The electron cyclotron instability and high-frequency ionization in a beam-plasma experiment

Abstract

A beam-plasma experiment has been achieved using an electron beam (~2-3keV, ~10mA) guided by a magnetic field of 100-1200 x 10-4 Wb/m2, over a pathlength of about 70 cm through He gas (~10-4 torr). Plasma densities were determined by means of an 8 cm microwave cavity. By a proper choice of the parameters it is possible to have both the electron cyclotron and plasma instability present. Under these conditions the hf breakdown by the beam-generated waves is caused by the cyclotron instability. The related density fluctuations distort all measurements on the cyclotron instability, like peak width, peak amplitude, and the dependence on parameters, for which we have chosen magnetic field and gas pressure. Only in a very limited parameter range, with a weak instability, it is possible to compare results with theory. Despite the dc character of the experiment, peak amplitudes could be compared with a linear theory because the instabilities appeared to be pulsed, with pulses rising spontaneously from noise level. Each pulse grows for a fixed time of ≈5 x 10-8 s, regardless of the magnitude of the plasma density. The pulses appear in groups of about 10, taking together ≈20 μs, and in this time plasma electrons gain energies up to 200 eV. The repetition of such groups of pulses is quite regular, with a frequency of ≈10 kHz, depending on ne. Plasma electrons loose energy by ionizing the neutral gas. The increase in plasma density during a group of pulses is a quadratic function of the magnetic field and may be as large as a factor ten. If two or more interaction peaks are present in the time integrated spectra, the pulses of these instabilities alternate in time.