Beam deflection phenomena in a Pulselac post-acceleration gap

The post-acceleration of a 400-keV, 10-kA proton beam by a 200-kV magnetically insulated gap is investigated. The defections from self- and applied E and B fields are measured and compared to calculated values. Several important principles of post-acceleration gap operation are observed for the first time that are important for multigap operation. First, the beam is inadequately space-charge neutralized without gas puffs or preformed plasma in regions of transverse applied-B field to allow efficient transport. The beam is also noncurrent neutralized in these regions. Second, the applied-B field defines equipotential surfaces in the gap allowing the voltage on the gap to steer and focus the beam, and it has an axial field component that acts like a pair of solenoidal lenses to focus the beam. It is also pointed out how azimuthal asymmetries in the beam current density and cathode plasma cause beam self-field asymmetries that lead to growth of beam emittance. Finally, we discuss a model which can be used to extrapolate the experimental results to multigap operation.