Excitation and Transport of solitons and Their effects on Ion Beam Neutralization

The excitation and transportation of BGK mode electron acoustic (EA) solitons were first observed in two-dimensional particle-in-cell simulations of ion beam neutralization by electron injection. The EA solitons, which are excited by two-stream instability of trapped electrons, can reach more than tens of Debye length in longitudinal size and last more than ten microseconds. They periodically move back and forth in a pulsed ion beam, causing a circular movement of electron phase-space-density hole. The velocity of solitons is close to the thermal velocity of injected electron. The decreased electron density results in a local maximum in electric potential and the potential peak can be several times higher than that of other places without solitons. With the attenuation of solitons, the electrons captured by the pulsed ion beam gain energy from solitions and become totally thermalized. In the absence of additional electron supplement, some of hot electrons are escaped from the ion beam, leading to the increase of beam potential and the decline of neutralization degree.