  In a virtual cathode oscillator, the scattering of a high-current
relativistic electron beam by the anode mesh leads to formation of an electron
cloud near the anode. The cloud particles possess low energy and large spread
in velocities caused by multiple scattering and ionization losses. The
electrons captured by the cloud do not participate in the oscillations of the
virtual cathode and partially block a vircator. As a result, the amplitude of
the electric field oscillations is reduced. In order to increase the
oscillation amplitude, the thickness of the anode mesh should be equal to the
mean free path for electrons in the mesh material.
