High frequency bursts at a double layer

Summary form only given. The high frequency (hf) oscillations which are driven by the electron beam on the high potential side of an electric double layer are investigated in a laboratory experiment where the current is drawn out from a magnetic mirror. Two innovations in hf probe design have made it possible to achieve a combination of absolute amplitude calibration and spatial resolution which, for such high frequencies, have never been achieved before. The influence of the hf region on the surrounding plasma is important, for example, for the matching of space double layers to the surroundings. We have therefore investigated how the hf region influences the double layer discharge physics, focusing on the energy and particle balance. Up to 20% of the beam energy is converted to oscillations within a region which extends from 100 to 400 Debye lengths from the double layer, on the high potential side. This makes, with our discharge parameters, the hf waves more efficient than inelastic collisions to locally deposit beam energy in the anode plasma. On stationary probes, amplitude variations are found which closely resemble bursty plasma waves observed in space. The interpretation of such data is difficult in space where it might be difficult even to distinguish between spatial and temporal variations. Our experiment can be regarded as a detailed case study of one example of such bursts.