Guozhi Liu

A repetitive X-band relativistic backward-wave oscillator

A repetitive X-band relativistic backward-wave oscillator (BWO) driven by a SINUS-881 accelerator is described. Relativistic electron beams with peak current of 5.4 kA and voltage of 610 kV at a repetition rate of 100 Hz were generated by the SINUS-881 and then guided through the corrugated waveguide by an axial magnetic field of 3.0 T produced by a superconducting magnet. An electron collector was used to collect the electron beams in order to mitigate the effect of secondary emission electrons and to prevent ionization and breakdown near the electron beam dump. This BWO produces a microwave pulse power of 1.1 GW at a 100-Hz repetition rate, a frequency of 9.38 GHz, a pulse duration of 23 ns, and a power transforming efficiency of 33%.

The effect of grooved surface on dielectric multipactor

The effect of periodic rectangular grooves on vacuum multipactor has been theoretically and experimentally investigated. Dynamic calculation is applied to research the electron trajectory and impact energy under groove surface. Two-dimensional electromagnetic particle-in-cell simulation is used to analyze and compare multipactor scenario, statistic energy, and secondary emission yield on the flat surface with that on the corrugated surface. It has been found by computational and simulative analysis that grooved surface can explicitly suppress multipactor in the developmental stage of multipactor. S-band high power microwave (HPM) dielectric breakdown experiment under vacuum, with microsecond pulse length was conducted. It was confirmed by experiment that periodic grooves perpendicular to the major electric field can effectively increase transmitted power.

A Cerenkov generator with coaxial slow wave structure

Results of analytic theory, numerical simulation, and experimental studies on a Ceronkov generator with coaxial slow wave structure are presented. The dispersion curves and the field patterns of the axial modes are given in the analytic theory, showing the operation mode of quasi-TEM mode. The numerical simulation results indicate that the device is featured by the resonance characteristics, the microwave frequency spectrum illustrates high purity, the dominant frequency remains fairly constant over a wide range of diode voltage, and the optimized efficiency of more than 37% is obtained. In preliminary experiment, a microwave pulse with frequency of 7.7GHz, power of 1.3GW, and efficiency of 28% was obtained.

Inducing phase locking of multiple oscillators beyond the Adler’s condition

To achieve phase locking of high power microwave oscillators on a relatively weak coupling strength, an inducing phase locking method is investigated. With an external signal injected from the end of relativistic backward wave oscillator, the frequency and phase of the output microwave is pulling in the starting oscillation process and remaining stable in the later stationary oscillation process. The simulation results indicate that injecting of inducing signal prior to the onset of natural oscillations is necessary, while the duration of injection, power of injection and locking bandwidth are not limited by classic Adler’s law.

Research on an improved explosive emission cathode

This paper presents a physical description of the cathode plasma process of an explosive emission cathode (EEC) and experimental results on a type of oil-immersed graphite EEC. It is believed that the generation of a cathode plasma is mainly dependent on the state of the cathode surface, and that adsorbed gases and dielectrics on the cathode surface play a leading role in the formation of the cathode plasma. Based on these ideas, a type of oil-immersed graphite EEC is proposed and fabricated. The experiments indicate that the oil-immersed cathodes have improved emissive properties and longer lifetimes.

Limitation of cross-excitation instability in a relativistic Cerenkov generator with coaxial slow wave structure

The operation in the cross-excitation instability regimes for relativistic backward wave oscillators has been reported and studied in some earlier papers. In the experiment of a relativistic Cerenkov generator with coaxial slow wave structure (CRCG), the cross-excitation instability has also been observed by us. In this paper, experimental results for the CRCG operating in this regime are given, and an explanation is provided to account for the appearance of the instability. Based on this explanation, two types of reflectors, i.e., coaxial Bragg reflectors and cutoff necks, are adopted to restrain the cross-excitation instability by increasing the reflection coefficient at the entrance of the coaxial slow wave structure.

Coaxial cavity vircator with enhanced efficiency

A vircator with a coaxial cavity has the potential to increase the beam–microwave conversion efficiency. According to the E -field distribution pattern of the modes in the anode cavity of a coaxial vircator, the resonant frequency band of the injected electron beam and the lowest two operating modes are derived. The main frequency of the virtual cathode is also deduced. The optimal operating frequency and high-efficiency designing method of a coaxial cavity vircator is discussed. An experimental setup is designed and built to test the high-power microwave (HPM) generation mechanism described by theoretical analysis as well as increase the power efficiency. HPM frequency obtained in the experiment is in good agreement with the analysis. The power and energy efficiencies obtained in the experiment are, respectively, 8.7% and 6.8% with 50 ns pulse width. Frequency and phase stable HPM radiation is observed as well as pulse shortening is evidently depressed.

Research on the Emission Uniformity of Explosive Emission Cathodes in Foilless Diodes

Some factors that influence the emission uniformity of explosive emission cathodes (EECs) in foilless diodes such as the guiding magnetic field strength and the rise rate of electric field have been researched in former literatures. This paper is concentrated on another factor that has been overlooked previously, i.e., the defects with dimensions of tens of micrometers on cathode surfaces, especially at blade edges. It is shown that these defects will significantly worsen the emission uniformity of EECs by introducing extraordinarily large microscopic field enhancement factors, and thus they should be eliminated. The micrometer-scale irregularities, however, should be maintained to provide effective emission micropoints. Meanwhile, the outer edge of an annular cathode blade should be rounded off to improve the emission uniformity. After being disposed like this, the emission uniformity of annular graphite cathodes in a foilless diode is obviously improved, which leads to an increase of energy efficiency by more than 20% by broadening the microwave pulse duration under a power level of 2.8 GW for an X-band relativistic backward wave oscillator.

Investigation on Dielectric Window Treelike Breakdown and Suppression Under HPM in Vacuum

Surface discharges of dielectric window seriously limit the generation and transmission of high-power microwaves (HPMs), which block the development of microwave technology. An S-band (2.856-GHz) HPM experimental system is established. Several kinds of dielectric window materials, i.e., polyethylene, polytetrafluoroethylene, and polymethyl methacrylate, are investigated under S-band HPMs in vacuum, and the characteristics of destroyed dielectric samples are analyzed with macroscopic and microcosmic observation. Treelike breakdown channels are found on the upstream face of dielectric window along the direction of microwave electric field, and a breakdown development model with an increase of cumulative breakdown times is proposed. Dielectric surface breakdown suppression technologies are also studied, i.e., surface-polishing and surface-notching treatments. The experimental results show that the polishing treatment on the dielectric surface can decrease breakdown degree and increase surface breakdown threshold. The surface grooves that are parallel to the direction of electric field will reduce the surface breakdown threshold, while the grooves that are perpendicular to the direction of electric field can suppress the surface breakdown obviously. The effect is influenced by the depth and width of grooves on the dielectric surface, and correlative theoretical analysis is discussed.

Characteristics of Space-Charge-Limiting Current for the Magnetically Immersed Foilless Diode

The effects of diode structural parameters, input voltage, and external magnetic field on the space-charge-limiting currents of a magnetically immersed foilless diode are studied by analysis and particle-in-cell (PIC) simulations and are verified with experiments. The effects of longitudinal anode-cathode (A-K) gap and anode-cavity radius on space-charge-limiting current and critical external magnetic field are obtained. A scaling law for diode current is given, which is in good agreement with the PIC simulations and experiments. This is important for the design of this kind of diode because the expected current and impedance of the foilless diode is obtained by changing the longitudinal A-K gap in practice. The I-V relation obtained is applicable in practice and in good accordance with the experiments.