Collective ion acceleration and power balance in intense electron beams in neutral gas

The documented collective acceleration of background gas ions by intense electron beams has achieved proton-to-electron energy ratios above 15, corresponding to 14 MeV protons, and is characterized by equal ion and beam front velocities. In order to guide experiments aimed at scaling to higher particle energies, it has been important to adequately understand the physics controlling the beam front velocity, β/sub f/c, which is experimentally found to be considerably less than the injected electron velocity. New proton collective acceleration data are presented that disagree with the space-charge-limited β/sub f/ models, but are in accord with a power balance β/sub f/ model that emphasizes the beam electron energy losses connected with self-magnetic field generation. Independent measurements are also presented that qualitatively confirm the predicted electron energy loss and its variation with experimental parameters