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Description
The agreement in the EDGE2D-EIRENE predicted and the measured Balmer-$\alpha$ line emission and the molecular Fulcher band emission for a set of deuterium Ohmic plasmas in JET-ILW validates the neutral and the target recycling models in EDGE2D-EIRENE. Since the plasma-neutral interactions are the strongest in the divertor, a validated EDGE2D-EIRENE neutral model provides higher fidelity to predicting the divertor performance in future fusion devices. In low-recycling and detached conditions EDGE2D-EIRENE predicted Balmer-$\alpha$ emission is within 10% of the measurements, and in high-recycling higher by a factor of 2. The discrepancy in the predicted Balmer-$\alpha$ emission in high-recycling conditions is, in part, explained by the lack of thermo-electric currents and drifts in the current EDGE2D-EIRENE simulations. The predicted Fulcher band emission is within 20% of the measurements in all divertor conditions. The magnitude of the Fulcher band emission averaged over the low-field side (LFS) divertor is found to be within a factor of 2 from low-recycling to detached plasma conditions, which is explained by the strong dependence of the population density of the $N=3$ molecular excited state on the local electron temperature. Since the target electron temperature decreases, and the molecular density increases, with deepening detachment, the decrease in the relative population density of the upper Fulcher band state is approximately compensated by the increased molecular density.
To calibrate the molecular influx, and to validate the fuel recycling model in EDGE2D-EIRENE, a deuterium gas injection module was set up in JET-ILW Ohmic deuterium plasmas to obtain a localized calibrated gas puff into the divertor. The Fulcher band spectrum is measured using two spectrometers viewing the LFS divertor, with one viewing the injection point. The intensity of the diagonal transitions are determined [1,2], from which the rotational and vibrational temperatures are derived, and the total Fulcher band emission is estimated. The EIRENE predicted Fulcher band emission is within 30% of the measurements at the injection site in high-recycling ($T_{e,t} \approx 5$eV, $n_{e,t} \approx 1.5 \cdot 10^{19} m^{-3}$) and detached conditions ($T_{e,t} \approx 1.0$eV, $n_{e,t} \approx 3.0 \cdot 10^{19} m^{-3}$), when a gas puff is applied on the predicted plasma. Using the inverse photon efficiency [3], the estimated molecular influx is within 20% of the analytically estimated gas-injection rate, validating the inverse photon efficiency technique.
[1] E.M Hollman et al., 2006 Plasma Phys. Control. Fusion 48 1165
[2] G. Sergienko et al., 2013 JNM 438
[3] S. Brezinsek et al., 2003 JNM 313–316