Excitation Of Microwave Radiation By A Longitudinal RF Probe In A Penning Reflex Discharge

A Dicke radiometer (f = 8.8 GHz) is employed to detect the emissions from a Penning Reflex Discharge (PAD). The source of the radiation is a shielded RF probe immersed in the air plasma. This straight probe has a molybdenum center-conductor that is oriented along the plasma radius. The excitation frequencies are between 0.45 and 1 .30 GHz. Observed radiation is measured under a variety of plasma conditions and as a function of radial probe position. The relative power level of the radiation, as a function of probe signal level, is also obtained as plasma parameters are varied. Results will be presented which clearly show that these emissions occur well below the required threshold for conventional cyclotron-harmonic radiation which is about 1.1 0 A. Measurements indicate that the radiated power level does not exhibit the same variation with magnetic field as the cyclotron-harmonic emissions. From the results of the experiments mentioned above, conclusions are drawn regarding the possible non-linear effect of the plasma, on waves excited by the probe. The relationship between this and other possible sources for plasma waves in a Penning discharge will also be discussed.