[en] Numerical and analytic studies of collective ion acceleration in intense relativistic electron beams are presented. Investigation of autoresonant acceleration has shown that radial beam inhomogeneities distort the slow cyclotron wave into a surface-localized mode. Variation of linear wave fields in inhomogeneous magnetic fields is strongly affected by this. Numerical studies of self-consistent cyclotron waves show the persistence of linear characteristics even at large amplitudes. Propagation of large amplitude waves has been observed over moderate distances in simulations without significant attenuation or nonlinear disruption. Ion acceleration in virtual cathodes has also been studied. Insights into both formation and late time dynamics have been gained. Increased virtual cathode understanding is being pursued toward defining optimal configurations. Future plans are outlined