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

Abstract

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 Hα, 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.