Thank you Baker
o7
U.S. Patent 4,345,220
Abstract
Coherent microwave radiation is generated in and transmitted through a waveguide/drift tube by means of an injected relativistic electron beam the parameters of which, in combination with the waveguide/drift tube geometry and dimensions effect a virtual cathode oscillation condition within the waveguide/drift tube. Tuning of the device is accomplished by manipulation of the relativistic electron beam parameters and/or waveguide/drift tube physical dimensions.
https://patents.google.com/patent/US4345220A/en