The relativistic magnetron with diffraction output (RMDO) has demonstrated nearly 70% efficiency in recent simulations. This letter reports a rapid mode switching technique in the RMDO using a low power, short-pulse, external single frequency signal. The MAGIC electromagnetic finite-difference-time-domain particle-in-cell code used in simulations demonstrated that an input signal of 300 kW is sufficient to switch neighboring modes in a gigawatt output power A6 RMDO with a transparent cathode, whereas for the original A6 magnetron configuration with radial extraction driven by a transparent cathode 30 MW is required. This frequency agility adds additional versatility to this high power microwave source.

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.

The possibility of using an external RF signal to rapidly switch the operating mode in relativistic magnetrons is studied using the PIC code MAGIC 1 for the A6 magnetron 2 operating at 360 kV with anode block radius = 2.11 cm. In the A6 magnetron with asymmetric extraction of radiation only nondegenerate modes (π−mode and the 2π−mode) can be used as the operating mode. Mode hopping to any neighboring mode leads to magnetron operation with an unloaded mode, resulting in overheating and erosion of its electrodes.We show that for the A6 magnetron driven by an ordinary solid cathode stable 2π−mode generation occurs in a narrow gap 0.53 cm between the anode and cathode, whereas π−mode generation occurs when 1.11 cm; mode competition takes place for any magnetic field when the cathode radius is 1.58 1.0 and the minimum input RF signal 200 MW switching the operating modes is for cm. For the transparent cathode 3 the interval of cathode radii where mode competition takes place is very small, 1.48 1.38, and the minimum power of the external signal for switching is about two orders of magnitude less.The external RF signal expands the region of applied axial magnetic field for cathode radii where pure operating modes are generated. In this case, RF priming 4 takes place even when the frequency of the input signal is different from that of the operating wave.