Experimental results indicate that, under certain conditions, ion beams undergo severe deflection preceding the region where neutralization of the beam occurs. It is believed that this effect is related to the existence of a conducting plasma which forms in the neutral region of the beam. This effect is analyzed in terms of an axially symmetric model. The plasma is simulated by assigning appropriate boundary conditions at the plasma surface. The method consists of specifying the electric field on a trial boundary, then examining the resulting potential distribution for a unipotential surface on this boundary. A sequence of numerical solutions is found in which the trial boundary is perturbed for each iterate until the Cauchy conditions are satisfied. After the location and shape of this boundary are determined, the effect of variations in electrode potentials is examined and compared with observation. It is shown that under some conditions the plasma can significantly influence the structure of the ion beam.