simulations were used to manage the second poloidal field (PF) null location resulting in inter-null distances 0.01-0.20 m. The H-mode confinement was maintained albeit with a plasma stored energy reduction of 10-30%. The ELM regime changed from small or no ELMs to medium-sized ELMs during the SF phase. The SF variants, namely the SF-exact, SF-minus, and SF-plus configurations, were obtained with the expected geometry enhancements, e.g., the connection lengths longer by a factor of 1.5-3 (cf. standard and Super-X configurations). Divertor results included: 1) Evidence of particle and heat flux sharing over the SF strike points, from Langmuir probes, infrared and filtered divertor cameras; 2) Observations of turbulence correlation between inner and outer SOL regions in the forming SF configuration, suggestive of a shared plasma SF region, based on fast divertor imaging; 3) Plasma radiation peaking in the PF null region suggesting a potential for X-point radiator regime.
Experimental results are interpreted using an improved reduced-magnetohydrodynamic model of the churning mode and the multi-fluid code UEDGE with drifts and poloidally non-uniform transport coefficients. Two likely transport candidates for the SF zone plasma mixing, classical electromagnetic drifts and the churning mode, provide fluxes whose relative magnitudes and contributions scale differently over the range of experimental plasma currents (and PF) and SOL power widths. The UEDGE model is also used to project safe divertor operations in future 1-2 MA NSTX-U SF experiments with input power up to 10 MW. In SF configurations, the magnetic field in the additional strike points is in the opposite direction and small sections of the “fish-scaled” graphite tiles are exposed to nearly normal heat fluxes. The modeling shows that even with high degree of power exhaust redistribution into the additional strike points, heat fluxes remain below 5-7 MW/m$^2$.