Microwave measurement of decaying plasma in liquid helium

Summary form only given, as follows. a pulsed discharge in liquid helium (LHe).\´ Novel results of measurement by microwave cavity method on decaying plasma in liquid helium are described. An X-band cylindrical TE mode cavity is immersed in a 50 liter LHe tub. Tungsten needle electrode is installed in a small room separated by stainless steel thin mesh from the cavity filled with LHe with saturating vapor pressure at temperature range from 4.2 to I .7 K. High-voltage pulse discharge, less than 20 kV, current 400 A and duration 6 ysec, is tired between a needle electrode and the mesh. Response of microwave cavity is measured after the pulsed discharge. The localized cryogenic plasma is produced near the stainless-steel mesh. The plasma is observed by microwave transmission through the cavity. The decaying plasma is characterized by two distinct signals for each of single shot, repeatedly. The first is prompt response with typical decay time of 100 ys that accompanies low-frequency vibration of 500 KHz. The response is considered to result from a localized high density decaying plasma near the mesh. The second is slow response from slowly decaying afterglow plasma appearing with time on the order of 500 ms after the pulsed discharge. The fast signal appears as a result of recombination in the decaying plasma, while the slow response results from diffision loss mechanism. Slow response is observed only for temperature above 2.3 K at saturating vapor pressure. The plasma density is estimated from the shift of resonant curve caused by decrease in the volume due to the presence of localized plasma to be on the order of 10" cm~\´. In OUT estimation of plasma density, the conventional large effective masses of ions in LHe obtained by Poitrenaud and Williams [2] were assumed. The mass of the charged carriers in the plasma could he much smaller than the conventional values, since the plasma density related to the slow response is expected to be much lower than 1014 cm".