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Description
In JET ITER-like-wall low-confinement mode plasmas the radial scrape-off layer (SOL) measured profiles of electron temperature and density at the low-field side (LFS) midplane and the ion fluxes to the LFS target plates were the same, within the uncertainties of the measurements, for the same volume-averaged core density profiles, independently of the LFS strikepoint connected to a horizontally or vertically inclined target plate. The similarity of the profiles is in contrast with simulations performed using the coupled edge fluid, kinetic neutral Monte-Carlo code EDGE2D-EIRENE [1,2], predicting electron temperatures at the LFS strikepoint ($T_{e,LFS-sp}$) significantly lower in the vertical than in the horizontal divertor plasma configuration for the same high-density conditions at the LFS midplane. Understanding the impact of the divertor geometry on the neutral particle confinement and the reduction of power loads to the plasma facing components are critical for validating divertor design tools for next-generation fusion-power plants.
For low-recycling conditions, i.e., $T_{e,LFS-sp}$ above 20 eV, reconstructions of the LFS plasma using the OEDGE code constrained by the measured ion saturation current, $j_{sat}$, and $T_e$ profiles at the LFS target predict the radial profiles of $T_e$ and electron density ($n_e$) in the SOL at the LFS midplane within 10% of the measured profiles for both divertor plasma configurations. For high-recycling conditions, $T_{e,LFS-sp}$ of 5-10 eV, OEDGE predicts $T_e$ at the LFS midplane for both divertor plasma configurations, but overpredicts $n_e$ at the LFS midplane by factor of 2. For the vertical divertor configuration, lowering the assumed $j_{sat}$ by 30% improves the agreement of the simulated $n_e$ with the LFS midplane measurements. Furthermore, the agreement between the simulated and the measured line-integrated Balmer-๐ผ emission across the LFS divertor plasma is improved with lowering the assumed $j_{sat}$.
Scans of the OEDGE input and model parameters for both divertor plasma configurations are carried out to characterize their impact on the 2D poloidal $T_e$ and $n_e$ profiles in the divertor and to assess their validity against 2D poloidal $T_e$ and $n_e$ profiles inferred from tangentially viewing cameras [3]. The role of recombination and molecular processes on the predicted Balmer-๐ผ and Balmer-๐พ emission is assessed.
[1] Groth et al., J. Nucl. Mater., 463 (2015), 471.
[2] Moulton et al., Nucl. Fusion 58 (2018) 096029.
[3] Karhunen et al., Nucl Mater. Energy., 25 (2020) 100831.