Estimation of 2D distributions of electron density and temperature in the JET divertor from tomographic reconstructions of deuterium Balmer line emission

Abstract

• 2D n e and T e estimates obtained from deuterium Balmer line ratios in JET divertor. • High n e observed to shift from strike point to above X-point in L-mode detachment. • Divertor T e observed to fall from 3—4 eV to 0.5—1.0 eV in L-mode detachment. • 2D estimates within 25% agreement with other spectroscopic n e and T e measurements. Estimates for 2D distributions of electron density, n e , and temperature, T e , have been obtained using reflection-corrected tomographic reconstructions of deuterium Balmer line emission in the JET divertor. The solutions are obtained by a two-dimensional Monte Carlo optimization model, based on intensity ratios of the Balmer D α , D γ and D ε emission and the ADAS photon emission coefficients with additional constraints provided by line-integrated spectroscopic measurements of n e and T e . Synthetic benchmarks utilizing EDGE2D-EIRENE simulation outputs yield reproduction of reference n e and T e distributions within 25% with statistical error estimates of 5–15%. Experimental analysis of an L-mode n e ramp shows extension and movement of the outer divertor high-n e region with n e,div up to 1.5 × 10 20 m −3 from the outer strike point to above the X-point and decrease of the divertor T e to the range of 0.5–1.0 eV within 25% agreement with the line-integrated measurements. Comparison between the local estimates and their corresponding line integrals suggests underestimation of n e and T e by up to 75% and 50%, respectively, due to the geometric effects of line integration.