Speaker
Description
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Scrape-off layer (SOL) transport simulations including magnetic and ExB drifts flows, performed with the multi-fluid edge-plasma code UEDGE, predict passive stabilization of the detachment front along the low-field side (LFS) divertor leg when main-SOL pumping upstream of the LFS divertor target is applied. With the ion $\mathbf{B}\times\nabla\mathrm{B}$ drift directed into the divertor, the $\mathrm{T_e}=1.5~\mathrm{eV}$ detachment front stabilizes at the pumping plenum with increasing main ion gas injection, maintaining $\mathrm{T_e}\gt80~\mathrm{eV}$ at the X-point, when the pump duct entrance is located 7-9 cm upstream of the divertor target. Neutral pressure buildup downstream of the pumping plenum, necessary to remove the injected particles and obtain particle-balance, induces particle, momentum, and power losses through plasma-neutral interactions. These plasma-neutral losses detach the LFS divertor without the radiation front moving upstream to the X-point, as typically observed for DIII-D [1,2]. UEDGE predicts plasma burn-through of the target-shielding neutrals for pump-to-target distances below 7 cm, whereas pump-to-target distances longer than 9 cm are predicted to reduce the interval of gas-injection rates for which $\mathrm{T_e}\gt80~\mathrm{eV}$ at the X-point are sustained.
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The simulations are based on a 12.5 MW DIII-D Stage 2 divertor reference geometry with a 6 cm wide pumping plenum situated 25-31 cm downstream of the X-point [3,4]. Predictive simulations performed with reduced X-point–to–pump distances indicate passive stabilization of the detachment front can be sustained for 17 cm X-point–to–pump separation with similar development of the X-point Te as a function of gas injection rate in the reference geometry. Passive stabilization of the detachment front is also predicted by UEDGE when the divertor is pumped upstream of the target on the private-flux side but at higher gas injection rates. The UEDGE simulations, evaluated for steady-state conditions, consider intrinsic carbon and seeded neon impurities as separate neutral and ion species in the simulations. Experimental measurements from the highly diagnosed dissipation-focused Stage 2 Chimney divertor, expected operational in 2027, will provide critical validation of the predictive UEDGE simulations, used in its design, underpinning FPP-relevant divertor design. Carbon-free UEDGE simulations of a DIII-D–like metallic FPP, assessing detachment-front stabilization with upstream pumping in the absence of intrinsic carbon, will also be presented to guide FPP design.
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[1] A.E. Jaervinen, et al., Nucl. Fusion $\mathbf{60}~(2020)~056021$
[2] A.G. McLean, et al., J. Nucl. Mat. $\mathbf{463}~(2015)~533$
[3] J. Yu et al., Nucl. Mat. Energy $\mathbf{41}~(2024)~101826$
[4] A. Holm et al, Mat. Energy $\mathbf{41}~(2024)~101782$
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$\mathrm{This}\;\mathrm{work}\;\mathrm{was}\;\mathrm{supported}\;\mathrm{in}\;\mathrm{part}\;\mathrm{by}\;\mathrm{the}\;\mathrm{US}\;\mathrm{Department}\;\mathrm{of}\;\mathrm{Energy}\;\mathrm{under}$ $\texttt{DE-FC02-04ER54698}$, $\texttt{DE-AC52-07NA27344}$, and $\texttt{DE-AC05-00OR22725}$. $\texttt{LLNL-ABS-2014170}$