Snowflake divertor experiments in NBI-heated H-mode plasmas in MAST-U.

Following first successful snowflake (SF) divertor experiments in ohmic L-mode plasmas in MAST-U, recent SF divertor experiments focused on H-mode plasmas. A 3 MW NBI-heated, 0.6 MA, 1 s long H-mode discharge scenario with a higher elongation was used. A novel feed-forward plasma control algorithm aimed at controlling the second poloidal field (PF) null enabled the attainment of SF configurations for up to 200 ms. Analysis utilizing infra-red video bolometer diagnostic, target Langmuir probes, filtered divertor cameras and infra-red thermography indicated consistent divertor plasma characteristics: peaked radiated power around the X-point(s), increased divertor ion fluxes in secondary SF strike points, and the re-appearance of ELMs in the SF phase of the discharge. Slight up-down heat flux asymmetry toward the lower divertor was measured. Previously reported simulation results, obtained using the UEDGE multi-fluid transport code with poloidally-varying transport coefficients and H-mode edge kinetic profiles, are compared with experimental data.