Energetic Electrons from a Beam-Plasma Overstability

Publisher Website
Google Scholar
Abstract
A detailed examination is made of a large‐amplitude beam‐excited electron plasma wave, and of the acceleration to high energies of favored electrons by the electric field of this wave. In laboratory experiments x rays with energies greater than 100 keV have been observed coming from a plasma in a magnetic field, typically 2000 G, penetrated by an electron beam, typically one ampere at 10 keV. To account for the impressive energies reached by the electrons producing these x rays, an acceleration process is invoked in which individual steps of coherent cyclotron acceleration are summed stochastically. The increase in the perpendicular energy of the favored electrons enhances their magnetic mirror confinement and enables the acceleration process to occur within a small volume of plasma. The overstable mode under consideration is the double‐hump‐excited electrostatic wave in a magnetic field, with k and E not quite parallel to B0. The wave frequency is close to the electron plasma frequency, the phase velocity is close to the beam velocity. It is electrons, often with low parallel energy, which feel the wave at their own cyclotron frequency (or a harmonic thereof) which are favored in the acceleration process. The rapid growth of the electric fields to the large amplitudes requisite for the acceleration mechanism is difficult to account for, however, since electron trapping would be expected to occur and the linearized Vlasov‐equation theory loses validity. An efficient untrapping process is therefore invoked: The plasma is thought of as a conglomerate of wave regions which are individually coherent but mutually incoherent. As an electron passes from one coherent region to another it is subjected to a new electric field randomly phased with respect to the first. A short correlation length improves the ability to account for overstability growth to large amplitude, but lengthens somewhat the calculated time for electron acceleration. Computations show efficient acceleration when the electron plasma frequency is of the same order or larger than the electron cyclotron frequency.

This publication has 4 references indexed in Scilit: