Simulation of Returning Electrons from a Klystron Collector
  Spurious oscillations in klystrons due to returning electrons from the collector into the drift tube were observed and studied at KEK. Simulations of returning electrons using EGS4 Monte Carlo method have been performed. And the oscillation conditions are described in this paper. 
Z. Fang, S. Fukuda, S. Yamaguchi, and S. Anami