Effects of axial momentum spread on the electron-ion two-stream instability in high-intensity ion beams

Use is made of the Vlasov-Maxwell equations to describe the electron-ion tw o-stream instability driven by the directed axial motion of a high-intensit y ion beam propagating through a stationary population of (unwanted) backgr ound electrons. The ion beam is treated as continuous in the z-direction, a nd the electrons are electrostatically confined in the transverse direction by the space-charge potential produced by the excess ion charge. The analy sis is carried out for arbitrary beam intensity, consistent with transverse confinement of the beam particles, and arbitrary fractional charge neutral ization by the background electrons. For the case of overlapping step-funct ion ion and electron density profiles, corresponding to monoenergetic elect rons and ions in the transverse direction, detailed stability properties ar e calculated, including the important effects of an axial momentum spread, over a wide range of system parameters for dipole perturbations with azimut hal mode number l=1. The two-stream instability growth rate is found to inc rease with increasing beam intensity, increasing fractional charge neutrali zation, and decreasing proximity of the conducting wall. It is shown that L andau damping associated with a modest axial momentum spread of the beam io ns and background electrons has a strong stabilizing influence on the insta bility. (C) 2000 Elsevier Science B.V. All rights reserved.