EFFECT OF TARGET-EMITTED IONS ON THE FOCAL SPOT OF AN INTENSE ELECTRON-BEAM

Intense electron beams are known to produce surface plasmas when they strike typical target materials. The beam space charge then draws an i on current that can substantially affect the beam dynamics. We conside r these effects for two intense-beam sources: (I) a needle-tip cathode immersed in a strong magnetic field, and (2) an induction accelerator -produced beam that is focused onto a field-free target. For the first case, the dominant effect is an electrostatic dipole instability betw een the beam and the ions. A sufficiently large magnetic field can kee p the oscillation amplitude to a low level. For the second case, the d ominant effect is that the focal length changes in time. For the ''wor st case'' of space-charge-limited proton emission, the predicted effec t is severe. The actual abundance and charge state of the target-plasm a ions will need to be measured or inferred from planned experiments. Mitigation schemes also are discussed briefly.