A collisional-radiative model including sublevel parameters (CRISP) for H-alpha radiation

We have developed a new kind of collisional-radiative model for a hydrogen beam which could find wide application in fusion-related experiments. The model differs from previously reported models in that it incorporates detailed calculations of the variation with the magnetic field of the transition probabilities and energy levels of all the fine-structure sublevels of the n = 1, n = 2 and n = 3 states of hydrogen. This technique is termed CRISP, for collisional-radiative including sublevel parameters. The model can treat separately all 18 sublevels of the n = 3 level, all 8 sublevels of the n = 2 level and both 2 sublevels of the n = I level. In this paper, we describe the model, which includes the effects on electronic level populations of collisional processes with background gas as well as radiative transitions. The dependence of the radiative transition terms in the collisional-radiative model on background fields is calculated using the perturbation theory and includes the fine structure of the transitions as well as the Zeeman and motional Stark effects. We also present results which agree well with experimental data for an enhanced laser-induced fluorescence signal seen from a hydrogen neutral beam in a perpendicular applied magnetic field of 0-10 mT with a background of neutral gas. This work may have application in the measurement of magnetic fields in environments that are not appropriate for magnetic probes. We expect the CRISP model approach to have application in calibration of motional Stark effect diagnostics on existing fusion experiments, and an extension of the modelling may enable an alternative motional Stark effect measurement of the magnetic field magnitude and direction on many devices.