A comparison between theory and laser spectroscopic measurements for a hydrogen plasma under high-intensity resonant Balmer line irradiation

Abstract

Observations are reported of a well diagnosed 1 eV hydrogen plasma before, during and after irradiation by a long-lasting high-intensity laser pulse resonant with a Balmer transition. The observations include time-resolved measurements of fluorescence scattering observed on H alpha , beta and gamma during H alpha irradiation, H beta scattering during H beta irradiation, and the H beta optical depth during H alpha irradiation. Comparisons are made with predictions based on a time-dependent solution of the collisional-radiative rate equations for a multilevel model of laser-pumped hydrogen atoms in the plasma, using widely accepted electron collision rates. Some aspects of the plasma's response to resonant irradiation, e.g. the initial transient behaviour of both direct and collision-induced fluorescence, are in good agreement with theory. However, the theoretical model fails by substantial amounts to predict the absolute (as opposed to relative) initial level populations, the relaxation of level populations both during and after the laser pulse, and the long-time quasi-equilibrium fluorescence intensity during laser irradiation. Possible explanations are discussed.