Ion divergence generated by nonuniform current density of intense ion beams

Abstract

Large electric and magnetic fields are generated by the space charge and current of intense ion beams. These fields induce ion divergence if the beam current density is not uniform. Within the accelerating region the deflection of ions due to the electric fields is much larger than ion deflection from the self-magnetic fields. An analytic technique is presented to calculate the electric fields generated by the static nonuniform beam space charge and the resulting ion trajectories during beam acceleration. It is shown that modest nonuniformities in the ion current density can result in significant ion divergence. Downstream of the ion acceleration region, the ion space charge is neutralized by the presence of electrons. Consequently, the self-magnetic field of the beam dominates the ion trajectories in this region. Analytic estimates of the beam divergence produced by the self-magnetic fields indicate that significant ion divergence can be generated by this process as well. A distinguishing feature between these two mechanisms is that the space-charge-induced divergence is largest for perturbation scale lengths smaller than the acceleration gap, while the converse is true for the divergence generated by the self-magnetic field of the beam. Both of these effects place uniformity requirements on ion sources.