  Unusually long electrostatic solitary waves (ESWs) are discovered in 2D and
3D Particle-in-Cell studies of the process of ion beam neutralization by
electron emission from filaments. These ESWs are long because trapped and
untrapped electron density perturbations nearly compensate each other. Surface
waves were discovered in the process of neutralization but were only observed
in 3D simulations. This is because the phase velocity of surface waves in a 2D
geometry is higher than in 3D giving the high-energy electrons generated
upstream near the electron source enough energy to excite such waves only in 3D
cylindrical beams.
