<title>Magnetic field effects on plasma-filled backward-wave oscillators</title>

Abstract

ABSTRACT Computer simulations of plasma-filled backward wave oscillators reveal that the gain of voluminous beam-plasma cyclotron interactions is significantly larger than that of the conventional backward wave oscillation. This stow plasma cyclotron wave is emiued from plasmas and its gain is substantially enhanced only if it also satisfies the dispersion relationof a ripple wall waveguide which couples the slow wave to fast wave. The observation may be employed to interpret previousexperiments which display a strong dependence of output power on the magnetic field and can be utilized to build high gainmicrowave beam-plasma devices. 1. INTRODUCTION Recently, a considerable amount of effort15 has been undertaken towards the production of ultra high power microwave sources based on backward wave oscillator (BWO) configurations. In which a rippled wall slow-wave structureprovides a very efficient way of converting the kinetic energy of relativistic electron beams into microwave radiations.One of the schemes to raise the relativistic beam current and hence the output power of relativistic electronic devicesis to use a background plasma to provide space charge neutralization. The plasma may also induce some other effects whichcould influence the outcome of backward wave oscillators. Experimentally, it has been observed6 that the efficiency can beincreased by a factor of eight when the plasma frequency o 0.2 cko (k is the ripple wall wavenumber and c is the speed oflight) and the enhancement mechanism was attributed to the excitation of beam-plasma backward wave instability. Veryrecently, computer simulation results7 have demonstrated that a more plausible explanation for the efficiency enhancement isthat a dense background plasma tends to reduce the phase velocity of the most unstable mode in a BWO and causes the beamto convert more energy to the wave.The investigations described above all impose a strong magnetic field to confine the electron motion to only onedimension. In vacuum BWOs8, the beam cyclotron resonance absorbed the BWO emission and introduced a dip in the curve ofthe output power versus the magnetic field. This issue will not be treated in this paper. On the other hand, experiments9 inrelativistic plasma-filled BWO illustrated that the output power peaked at a certain magnetic field and they conjectured that theresonance was due to