Spatially resolved temperature measurements in a CO2 laser discharge by folded BOXCARS

Broadband nitrogen CARS thermometry is used to study a discharge plasma of a CO2 laser in a folded BOXCARS arrangement. The CO2 laser discharge is set up as a longitudinal fast flow system with a high frequency excitation of 27.1 MHZ and galvanic coupling to the discharge. The total discharge pressure amounts to 4 kPa and the partial pressure of nitrogen 900 Pa. In this discharge plasma sheaths occur close to the electrodes. The physical features of these sheaths in terms of temperature and temperature gradients influence efficiency and beam quality of the CO2 laser. The spatial resolution of the temperatrue measurements of + - 0.075 mm rectangular to the optical axis of the CARS beam allows a temperature determination in the small region of the discharge sheaths with a thickness of about 1 - 2 mm. The rotational and vibrational temperature profile perpendicular to the discharge electrodes shows a drop of the vibrational temperature of nitrogen in the regions of the plasma sheaths. Due to stimulated emission the vibrational temperature decreases by 970 K, where as the rotational temperature increases by 50 K in the maximum of laser oscillation.