Radial Distribution of Absorption in a Cesium Heat Pipe with Axial Laser Heating

Diode Pumped Alkali Lasers (DPAL) have been scaled to greater than 100 W and exhibit slope efficiencies exceeding 80%, offering application for tactical laser weapons. The hybrid DPAL system combines efficient diode pumping with the good beam quality and thermal characteristics of gas lasers. Thermal effects on alkali concentration have been observed to degrade performance, while low speed flowing systems are in development. However, spatial gradients in temperature and concentrations have not previously been observed. In the present work, a 0.8 W/cm2 pump laser at the D1 frequency heats the medium in a T=50-100 deg C cesium heat pipe with 5 Torr nitrogen used for quenching. A 31 muW/cm2 diode laser probes the spectral absorbance of the cesium cell on the D2 transition with radial spatial resolution. The 300 kHz linewidth probe laser is scanned 20 GHz across the optically thick hyperfine structure, revealing absorbances of 1-5. The absorbance outside of the pumped volume is modulated by up to a factor of 2 when the pump beam is blocked, suggesting significant temperature gradients. The radial temperature profile is observed across the 1.5 cm pipe with resolution of 2 mm. The variation of pump power, nitrogen pressure, and heat pipe temperature has been provided showing distinct trends. Cesium D2 lineshapes have been obtained for several heat pipe spatial locations with the pump laser actively heating the gaseous medium.
The original document contains color images.