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

Abstract

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.

Numerical and laboratory simulations of auroral acceleration

“…19 Electron waves appear on the high potential side appear also in the simulations presented in this paper and in our recently published simulations of auroral flux tubes. 28 Such waves can scatter particles in velocity space, forming populations that are trapped between the double layer and the magnetic mirror.…”

“…Experiments, simulations, and theory showed that these spikes formed as electrons accelerated in a double layer entered a density gradient on its high potential side. [19][20][21][22] The modelling of auroral flux tubes has been approached in different ways. Knight 23 used a stationary kinetic model to find the current-voltage relationship of the flux tube, assuming a monotonic potential profile.…”

Radiation from an electron beam in a magnetized plasma: Whistler mode wave packets

“…The experimental device is named the Green Tank and is shown in Figure 1. It is described in detail elsewhere [ Gunell et al , 1996a]. It is operated in a cathode sheath configuration that was also used in several of the previous studies of the HF spike [ Gunell , 1997; Gunell et al , 1996b; Löfgren , 1999; Wendt , 2001].…”

Can the downward current region of the aurora be simulated in the laboratory?

“…The beam travels through a narrow gap region, without significant electric fields. Such a gap region has been observed by satellites [3] and in laboratory experiments [14,18]. When a distinct gap region exists, there is a laminar flow of electrons through the double layer.…”