The interaction of an electron beam with a relatively dense background plasma was examined through numerical experiments, where the beam velocity is much greater than the thermal velocities. It is shown that the initial growth of the unstable waves is described by the two-stream instability and that the phenomena can be separated into two regions, (high and low density) depending on whether or not merging of space-averaged velocity distributions occurs during the growth of the initially most unstable wave. The occurrence of these regions is shown to follow from energy and momentum considerations. The long-time behavior of the low-density regime, where the merging does not take place during the initial growth of the instability is shown to be described quite well by quasilinear theory.
A ring-beam distribution function of moderately relativistic electrons is unstable to electromagnetic and electrostatic waves. The results obtained in numerical simulations show that electromagnetic radiation corresponding to the normal modes of the background plasma is observed to grow even in the presence of a strong electrostatic instability and becomes very strong when the growth of the electrostatic Langmuir waves is minimized, and that the instability process is best described as a beam cyclotron resonance. Another strong radiation generated by the ring beam is the Z mode which is coupled to the electrostatic Langmuir wave. Under certain circumstances, these mechanisms may be significant in astrophysical situations.
The axial drift of a relativistic electron beam along a uniform magnetic field is shown theoretically to lead to significant frequency up-shifts for linear cyclotron-wave instabilities driven by velocity-space anisotropy. This mechanism suggests novel means for the development of devices for generation of intense far-infrared power.
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