Time-resolved gas temperature measurements by laser absorption in a pulsed microwave hydrogen discharge

Diode laser absorption spectroscopy is used for determination of the translation temperature of hydrogen in surface-wave-created microwave discharges in a 16 mm inner diameter quartz tube, both in pulsed and continuous modes. It is shown that the gas temperature, deduced from the Doppler profile of H α , 656.3 nm line, is almost constant along the plasma column. It increases from 1000 to 2500 K as the pressure increases from 1 to 10 Torr for microwave powers ranging from 150 to 700 W. In a steady-state discharge, the influence of pressure and power is explained by considering thermal diffusion and power balance via an analytical calculation. Over a wide range of power and pressure, the gas temperature is proportional to the injected power density. In time-resolved temperature measurements with pulsed discharges, the characteristic time for the gas heating and the establishment of the steady-state is found to be of the order of 80 µs.