  Ion beam charge neutralization by electron injection is a complex kinetic
process. Recent experiments show that resulting self-potential of the beam
after neutralization by plasma could be much lower than the temperature of
plasma electrons [Physics of Plasmas 23, 043113 (2016)], indicating that
kinetic effects are important and may affect the neutralization of ion beam. We
performed a numerical study of the charge neutralization process of an ion beam
making use of a two-dimensional electrostatic particle-in-cell code. The
results show that the process of charge neutralization by electron injection is
comprised of two stages. During the first stage, the self-potential of the beam
is higher than the temperature of injected electrons (Te/e). Therefore, the
neutralization of the ion beam is almost unaffected by Te, and all injected
electrons are captured by the ion beam. At the second stage, with the decline
of the beam potential (), hot electrons escape from the ion beam, while cold
electrons are slowly accumulated. As a result, can be much lower than Te/e. It
is found in the simulations that during the accumulation of cold electrons,
scales as . In addition, the results show that the transverse position of
electron source has a great impact on ion beam neutralization. Slight shift of
electron source leads to large increase of the beam potential because of
increase in potential energy of injected electrons.
