Virtual Cathode Theory and Design of a Millimeter Wave VIRCATOR (Virtual Cathode Oscillator).

摘要

The theory of a virtual cathode oscillator and detailed experimental design of a millimeter wave virtual cathode oscillator (VIRCATOR) are presented. It is found that second order analytical approximations give an accurate value for the space-charge limiting current of an electron beam in cylindrical geometry. The same Green's function approach could be used on other geometries as well. Exceeding the space-charge limiting current is the necessary condition for virtual electrode formation. However, it does not explain the nature of the space-charge limit instability. The oscillatory stable state of the virtual cathode is found to originate at the bifurcation point of several stable and unstable electron beam states. An analysis of the nonlinear growth rate of the instability is derived. The virtual cathode is in effect a relaxation oscillator. The advantage of the vircator over a reflex klystron is that the virtual cathode moves opposite to the electron beam propagation direction during the bunching phase resulting in enhanced electron densities unobtainable otherwise. The periodic time-dependent nature of the bunching results in efficient microwave generation. (rrh)