First snowflake divertor experiments in MAST-U tokamak br

First snowflake (SF) divertor experiments in the MAST-U tokamak demonstrated steady-state snowflake-plusdivertor configurations in 450 kA ohmic L-mode plasmas. The SF divertor configuration features a secondpoloidal field (PF) null in the divertor region close by or overlapping with the main X-point. The resultinglow PF region and two additional divertor legs (strike points) may lead to additional power and particle fluxsharing via a hypothesized convective cell, and increased plasma-wetted area and radiation. The free-boundaryGrad-Shafranov equilibrium code FIESTA was used to design SF configurations with several inter-null distancesand orientations. In the experiment, the SF configurations with inter-null distances 0.13-0.20 m and lasting0.2-0.3 s were obtained. Parallel connection lengths between the midplane and the outer strike point in theSF configurations (evaluated at field lines 1-2 mm from the separatrix in the midplane) were 25-30 m, higherthan in the standard divertor (20-25 m) or the Super-X divertor (25 m). Diagnostic measurements highlightedsalient SF features. The infra-red video bolometer diagnostic showed that the radiated power peaking in the PFnull region was not as pronounces as in the standard divertor. Divertor ion fluxes measured by target Langmuirprobes showed increased ion flux in the plate region where a secondary SF strike point landed, concomitantlywith the SF formation. These measurements may suggest that some particle and heat redistribution was takingplace in the convective SF zone. The first SF experiments provide a basis for future SF studies in MAST-Utokamak with higher input power, improved plasma control and diagnostic measurements, to be comparedwith the modeling predictions of plasma convective SF mixing and lower density strike point detachmentthreshold.