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

Abstract

In NSTX Upgrade (NSTX-U), power exhaust is expected to challenge available material and heat removal technologies, which are limited to ∼10 MW/m 2 . 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/m 2 ) 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.

Modeling divertor concepts for spherical tokamaks NSTX-U and ST-FNSF

“…Prior UEDGE modeling of the NSTX-U SFD [16] indicated that SFD divertor optimization should be considered to achieve favorable neutral confinement and thus avoid sheath-limited outer target plasma conditions. In pursuit of such optimization, several different SFD configurations are studied and compared.…”

“…The divertor volume (V div ), however, varies little between the five configurations: SFD-A and -B have the largest and smallest volumes, V div = 0.19 m 3 and V div = 0.15 m 3 , respectively. Earlier research [16] studied only the CD and SFD-A. The present work evaluates an additional range of topologies given by SFD-B, -C, and -D, in which the secondary X-point is translated radially outward.…”

“…The χ i,e values of 2 m 2 s −1 at the CEI are typical of NSTX H-mode transport barriers [14,18]. In the SOL, values of 4 m 2 s −1 were determined using the same approach as [16], i.e. targeting a 3 mm heat flux width in the CD case.…”

“…The expected unmitigated divertor target heat fluxes will be more than twice as high as observed in NSTX [15]. Initial NSTX-U SFD and conventional divertor (CD) configurations presented in [3] have been evaluated with UEDGE [16]. Limited advantage was seen for the SFD compared to the CD.…”

“…Section 2 presents an optimization of the SFD for NSTX-U, building on earlier modeling [14,16]. In particular, the effect of snowflake X-point position on neutral confinement, and thus on detachment properties, is explored.…”