Abstract:

Spectroscopy of the hydrogen atom in magnetic and electric fields has been a subject of interest and research for nearly a century. The work presented in this thesis builds on that strong tradition and presents a novel result for the dependence of a laser-induced fluorescence (LIF) signal from the H-alpha transition in a hydrogen neutral beam passing through a background of neutral hydrogen gas. The total integrated LIF signal is found to be enhanced and the fine-structure lines present in the H-alpha spectrum are seen to vary significantly with applied perpendicular magnetic field over the range of 0-0.01 Tesla. The observation is understood as the result of mixing of the atomic fine structure levels in the presence of the Lorentz electric field perceived in the beam reference frame, and the resultant changes in the electronic state lifetimes and transition probabilities. A collisional-radiative model is presented which predicts the beam fluorescence in the presence of magnetic and electric fields by including 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 perturbation theory and includes the fine structure of the transitions as well as the Zeeman and motional Stark effects. This model is compared to experimental data under various conditions and is found to match well. The H-alpha signal enhancement in low fields could be exploited as a precise and non-perturbative magnetic field diagnostic in very low field situations. The work done in the course of this thesis has provided the foundation for the motional Stark effect with laser-induced fluorescence diagnostic (MSE-LIF) which will make available a measurement technique for magnetic field pitch angle and magnitude in plasma devices with fields lower than 1 Tesla. This direct magnetic diagnostic will allow equilibrium pressure profile reconstruction with unprecedented accuracy, and extend the realm of applicability of the established motional Stark effect (MSE) diagnostic, which has been highly successful in measuring the q-profile and radial electric fields in large tokamaks, to a new class of experimental configurations.