Dan Cai

Characterization of plasma expansion dynamics in a high power diode with a carbon-fiber-aluminum cathode

Thermal plasma expansion is characterised during the operation of a high power diode with an explosive emission carbon-fiber-aluminum cathode driven by a 250 kV, 150 ns accelerating pulse. It is found that a quasi-stationary state of plasma expansion is obtained during the main part of the accelerating pulse and the whole plasma expansion exhibits an “U”-shape velocity evolution. A theoretical model describing the dynamics of plasma expansion is developed, which indicates that the plasma expansion velocity is determined by equilibrium between the diode current density and plasma thermal electron current density.

Characterization of Cesium Iodide-Coated Carbon-Fiber Aluminum Cathode for an

Carbon-fiber-aluminum (CFA) cathode is characterized to generate high-power microwave (HPM) from a high-efficiency three-cavity virtual-cathode oscillator (vircator). Plasma expansion velocity associated with the cesium iodide (CsI)-coated CFA cathode is determined in the experiment. It is found that the stationary plasma expansion velocity is diminished by CsI-coating to about 3 cm/μs when driving by a 300-kV electrical pulse. Particle-in-cell (PIC) simulations indicate that the proposed vircator is capable of generating 550-MW S-band HPM at a central frequency of 2.1 GHz, when the diode voltage and current are 420 kV and 9.2 kA, respectively. The corresponding power conversion efficiency is as high as 14%. The generated HPM is with the TE10 mode of rectangular waveguide, which can give rise to an on-axis radiation pattern without requiring mode converter.

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The solution utilized to design a compact transverse electromagnetic mode (TEM)-CPT (circular polarized TE11) mode converter with high power handling capability and high efficiency, especially at high frequency bands is proposed in this paper. The analysis shows that increasing the quantity of the arms of the converter could make the converter work under overmoded state. The problem that the power capability of the converter working at high frequency bands is low will be addressed. To verify the feasibility of this solution, simulations and experiments on a 20 arms converter are carried out, and the results show that the converter would convert the TEM mode into CPT mode efficiently in a 2–3 free wavelengths longitudinal size and that the power capability is in gigawatt (GW) class. Moreover, this kind of converter can also be utilized in the design of GW TEM phase shifter, whether at low or high frequency bands, which plays a key role in the power combining technology.

-Band High-Efficiency Vircator

In this paper, a gigawatt (GW) transverse electromagnetic wave (TEM)-mode phase shifter (TPS) used to adjust the phase of output of high-power microwave is investigated. The phase shifter is composed of two identical coaxial TE11 circular polarizers, and can adjust the output microwave phase in a range of 0°-360°. The electrical behaviors are analyzed in theory and simulated by the CST software, the results show that the TPS at frequency 1.79 GHz can adjust the microwave phase in a range of 0°-360°, the transmission in this range of phase-shifting process is over 97.5%, and the power-handling capacity is as high as 5.3 GW.

Solution to GW TEM-Circular Polarized TE 11 Mode Converter Design for High Frequency Bands

The carbon nanotube (CNT)-based materials can be used as vacuum device cathodes. Owing to the excellent field emission properties of CNT, it has great potentials in the applications of an explosive field emission cathode. The falling off of CNT from the substrate, which frequently appears in experiments, restricts its application. In addition, the onset time of vacuum breakdown limits the performance of the high-power explosive-emission-cathode-based diode. In this paper, the characteristics of the CNT, electric field strength, contact resistance and the kind of substrate material are varied to study the parameter effects on the onset time of vacuum breakdown and failure mechanism of the CNT by using the finite element method.

GW TEM-Mode Phase Shifter for High-Power Microwave Applications

Anode plasma generated by electron beams could limit the electrical pulse-length, modify the impedance and stability of diode, and affect the generator to diode power coupling. In this paper, a particle-in-cell code is used to study the dynamics of anode plasma in the high-power electron beam diode. The effect of gas type, dynamic characteristic of ions on the diode operation with bipolar flow model are presented. With anode plasma appearing, the amplitude of diode current is increased due to charge neutralizations of electron flow. The lever of neutralization can be expressed using saturation factor. At same pressure of the anode gas layer, the saturation factor of CO2 is bigger than the H2O vapor, namely, the generation rate of C+ ions is larger than the H+ ions at the same pressure. The transition time of ions in the anode-cathode gap could be used to estimate the time of diode current maximum.

Initiation of vacuum breakdown and failure mechanism of the carbon nanotube during thermal field emission

The tunable capability expands the application fields of backward wave oscillator (BWO), especially for large range modulation. This paper presents analysis, PIC simulation, and preliminary design of a novel relativistic BWO which achieves the purpose of modulation among three or more frequencies within two bands. A new dielectric slow-wave structure (SWS) with hollow section was designed in the novel BWO instead of the conventional SWS with fixed solid conductors. The wide range of adjustment of propagation constant and output frequency could be easily achieved by modulating the concentration (permittivity) of the dielectric filled in the hollow section. The results of PIC simulation show the output has three stable situations at two bands with a magnetic field of 3T: 6.9 GHz, 0.9 GW; 7.3 GHz, 1.1 GW; and 10.0 GHz, 1 GW. The specific permittivities of the corresponding SWSs are 15.7, 34.3, and 42.0, respectively.