The transparent cathode: Rejuvenator of magnetrons and inspiration for new RF sources

The relativistic magnetron has been studied as a high power microwave source since the 1970's. It is a simple, passive device requiring only a uniform magnetic field. It is possible to design a relativistic magnetron with center frequency from L-band to X-band, and it is possible to provide some modest frequency agility about the center frequency as well. Recently, researchers at the University of New Mexico proposed that a transparent cathode can significantly improve the output characteristics of magnetrons. Since then, a comprehensive study using particle-in-cell simulations has identified the many benefits of using this cathode. This paper will review the salient features of operating relativistic and non-relativistic magnetrons using a transparent cathode. In addition, progress from ongoing experiments at the University of New Mexico on an A6 magnetron driven by a short-pulse Sinus-6 accelerator will be presented. The presentation will conclude with a discussion of novel variants of the rippled-field magnetron (what we call the "Simplest Ubitron") that can produce coherent radiation as well as noise. Other devices benefiting from the transparent cathode will be discussed as well.