The interaction of intense proton beams with low-pressure (0.25 to 4 T
orr) background gases is studied to evaluate beam-current neutralizati
on during transport. Electrons to neutralize the beam are provided by
beam-induced ionization of the gas. In experiments with 1 MeV, 1 kA/cm
2 protons, net currents outside the beam envelope and electron densiti
es within the beam envelope are measured for helium, neon, argon, and
air. Net-current fractions are 2% to 8% and ionization fractions are 0
.6% to 5% for 5 to 7 kA beams. Simulations of the experiments for heli
um and argon suggest that fast electrons play an important role in gen
erating a significant fraction of the return current in a halo outside
the beam. As a result, net currents inside the beam may be larger tha
n inferred from magnetic-field measurements outside the beam. Ions at
the head of the beam are observed to lose more energy than expected fr
om collisional energy losses in the background gas.