Xuelong Zhao

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

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.

All-Metal Transmit-Array for Circular Polarization Design Using Rotated Cross-Slot Elements for High-Power Microwave Applications

Transmit-array (T-A) design for circular polarization made by all metal is presented in this communication. The proposed T-A consists of numerous cross-slot elements. The simulation results indicate that rotating the cross slot on the element, a full transmission phase range of 360° can be achieved for a transmission magnitude better than −0.6 dB at 9.375 GHz. A T-A has been designed, simulated, and measured in x-band. The measurement results show that the T-A can radiate the wave in the desired direction with an enhanced gain. They are in agreement with the theoretical and simulation results. Furthermore, the all-metal T-A presented in this communication has the application potentials for high-power microwave applications.

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

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.

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

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.

Simulative research on the anode plasma dynamics in the high-power electron beam diode

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.

Variation of anode grid surface morphology and its effect on operation of a triode virtual cathode oscillator

After repeatedly operation of a triode virtual cathode oscillator, the surface morphology of anode grid is studied by a scanning electron microscope. It is found that there are many quasi-periodic sawteeth formed on the anode grid, which are about 300-500 μm in height, ~200 μm in width, and 150-200 μm in period. The formation of this sawteeth implies that there is possible Rayleigh-Taylor-like instability on the anode grid during the irradiation by high-current relativistic electron beam. These sawteeth enhance the electric field on anode grid, leading to more feasible of anode plasma generation, and more rapidly expansion of that plasma. As a result, the electron transmissivity of anode grid is decreased, the output microwave power of the virtual cathode oscillator is lowered and its operational performance is degraded.

All-Metal Beam Steering Lens Antenna for High Power Microwave Applications

All-metal beam steering antenna based on the concept of Risley prism is presented in this communication. The antenna is composed of a feed antenna and two metal lenses. Mechanically rotate two lenses along the same axis independently can steer the radiation beam in both azimuth and elevation planes. Simulations and measurements were carried out in X-band to verify its performance, and the results agree each other well, which are also in agreement with the theoretical results. Furthermore, the power handling capability of the lens antenna with a dimension of

A Novel

1~\text {m} \,\, \times \,\, 1 \,\, \text {m}

\text{TM}_{0{n}}

is estimated about 4 GW, which indicates the all-metal beam steering antenna has the application potentials for high-power microwave applications.

Mode Content and Relative Phase Analysis Technique

Overmoded relativistic backward wave oscillators (RBWOs) are usually difficult to achieve pure mode ouhowever, until now, radiation equipments for high power microwave (hPM) devices are only matched to a single operating mode. So it is unsuitable to utilize the ready-made hPM antenna to radiate mixed TM0n modes with high radiation efficiency, and thus practical applications of the overmoded RBWO device becomes limited. For converting these hybrid TM0n modes into a pure one (e.g., TM01 or TEM mode), diagnostics of the overmoded RBWOs output mode is required. In this letter, a practical measurement technique without requiring high precision calibrations of the power distribution and relative phases between the TM0n modes propagating in overmoded waveguide is described. This method is validated by measuring the output modes of a specially designed TM01-TM0n mixed modes exciter.