Investigation of microwave excited CO2 laser discharges

Abstract

Microwave discharges for the excitation of CO2 lasers have been investigated. A waveguide structure with closed front and end, and with the discharge tube located parallel to the waveguide axis is used. The field distribution within the waveguide but outside the discharge tube is measured before and after ignition of the discharge. The wavelength of the microwave is reduced by the discharge plasma. The side-on light emission is detected. Light emission is detected over a distance within which the measured field strength in the waveguide varies fivefold. The experimental results are compared to the results of a self-consistent one-dimensional microwave discharge model. The model includes an equation for the electron density, the neutral gas density and a simplified wave equation for the electric field. The results show, in accordance with experimental results, a reduction of the wavelength due to the discharge plasma and the discharge excited over a range within which the electric field outside the plasma varies appreciably. In contrast to the electric field outside the plasma, the electric field inside is almost constant within the excited region. This is due to the high plasma conductivity which is a result of the high electron density which reaches values of up to 1012 cm-3. This leads to a power density up to several tens of W cm-3. In order not to overheat the gas the discharge has to be operated in a pulsed mode. Another method of limiting the gas temperature is to use a waveguide structure with a small electrode spacing or with a fast gas flow.