Influence of self-fields on the Vlasov equilibrium and stability properties of relativistic electron beam-plasma systems

Abstract

The influence of self-fields on the equilibrium and stability properties of relativistic beam-plasma systems is studied within the framework of the Vlasov-Maxwell equations. The analysis is carried out in linear geometry, where the relativistic electron beam propagates through a background plasma (assumed nonrelativistic) along a uniform guide field B 0 e ̂ z , It is assumed that ν γ 0 ⪡ 1 for the beam electrons (ν is Budker's parameter, and γ 0 mc 2 is the electron energy), but no a priori assumption is made that the beam density is small (or large) in comparison with the plasma density, or that conditions of charge neutrality or current neutrality prevail in equilibrium. It is shown that the equilibrium self-electric and self-magnetic fields, E r s(r) e ̂ r and B θ s(r) e ̂ θ , can have a large effect on equilibrium and stability behavior. Equilibrium properties are calculated for beam ( j = b) and plasma ( j = e, i) distribution functions of the form f b 0( H, P θ , P z ) = F( H − ω rb P θ ) × δ( P z − P 0)( j = b), and f j 0(H, P θ, P z) = f j 0(H − ω rjP θ − V jP z − m iV j 2 2 ) ( j = e, i), where H is the energy, P θ is the canonical angular momentum, P z is the axial canonical momentum, and ω rj (the angular velocity of mean rotation for j = b, e, i), V j (the mean axial velocity for j = e, i), and P 0 are constants. The linearized Vlasov-Maxwell equations are then used to investigate stability properties in circumstances where the equilibrium densities of the various components ( j = b, e, i) are approximately constant. The corresponding electrostatic dispersion relation and ordinary-mode electromagnetic dispersion relation are derived (including self-field effects) for body-wave perturbations localized to the beam interior ( r < R b ). These dispersion relations are analyzed in the limit of a cold beam and cold plasma background, to illustrate the basic effect that lack of charge neutrality and/or current neutrality can have on the two-stream and filamentation instabilities. It is shown that relative rotation (induced by self-fields) between the various components ( j = b, e, i) can (a) result in modified two-stream instability for propagation nearly perpendicular to B 0 e ̂ z , and (b) significantly extend the band of unstable k z -values for axial two-stream instability. Moreover, in circumstances where the beam-plasma system is charge-neutralized but not current-neutralized, it is shown that the azimuthal self-magnetic field B θ s(r) e ̂ θ has a stabilizing influence on the filamentation instability for ordinary-mode propagation perpendicular to B 0 e ̂ z .