A ring reflector was experimentally investigated using an axial virtual cathode oscillator (vircator). The ring reflector was installed behind the mesh anode of the axial vircator to enhance the microwave power output by forming a resonant cavity and increasing the electron beam to microwave energy conversion efficiency. The optimum position of the ring reflector is analyzed through simulations and experiments by varying the anode to reflector distance from 6 mm to 24 mm in 3 mm steps. PIC simulations show that the ring reflector enhances the microwave power of the axial vircator up to 220%. Experiments show that the microwave power from the axial vircator without the ring reflector is 11.22 MW. The maximum average peak microwave power of the axial vircator with the ring reflector is 25.82 MW when the anode to ring reflector distance is 18 mm. From the simulations and experiments, it can be seen that the ring reflector yields decaying enhancement that is inversely proportional to the anode to ring reflector distance and there is no noticeable microwave enhancement after 24 mm. The frequency range attained from the simulations and experiments is 5.8 to 6.7 GHz and 5.16 to 5.8 GHz, respectively. The difference between the simulation and experimental results is due to the error in the anode to cathode gap distance. Although the frequency is slightly changed, the ring reflector seems to have no influence on the frequency of the generated microwave.

The operation features of the coaxial virtual cathode oscillator emitting electrons in the outer radial direction were investigated through simulations and experiments. A coaxial vircator was compared with an axial vircator when the anode to cathode distance of both vircators was 6 mm. The proposed coaxial vircator was operated when the anode to cathode distance was 5 mm, 6 mm, and 7 mm. The peak power and frequency of the microwave generated from the proposed coaxial vircator when the anode to cathode distance was 6 mm were 20.18 MW and 6.17 GHz, respectively. The simulations and experiments show that the proposed coaxial vircator generates 80% more microwave power than the axial vircator with the same anode to cathode distance. According to the simulations and experiments, the proposed coaxial vircator tends to generate a higher power average when the anode to cathode distance was larger than 5 mm. The frequency of the proposed coaxial vircator when the anode to cathode distance was 5 mm and 7 mm was approximately 8 GHz and 5 GHz, respectively. The geometric factor of the proposed coaxial vircator was considered to be the reason for the greater microwave power generation than the axial vircator. The frequency of the proposed coaxial vircator decreases inversely proportional with the anode to cathode distance as observed in the axial and basic coaxial vircators.