Electron beam injection experiments: The beam‐plasma discharge at low pressures and magnetic field strengths

Abstract

Abslract. A column of enhanced density plasma,exceeding the density expected from ionization by primary beam electrons, has been observed in a large vacuum system at low magnetic fields (1 to 1.5 G) and low ambient pressures (10 -6 to 10 -s torr). The peak luminosity of the discharge is about 10 times that of the beam alone, with a radius increase by a factor of 3. In the absence of the discharge, RF emission is observed at 1.1 to 1.2 fc-A strong band of RF noise with upper frequency cutoff at about fc is observed in the discharge mode, along with higher frequency noise at or near the plasma frequency.The onset of the plasma discharge is critically dependent on beam current.The present results agree with observations made at much higher densities and magnetic fields in fusion research studies. There has been difficulty in explaining many of the observational results obtained in rocket flights with modest and high current electron accelerators (Winckler, 1976, 1977). In particular, these include 1) the apparent neutralization of the vehicle under conditions where the return electron flux from the ambient plasma or the beam-produced ionization (based on the classical ionization rate of the local neutral gas) to the vehicle should have been much less than the emitted flux, and 2) the presence of a plasma cloud with increased Te and density surrounding the vehicle (Cartwright et el., 1977). We have recently concluded a series of electron beam experiments in the very large vacuum chamber at Johnson Space Center which can provide a plausible explanation for the flight observations. The experimental configuration is basically similar to that described by Bernstein et al. (1975, 1977) and is shown in fig. 1. The experimental conditions were as follows: 1. A tungsten cathode, convergent flow electron gun was used for most measurements; although the gun perveance is % 1.4x10 -6, the maximum beam currents and voltages employed were 100 ma and 2 kV respectively. The gun was operated DC. A pulsed electron gun was also operated for a short period of time. Although the guns could be isolated electrically, the present measurements were made with both the gun and collector grounded to the chamber walls. 2. A set of three coils have been added around the chamber periphery; the total variation in field strength along the beam path was % 15%. Most measurements were made at total mean field strengths ranging from 1.0-1.45 G. Typical beam injection pitch angles were <20 o . The path length between gun and collector was % 20 m. 3. The base pressure in the system was lx10 -6 torr, consisting primarily of water vapor (30%) and N 2. Increases in pressure to lx10 -s torr were accomplished with the addition of dry N 2.This pressure range corresponds to the altitude range 120-180 km; although rocket-borne accelerators have been flown at higher altitudes it is probable that the rockets are always surrounded by gas clouds of similar density which are produced by outgassing and residual motor exhaust. As shown in fig.

Plasma Waves Stimulated by Electron Beams in the Lab and in the Auroral Ionosphere

“…Lab studies of BPD at the NASA/JSC chamber have been conducted for some time (Bernstein-et al, 1978). These investigations have shown that the radio frequency waves above 1 MHz may not be large enough in amplitude to provide the necessary energization for BPD (Bernstein and Kellogg, 1980).…”

Time-Dependent Plasma Behavior Triggered by a Pulsed Electron Gun under Conditions of Beam-Plasma-Discharge

“…This critical current level yields a marked increase in ion-pair-production, a greatly enhanced 3914 X emission, a modification of the primary beam velocity distribution and the emission of intense RF waves. The BPD has been the subject of a continuing series of space-simulation experiments (Bernstein et al, 1978;Bernstein et al, 1980and Jost et al, 1981) One such recent work (Walker et al, 1981) has determined that the density-related plasma condition for BPD threshold +1.3 can be expressed as w = (5.8 -1.9 ) Wc, where (w ,w ) are p -1 9p…”

“…One of the areas in space-related beam-plasma interactions to receive considerable attention in recent years has been the collective plasma process called the beam-plasma-discharge (Bernstein et al, 1978;Bernstein et al, 1980;and Jost et al, 1981).…”

Electrostatic wave observation during a space simulation beam‐plasma discharge

“…The Beam-Plasma-Discharge (BPD) instability (Getty and Smullin, 1963) has re-emerged in recent years (Smullin, 1981;Bernstein et al, 1978) as a fundamental process to be considered by designers of space vehicle-borne electron accelerator experiments. Because of concerns over vehicle neutralization during these experiments and an interest in further understanding of the BPD process (Cambou, 1975(Cambou, , 1978) a series of space simulation investigations of the BPD phenomenon was undertaken in the large vacuum chamber facility at…”

“…Johnson Space Center (Bernstein 1978(Bernstein , 1979Szuszczewicz 1979Szuszczewicz , 1982aPapadopoulos, 1982;Anderson, 1981 where VB is the beam energy in volts, B the magnetic field in gauss, P the ambient neutral particle pressure in torr, and L the beam length in meters.…”