Very slowly decaying afterglow plasma in cryogenic helium gas

Summary form only given. The purpose of the present experiment is to observe extremely slowly decaying cryogenic afterglow plasmas. It is well known that decay time of afterglow plasma in cryogenic helium gas is elongated by plasma production caused by collisions between metastable molecules. As a result, slow decay times on the order of a few msec were observed in cryogenic helium gas at 4.2 K. The loss mechanism was electron-ion recombination, since they observed densities more than 10/sup 9/ cm/sup -3/. We try to measure a late afterglow period with low densities less than 10/sup 8/ cm/sup -3/ in cryogenic helium gas where the dominant loss mechanism is ambipolar diffusion. We fabricate a comparatively large stainless-steel cylindrical discharge vessel with diameter 16.6 cm and 9.1 cm in length that is a TE(011) mode cavity for 2.84 GHz with Q value larger than 1000. The diffusion length, 2.2 cm, of the present discharge vessel is much larger than those in the previous experiments on cryogenic afterglow plasmas. The plasma is produced repeatedly between tungsten needle electrodes by high-voltage pulse of 15 kV, 600 A and duration 3 micro-sec. Gas pressure is varied from 0.06 to 3 Torr. The plasma decay with time constant longer than I sec in cryogenic helium gas at 4.2 K is measured by an improved method of microwave interferometer. We can see very slowly fading fluorescent light with our naked eyes. The decay time is elongated by increasing power of CW microwave for observing the plasma parameters. Also, the decay time increases if gas temperature is cooled below 4.2 K.