Atomic models for the motional Stark effect diagnostic

We present detailed atomic physics models for the motional Stark effect (MSE) diagnostic on magnetic fusion devices. Excitation and ionization cross sections of the hydrogen or deuterium beam travelling in a magnetic field in collisions with electrons, ions and neutral gas are calculated in the first Born approximation. The density matrices and polarization states of individual Stark-Zeeman components of the Balmer a line are obtained for both beam into plasma and beam into gas models. A detailed comparison of the model calculations and the MSE polarimetry and spectral intensity measurements obtained at the DIII-D tokamak is carried out. Although our beam into gas models provide a qualitative explanation for the larger pi/sigma intensity ratios and represent significant improvements over the statistical population models, empirical adjustment factors ranging from 1.0 to 2.0 must still be applied to individual line intensities to bring the calculations into full agreement with the observations. The analyses of the filter-scan polarization spectra from the DIII-D MSE polarimetry system indicate unknown channel and time-dependent light contaminations in the beam into gas measurements. Such contaminations may be the main reason for the failure of beam into gas calibration on MSE polarimetry systems.