A mode analysis is carried out with the MAFIA TS3 module to study the high-power microwave generation in the inward-emitting coaxial vircator with two opposed nonaxisymmetric emitting surfaces. The central angle thetas of the emittors, and the axial distance D between the cathode and anode reflector, are verified to have a large influence on the mode component. For a fixed diode voltage of 250 kV, the coaxial diode current is 13.7 kA when the beam injection is axisymmetric (thetas = 180deg). In this situation, the simulation gives the highest efficiency of 8.3% for D = 5.0 cm and 10.9% for D = 10.0 cm , comparing with the nonaxisymmetric beam injection. For the case of two opposed nonaxisymmetric emitting surfaces with central angle thetas, there exists a range for thetas to acquire a rather pure output of the TE11 mode (80% of the total power), which is 80deg < thetas < 160deg. However, the efficiency decreases with smaller thetas. The polarized direction of the main TE11 mode is consistent with the directional layout of the two opposed emitters. FFT transformation shows that the dominant frequency for thetas < 80deg can be estimated by the equations derived from the 2-D theoretical analysis of the symmetric coaxial vircator.

Mode Analysis of High-Power Microwave Generation in the Inward-Emitting Coaxial Vircator Based on Computer Simulation

The characteristic frequency and energy conversion efficiency of the coaxial vircator are studied by numerical simulation method. Comparing with planar vircator, the coaxial vircator generates much narrow bandwidth and higher center frequency. The simulation results also evince that by adjusting d can tune the output microwave frequency in about two octave range under certain conditions. The negative pulsed coaxial vircator can significantly improved efficiency from about 4% of positive pulsed coaxial vircator to about 10%. The experimental apparatus are setting up, the experimental results will be given in future.

Numerical simulation studies of coaxial vircators

A two-dimensional solution of space-charge-limiting current for a high current vacuum diode with a spherical cathode is presented. The relation between space-charge-limiting current and electric field enhancement factor at the cathode surface for the diode with a curved surface cathode is also discussed. It is shown that compared with the current given by the conventional Child–Langmuir law, which describes the one-dimensional space-charege-limiting current, the two-dimensional space-charge-limiting current in such a diode is enhanced due to the electric-field enhancement along the cathode surface. Among practical parameter ranges, enhancement factor ηb approximately satisfies ηb ≠ Aβn, where β is the electric field enhancement factor at the cathode surface, and n is a constant between 1 and 2, which is confirmed to be universal for the diodes with curved surface cathodes.

https://iopscience.iop.org/article/10.1088/1674-1056/19/7/075207/pdf

Relation between space-charge-limiting current and electric field enhancement factor at curved surface cathode

Complex Phase Dynamics of Overlimiting Electron Beams Propagating in Opposite Directions

Influence of the Conductivity of Resonator Walls on the Characteristics of a Microwave Generator with a Virtual Cathode: Numerical Experiments

An experiment of a virtual cathode oscillator (vircator) built on the low impedance intense electron beam accelerator Flash II is reported. A novel spectrum diagnosis method—a circulating dispersion line—is proposed. A thin oil layer coated graphite cathode is introduced in the experiment to decrease the delay time of the explosive emission process and obtain a homogeneous electron beam emission for improving the high-power microwave (HPM) generation efficiency. The effect of diode current on HPM generation in the vircator system is discussed. The HPM pulse width has a strong connection with the diode current waveform. For most shots, corresponding to the time that microwave emission starts, there is an inflection point in the diode current pulse. Compared with the case that no microwave is generated, the diode current increases more slowly following the inflection point. HPM generation terminates when the beam current reaches the self-pinching critical current of the diode.

Effects of diode current on high power microwave generation in a vircator

The experimental and theoretical results of coaxial diode and the theoretical results of coaxial vircator are presented in this paper. The cathode is a cold, field- emitting graphite ring and needle-shaped copper applied to a grounded cylinder. The anode is a semi-transparent cylinder located inside of, and concentric to the cathode cylinder. The anode cylinder is pulsed positive. The coaxial vircator generates microwave by injecting a radial electron beam into cylinder such that the space-charge limited current is exceeded. A virtual cathode forms and oscillates in radial position and amplitude, generating microwaves which are extracted by an attached waveguide with a circular cross- section. Analytic and PIC simulations were used to study coaxial diode and vircator, with aid of the two dimensional PIC code, KARAT. The comparisons between the theoretical and the experimental results for a coaxial diode are presented.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Coaxial diode and vircator

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Effects of diode current on high power microwave generation in a vircator

Coaxial diode and vircator