Experimental Study of the Effect of External Signal on Microwave   Oscillations in a Nonrelativistic Electron Beam with Virtual Cathode
  The effect of an external harmonic signal on the characteristics of microwave generation in a nonrelativistic electron beam with virtual cathode (VC) formed in a static retarding electric field (low-voltage vircator system) has been experimentally studied. A significant increase in the vircator generation power is observed when the frequency of the external signal is close to the frequency of VC oscillations. At large detunings, a broadband chaotic generation is observed. 
Yurij Kalinin, Alexander Hramov