Relativistic magnetrons’ development stages

A paper presents the main stages of relativistic magnetrons’ development. We describe the designs eliminating conventional magnetrons’ shortcomings and restrictions which are associated with a radial output of radiation through a narrow slot in one of the magnetron resonators. A low breakdown threshold and an operation in only nondegenerate modes are among these restrictions. In the paper we consider the design of the magnetron with a diffraction output of radiation, where all magnetron’s resonators are extended into conical antenna to the cross-section where the cutoff frequency is lower than the frequency of generation. This magnetron with axial symmetrical output of radiation can operate in any mode and a switch to the degenerate oscillation does not lead to a catastrophe which may occur in conventional magnetrons.We managed to increase an efficiency of the magnetron by optimizing its diffraction output with a depth of resonators increasing in the antenna. In the first experiment the electron efficiency of the magnetron achieved the value exceeding 60%. The replacement of a solid cathode to the cathode transparent to azimuthal electric field of synchronous wave, allowed us to shorten the leading edge of radiated wave to the duration of leading edge of accelerating voltage. Transparent cathode consists of separate emitters oriented along the axis and periodically placed at the circle with a radius of the cathode. High efficiency was also achieved in the magnetron with a long virtual cathode, the use of which allowed us to eliminate both plasma responsible for a pulse shortening and an electron bombardment reducing a cathode’s lifespan. We showed a possibility to transform the operating $\pi$-mode into the output radiation with a simplified structure including the radiation with the structure similar to Gaussian. This can be achieved in a compact design of the magnetron. In a regime of fast mode switching induced by external signal, we estimated the influence of noise leading to the blurring of the boundary magnetic fields intrinsic to different modes. In magnetic fields within these broaden boundaries the generation of neighboring magnetron operating modes becomes unpredictable. Alternate regions of magnetic fields with stable and unstable regimes of generation are observed on the map of generation regimes of the magnetron, which makes it difficult to switch the operating modes by small external signal.