Experimental Investigation into the Optimum Position of a Ring Reflector for an Axial Virtual Cathode Oscillator

Abstract

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.

Operation Features of a Coaxial Virtual Cathode Oscillator Emitting Electrons in the Outer Radial Direction

“…In most cases, a pulsed power source is used as the high-voltage source. A Pulse Forming Network (PFN)-Marx generator is a pulsed power source to operate HPM devices [12,[29][30][31]. The PFN-Marx generator generates and applies a rectangular high-voltage pulse into the designated load without an additional pulse-forming stage.…”

“…However, the vircator has relatively low efficiencies (typically ≈5%) compared with other HPM devices. A vircator is widely investigated in academic fields to improve its low efficiencies and modulate its output frequency by installing additional structures and changing the cathode and anode materials [8][9][10][11][12][13][14][15][16].…”

Investigation of the Optimum Diameter of the Ring Reflector for an Axial Virtual Cathode Oscillator

“…A vircator is classified as a space charge device which utilizes the cloud of space charge called the virtual cathode. The virtual cathode is formed when the operating current exceeds the space charge limited current [11][12][13]. A vircator generates microwaves through the oscillation of the virtual cathode formed in the drift tube and the reciprocating motion of electrons between the installed cathode and the virtual cathode [14][15][16].…”

“…Using reflectors is one of the microwave-enhancing methods used in vircator studies. Bar, disk, and ring reflectors have been studied and analyzed in various pieces of vircator research [11,[23][24][25].…”

“…The distance was chosen not to be close to the virtual cathode position and not to exceed 24 mm, which is the microwave power enhancement ceasing point. Previous research on the ring reflector showed that the distance from the anode to the ring reflector affects the microwave power [11]. From the point of view of the forming cavity, the hole diameter of the ring reflector may affect the optimum interaction between the microwave and the electrons in the cavity region.…”