  Propagation of charged particle beams in background plasma as a method of
space charge neutralization has been shown to achieve a high degree of charge
and current neutralization and therefore enables nearly ballistic propagation
and focusing of charged particle beams. Correspondingly, use of plasmas for
propagation of charged particle beams has important applications for transport
and focusing of intense particle beams in inertial fusion and high energy
density laboratory plasma physics. However, the streaming of beam ions through
a background plasma can lead to development of the two-stream instability
between the beam ions and the plasma electrons. The beam electric and magnetic
fields enhanced by the two-stream instability can lead to defocusing of the ion
beam. Using particle-in-cell (PIC) simulations, we study the scaling of the
instability-driven self-electromagnetic fields and consequent defocusing forces
with the background plasma density and beam ion mass. We identify plasma
parameters where the defocusing forces can be reduced.
