Bursts of high-frequency plasma waves at an electric double layer

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. A new HF probe design has made it possible to achieve a combination of absolute amplitude calibration and spatial resolution which, for such high frequencies, has not been achieved before. The HF waves convert about 20% of the beam energy to oscillations within a region extending from 100 to 400 Debye lengths on the high-potential side of the double layer. This makes the HF waves the dominant mechanism of local tapping of the beam energy, and the modified energy and particle balance is investigated. The HF waves propagate with an approximately constant phase velocity which is slightly smaller than the beam velocity. Time-averaged measurements locate them to a HF region, extending approximately from 5 to 15 cm from the double layer, with a typical half width of 10 cm for the time-averaged electric field amplitude. Time-resolved measurements show, however, a much more narrow structure. The envelope of the electric field amplitude has a single maximum within the HF region and a typical half width of 1 - 2 cm (about one wavelength) along the beam. We call this envelope `the HF spike'. The position of the HF spike changes in an apparently irregular fashion within the HF region and it moves with velocities in the range . The spatial increase and decrease in amplitude of the wave is exponential over nearly two orders of magnitudes, with electron-folding distances of only about 5 mm, so the maximum of the envelope is very sharp. The amplitude increase agrees approximately with the growth rate from linear beam-plasma theory and the maximum amplitude observed agrees with saturation by beam trapping. The strong spatial decrease in the wave amplitude is not understood. It is proposed that the motion of the HF spike is caused by fluctuations, on the ion acoustic time scale, both in the growth length and in the starting point for wave growth at the double layer, which moves back and forth. This motion is shown to be strongly correlated to the motion of the HF spike in a particular case. Amplitude modulations, as observed by a stationary probe, are found on two time scales. On a slower time scale of typically , the motion of the HF spike, together with its limited spatial extent, gives rise to a temporal burst. Within these bursts the waves are also modulated on a much faster time scale of 10 - 30 ns (the wave period is 3 ns). The HF spike cannot be interpreted as a linear superposition of waves with the constant phase velocity measured, because this wave packet would have a spatial extention at least ten times larger than the width of the HF spike.