Excitation of surface waves in 3D ion beam neutralization

Neutralization of beams with 2D and 3D geometries by the electrons emitted from an external source is studied using particle-in-cell simulations. Our work reveals that the high-energy electrons excite Trivelpiece-Gould (TG) surface waves in the beams with 3D axisymmetric geometries. These high-energy electrons are generated because of a large amplitude electrostatic solitary wave (ESW) that forms near the electron source and has an electric potential amplitude more than three times the electron thermal energy. We also find that surface wave excitation only happens when the beam radius is large enough at the ion source to attract enough electrons that could form the large amplitude ESW. A comparison of the 3D TG surface wave dispersion relation with an expression for 2D surface waves reveals that they become excited in 3D axisymmetric but not in 2D planar beam because of a higher phase speed requirement in the latter case.