Consistency of atomic data for the interpretation of beam emission spectra

Abstract

Several Collisional-Radiative (CR) models [1, 2, 3] have been developed to calculate the attenuation and the population of excited states of hydrogen or deuterium beams injected into tokamak plasmas. The datasets generated by these CR models are needed for the modelling of beam ion deposition and (excited) beam densities in current experiments, and the reliability of this data will be crucial to obtain helium ash densities on ITER combining charge exchange and beam emission spectroscopy. Good agreement between the different CR models for the Neutral Beam (NB) is found, if corrections to the fundamental cross sections are taken into account. First the H a and H b beam emission spectra from JET are compared with the expected intensities. Second, the line ratios within the Stark multiplet are compared with the predictions of a sublevel resolved model. The measured intensity of the full multiplet is ≈30% lower than expected on the basis of beam attenuation codes and the updated beam emission rates, but apart from the atomic data this could also be due to the characterization of the NB path and line of sight integration and the absolute calibration of the optics. The modelled n = 3 to n = 4 population agrees very well with the ratio of the measured H a to H b beam emission intensities. Good agreement is found as well between the neutral beam power fractions measured with beam emission in plasma and on the JET Neutral Beam Test Bed. The Stark line ratios and s/p intensity ratio deviate from a statistical distribution, in agreement with the CR model in parabolic states from Marchuk et al. [4]. MOTIVATIONPowerful neutral hydrogen or deuterium beams provide the dominant external heating and momentum input in most large scale tokamak experiments. For the interpretation of neutral beam (NB) heated discharges, detailed knowledge is required about the energy distribution of the neutrals (power fractions) and the attenuation of the beams in order to obtain radial proles of the fast ion deposition and hence of the heating, torque and beam driven current. For the quantitative interpretation of Charge eXchange (CX) spectra, the local NB fluxes and population of excited states in the beam are needed to convert CX emissivities into local impurity densities. All these calculations strongly rely on the accuracy of the atomic data for the NB that is provided by Collisional-Radiative (CR) models of the beam [1, 2, 3, 5, 6].When the Beam Emission Spectrum (BES) was recorded for the first time, it was immediately proposed to monitor the beam attenuation, and hence the accuracy of the effective beam stopping cross sections, by using the observed beam emission intensities [7, 8]. This replaces the accumulated error on the beam attenuation along the beam path [9], by a local error in the beam emission rate. Beam emission, when combined with Charge eXchange Recombination Spectroscopy (CXRS), also has the potential of reducing the need of an absolute calibration of the CXRS spectra and a calculation of the intersection integral between a lin...

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