Investigation Of A Virtual Cathode Oscillator With Coaxial Waveguide

The virtual cathode oscillator (VCO) has historically suffered from relatively low beam-wave power conversion efficiency, which is the major disadvantage of the device. In order to enhance the microwave conversion efficiency and output power, a new configuration of VCO with coaxial waveguide is proposed and studied in an all-round way. The power efficiency of 21.8% under an optimized configureation is obtained in simulation. Besides, preliminary experiment is conducted driven by the Torch-01 accelerator.This new VCO configuration is expected to have the following advantages over the traditional structures. Firstly, due to the presence of the inner conductor of the coaxial waveguide, the hollow electron beam can be located in the vicinity of the maximum axial electromagnetic field of TM01 mode, giving rise to effective beam-wave interaction. Secondly, the coaxial waveguide is expected to enhance the space-charge limiting current of hollow electron beam. Therefore, even with the equal power conversion efficiency, through enhancing the injecting electron-beam current, as well as the input power, microwave output power can be improved. Thirdly, the existing approaches improving the power conversion efficiency in previous various VCOs, such as applying feedback annulus and pre-modulating electron beam, also can be straightforwardly used in this new configuration.The transportation of relativistic electron beam is analyzed with the laminar-flow equilibrium model as well as another theoretical model based on the nonlinear Poisson equation. It is revealed that hollow electron beam propagating in coaxial waveguide has much higher space-charge limitting current than in cylindrical waveguide, the radial location of the electron beam in the waveguide and its thickness are key factors for the space-charge limitting current. In addition, it is found that the velocity and density of electrons are various in the electron beams and those located near the fringe of beam have much larger velocity.The characteristics of this new VCO configuration with coaxial waveguide and hollow electron beam are revealed. The charge-sheet model based on virtual cathode self-oscillation mechanism is employed to analyze the microwave power and dominant mode. The resonant frequency of this device is studied in detail by use of software. The small-signal model, single-electron model and modulating current model, which are 1-D and based on the reflexing electron mechanism, are compared with each other and revised.As in other high power microwave generators with slow-wave structures, the coupling impedance of the specific microwave mode in the VCO is derived and used to analyze the optimization of diode parameters. It is shown that the optimal values of diode parameters are reached when the diode impedance equal the coupling impedance of the generated microwave mode in the VCO with coaxial wave guide as well as in VCOs with other configurations.The configuration design and parameters optimization of this new device are carried out with 2.5-D full electromagnetic PIC code. The insights into the physics of the beam-wave interaction are obtained. The dependences of the microwave output power, power conversion efficiency and frequency on the parameters of waveguide structure, electron beam and guiding-magnetic field are analyzed intensively. After optimization,a microwave with frequency of 3.77GHz, power of 2.3GW and power efficiency of 18.2% in TEM mode is obtained under the condition of diode voltage 520kV, beam current 16.7kA and guiding-magnetic field 1.1T.Moreover, a VCO with both coaxial diode and coaxial waveguide is proposed. Under the optimized configuration, a microwave with frequency of 3.77GHz, power of 2.4GW and power efficiency of 21.8% in TEM mode is obtained under condition of diode voltage 659kV, beam current 16.6kA and guiding-magnetic field 1.1T.In addition, a foilless VCO with coaxial waveguide is put forward. After optimization,a microwave with frequency of 3.77GHz, power of 2.6GW and power efficiency of 11.3% in TEM mode is obtained under the condition of diode voltage 700kV, beam current 33kA and guiding-magnetic field 1.1T.Experiments are performed on the Torch-01 accelerator to demonstrate the characteristic of the VCO with coaxial waveguide. When the diode voltage is 671kV, the current is 16.7kA and the guiding-magnetic field is1.0T, the radiated microwave with frequency of 4.30GHz, power of 464MW, and pulse duration of 10ns is generated. Its power efficiency is 4.1% and the dominant mode is TEM mode.