On the theory of ion-beam plasma instability

High-and low-frequency convective instabilities of an ion beam in a longitudinal magnetic field in the presence of the gas entering the ion transportation channel from the plasma source are studied theoretically. Spatial growth rates of the beam instabilities are found and mechanisms for the nonlinear saturation of potential oscillations are investigated in the hydrodynamic approximation. The averaged (over the oscillation period) effective beam emittance associated with collective fast-ion heating is determined. The results obtained can be applied to calculating the dynamics of negative ion beams in the injectors of linear and cyclic accelerators.