Oscillation Conditions of the Vircator Klystron with External Delayed   Feedback: A Computer Simulation
  A new version of the virtual cathode oscillator, vircator klystron, is suggested and studied numerically. It essentially includes adjustable external feedback. The non-steady-state nonlinear dynamics of the oscillator is analyzed in relation to the beam current and feedback parameters. Physical processes in the electron beam during transitions between oscillation conditions are considered. 
V. G. Anfinogentov and A. E. Hramov