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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.