  A drastic cumulation of current density caused by electrostatic repulsion in
relativistic vacuum diodes with ring-type cathodes is described theoretically
and confirmed experimentally. The distinctive feature of the suggested
cumulation mechanism over the conventional one, which relies on focusing a
high-current beam by its own magnetic field, is a very low energy spread of
electrons in the region of maximal current density that stems from a laminar
flow profile of the charged-particle beam.
