Optimization of the snowflake divertor for power and particle exhaust on NSTX–U

•A model of diffusive transport and partitioning of power in the SFD is derived.•The best SFD configurations for heat flux reduction are determined.•The optimal location for the entrance to a divertor cryopump on NSTX-U is computed.•UEDGE simulations indicate that redistribution of SOL power occurs...

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Bibliographic Details
Published in:Nuclear materials and energy 2019-05, Vol.19 (C), p.516-523
Main Authors: Vail, P.J., Izacard, O., Kolemen, E.
Format: Article
Language:English
Subjects:
cryopump
ENGINEERING
NSTX-U
snowflake divertor
UEDGE
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Summary:•A model of diffusive transport and partitioning of power in the SFD is derived.•The best SFD configurations for heat flux reduction are determined.•The optimal location for the entrance to a divertor cryopump on NSTX-U is computed.•UEDGE simulations indicate that redistribution of SOL power occurs due to pumping. In this paper, simple analytical modeling and numerical simulations performed with the multi-fluid edge transport code UEDGE are used to identify optimal snowflake divertor (SFD) configurations for heat flux mitigation and sufficient cryopumping performance on the National Spherical Torus eXperiment Upgrade (NSTX–U). A model is presented that describes the partitioning of sheath-limited SOL power and particle exhaust in the SFD as a result of diffusive transport to multiple activated strike points. The model is validated against UEDGE predictions and used to analyze a database of 70 SFD-minus equilibria. The optimal location for the entrance to a divertor cryopumping system on NSTX–U is computed for enabling sufficient pumping performance with acceptable power loading in a variety of SFD-minus configurations. UEDGE simulations of one promising equilibrium from the database indicate that a significant redistribution of power to the divertor legs occurs as a result of neutral particle removal near one of the SFD-minus strike points in the outboard scrape-off layer. It is concluded that pump placement at the optimal location is advantageous as the large number of compatible equilibria reduces the precision required of real-time SFD configuration control systems and enables acceptable divertor solutions even if UEDGE-predicted power redistribution slightly reduces the achievable pumping performance.
ISSN:2352-1791
2352-1791
DOI:10.1016/j.nme.2019.03.003