Test of a high efficiency relativistic magnetron with diffraction output (MDO) and spherical cathode endcap

A previous computational study of a relativistic magnetron with diffractive output at UNM using MAGIC [1] indicated an electronic efficiency of 70% [2]. The original anode design in this study was tested in experiment. Some subtle changes in the source design were necessary, such as introduction of a Helmholtz-like magnetic coil pair and use of a polished stainless steel endcap on the downstream end of the transparent cathode. The purpose of these changes was to protect the dielectric output window from electron bombardment, an approach that is separate from the work of others to do the same with modified magnetic fields [3]. An ethanol calorimeter and L- and S-band waveguide detectors placed in the near-field were used to obtain RF pulse shape, frequency, and to calculate peak power. A self-integrating Rogowski coil and D-dot probe, which measured total current and voltage respectively, were then used to calculate a total efficiency of 40.4% for the π-mode. This lower efficiency is suggestive of a specific mechanism by which the spherical endcap has poor efficacy. Finally, images of radiated field pattern on a neon-bulb grid and of air breakdown outside of the 21 cm diameter output window were captured.