Transient Gas Motion in Nuclear-Reactor-Pumped Lasers: Computational and Experimental Results

Abstract

In some types of nuclear-reactor-pumped lasers, the fission fragments that are used to excite gaseous lasing species heat the gas in a spatially nonuniform manner. This heating nonuniformity induces transient gas motion, which results in density and refractive-index gradients that affect the laser's optical behavior. A model of the transient gas motion is developed using the acoustic filtering methodology, which neglects the spatial variation of the pressure. This model incorporates the effect of spatially varying gas density on fission-fragment heating. Gas motion out of the laser cell into small, rapidly cooled regions is treated as a volumetric mass loss distributed over the laser cell. Although these regions have a relatively small fraction of the total volume, a large amount of gas can flow into them during the heating because of the rapid cooling therein. This gas removal from the cell during pumping, neglected in previous analyses, is important because fission-fragment heating is strongly dependent on local gas density. To quantify the laser's optical behavior, experiments are performed in which a probe laser beam is passed through the laser cell. This probe beam acquires a wavefront distortion from the refractive-index gradients and is imaged onto a wavefront slope sensor, which yieldsmore » temporally and spatially resolved measurements of the angular deflection (wavefront slope) of the probe laser beam. Experimental and model results for this quantity exhibit reasonably good agreement over a wide range of pressures and heating amplitudes. 14 refs., 12 figs.« less