  We compare the transport properties of a well-characterized hydrogen plasma
for low and high current ion beams. The energy-loss of low current beams can be
well understood, within the framework of current stopping power models.
However, for high current proton beams, significant energy-loss reduction and
collimation is observed in the experiment. We have developed a new
particle-in-cell code, which includes both collective electromagnetic effects
and collisional interactions. Our simulations indicate that resistive magnetic
fields, induced by the transport of an intense proton beam, act to collimate
the proton beam and simultaneously deplete the local plasma density along the
beam path. This in turn causes the energy-loss reduction detected in the
experiment.
