Multi-Fluid Transport Modeling of NSTX Upgrade Standard and Snowflake Divertor Configurations

In NSTX Upgrade (NSTX-U), power exhaust is expected to challenge available material and heat removal technologies, which are limited to ∼10 MW/m2. Preliminary analysis of heat flux mitigation strategies, including the snowflake divertor configuration and radiative divertor operation, is performed with the multi-fluid edge transport code, UEDGE. Divertor recycling between 95% and 99%, and power exhaust between 7 and 9 MW are explored. Compared to the standard divertor, the particular snowflake divertor studied here strongly reduces outer target heat flux, but tends to have higher (>10 MW/m2) inner target heat flux. This suggests that snowflake divertor optimization should be pursued in future work. Both neon and argon impurity seeding scenarios mitigate high heat fluxes in both standard and snowflake configurations, but argon can cause radiative collapse of the core plasma at only 3% concentration. Divertor cryopumping is also considered, and modeling shows ∼50% more particle removal with the snowflake divertor. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)