Propagation of an intense electron beam through a gap

Experiments and simulations for the propagation of intense relativistic electron beams that pass through a gap are carried out. The radius of the drift tube increases at the gap entrance and it returns to the initial radius after the gap length. Though the injected beam current into the gap is larger than the space charge limiting current at the gap, it passes through the gap without loss when the gap length is short. A criterion is proposed for the gap length through which the beam current passes without loss.