Experimental measurements of bifurcations, chaos and three cycle behaviour on a neutralized electron beam

Publisher Website
Google Scholar
Abstract
An experiment involving the natural oscillations on an electron beam propagating parallel to a magnetic field in a low pressure gas is described. For conditions such that omega pe2<< omega ge2 and beam currents above a few mu A, the beam system behaves as if it were approximately neutralized by cold ions. Increasing the beam current, Ib, increases the frequency and amplitude of the oscillation until period doubling occurs. With a further small increase, many more period doublings occur in rapid succession until the spectrum of the oscillations is essentially flat out to the original frequency f0 above which it rapidly decreases. This state remains as Ib is increased until, suddenly, a perfectly coherent three cycle oscillation appears with a periodicity of three times that of f0; with a small further increase of Ib, the three cycle becomes a 6 cycle and then the chaotic regime returns. The frequency f0 agrees well with the ion plasma frequency of the neutralized electron beam and is probably driven by some form of shear instability. By considering only the amplitude of the oscillations the evolution of the system has been successfully modelled with a simple one dimensional difference equation which takes account of the competition between exponential growth of the instability and quadratic saturation leading to the final steady state.