Microwave Generation Power in a Nonrelativistic Electron Beam with   Virtual Cathode in a Retarding Electric Field
  The power of microwave generation in a nonrelativistic electron beam with virtual cathode formed in a static retarding electric field (low-voltage vircator system) has been studied experimentally and by means of numerical simulation within the framework of a one-dimensional theory. The limits of applicability of the one-dimensional theory have been experimentally determined. 
E.N. Egorov, Yu.A. Kalinin, A.A. Koronovskii, A.E. Hramov, and M.Yu.   Morozov