Frequency switching in a relativistic magnetron with diffraction output

Abstract

Symmetric axial extraction of radiation from a relativistic magnetron with diffraction output (MDO) facilitates the use of any eigenmode as the operating one. As a consequence, a relatively small input RF signal can be used for mode switching, unlike the case for asymmetric extraction when only non-degenerate modes (the π- mode or the 2π- mode) can be used as the operating one. Using the MAGIC particle-in-cell code we demonstrate that about 180 MW is required to switch these non-degenerate modes in the well-known 400 kV A6 magnetron with extraction of radiation from one of its cavities when driven by a solid cathode, and about 30 MW is required for the same device when driven by a transparent cathode. For the gigawatt A6 MDO with a transparent cathode, however, only 200–300 kW is sufficient for mode switching and the switched mode continues to be generated after elimination of the input short RF signal when the amplitude of the applied axial magnetic field is near the critical value corresponding to the boundary between synchronous regions for neighboring modes. In repetitively pulsed systems, in order to switch each subsequent pulse independent of the previous one, the time between voltage pulses must be chosen to be not less than 20–30 ns (the time for the stored electromagnetic energy to flow out of the cavity) so that decreasing the output power of the previous pulse cannot switch the subsequent pulse. Finally, using this mode switching technique, we demonstrate the possibility of generating short gigawatt microwave pulses with different frequency and polarization by using a short, weak, single frequency signal that is very attractive for radar applications.