A gas discharge is described which is powered by microwaves of frequency 2400 Mc/sec and which may be run at low power with sufficient stability for direct recording spectroscopy. Discharges in helium have been cooled with liquid helium; the rate of consumption of the coolant is very low and Doppler temperatures of 10deg;K have been obtained; under these conditions the discharge seems to be brighter than comparable dc discharges. The microwave discharge has also been operated cooled with liquid nitrogen and uncooled at room temperature.
J. M. VAUGHAN, "Microwave Discharge Tube with Liquid Helium Cooling," J. Opt. Soc. Am. 54, 318-319 (1964)