PPPS-2013: Modeling snowflake divertors for reduced surface heat-flux in tokamaks

Summary form only given. The high peak heat-flux of plasma exhaust in magnetic fusion devices incident on material surfaces could exceed material limits and must be controlled. The magnetic snowflake divertor¹ can alleviate this problem by introducing a null in the poloidal magnetic field, B_p, where B_p's local strength varies as the square of the distance from the null compared to the linear variation of the standard X-point divertor. The increased magnetic flux expansion of the snowflake and the high ratio of plasma-to-poloidal magnetic pressure, termed β_p, are predicted to reduce both steady state heat flux and that from large, intermittent edge localized modes (ELMs).^1,2 Experiments in TCV (Lausanne), NSTX (Princeton Plasma Physics Lab), and DIII-D (General Atomics) tokamaks have observed substantial heat-flux reduction when using a snowflake configuration. Some of these effects have been modeled by the 2D UEDGE edge plasma/neutral transport code.³ Here we present extensions to that modeling to include detailed comparison with the new experimental results from DIII-D.⁴ In addition, we add a feedback model such that the radial spreading of the ELM by plasma convection² near the null point is proportional to the local β_p, thus providing a dynamic description of the spreading during the ELM.