  Producing a dense background plasma for neutralization purposes is
experimentally difficult and requires a large amount of energy. We show that
even an underdense background plasma with a small relative density can achieve
high neutralization of intense ion beam pulses. Using particle-in-cell
simulations, we show that if the total plasma electron charge is not comparable
to the beam charge, electron emitters are necessary for effective
neutralization, but are not needed if the plasma volume is large. Several
plasma densities are investigated, including the case of electron emitters
without plasma, which does not effectively neutralize the beam. Over 95%
neutralization is achieved for even very underdense background plasma. We
compare with an analytic model of neutralization and find close agreement with
the particle-in-cell simulations. Further, we show experimental data from the
NDCX-II group that verifies the result that underdense plasma can neutralize
intense heavy ion beams effectively.
