Poloidal radiation asymmetries during disruption mitigation by massive gas injection on the DIII-D tokamak

Abstract

A comparison of radiated power poloidal peaking during disruption mitigation using massive gas injection at multiple poloidal positions on the DIII-D tokamak is presented. The two injectors are located poloidally above and below the low field side midplane and toroidally located within the quadrants to either side of the fast bolometry diagnostic used to measure the radiated power. Differing quantities of injected neon are compared. A strong dependence of impurity poloidal flows upon the injector location is observed. Injection from the upper half of the vessel results in strong poloidal flows over the top of the plasma to the high field side midplane, while lower injection exhibits far less pronounced poloidal flow that is oriented in the opposite direction. The poloidal location of both pre-thermal quench and thermal quench emissivity peaking shows a strong dependence upon the injector location, although the poloidal flow in the upper injection case results in a much broader distribution. The wall radiative heat flux mimics the emissivity, but the distribution is smoothed with lower poloidal peaking due to geometric effects. Thermal quench MHD appears to have little effect upon the poloidal phase of maximum emissivity in experiment or modeling, which can be attributed to the slower parallel transport of impurities along field lines in the poloidal versus toroidal direction. Poloidal peaking factors of ≤1.6 and ≤2.2 were observed for upper and lower injection, respectively. Under very conservative assumptions, the observed poloidal peaking factor will bring ITER near the melting limit for first wall stainless steel. However, further modeling is required to determine if those conservative assumptions are warranted.