[en] By using an electromagnetic particle-in-cell (PIC) code, an S-band two-cavity wide-gap klystron amplifier (WKA) loaded with washers/rods structure is designed and investigated for high power injection application. Influences of the washers/rods structure on the high frequency characteristics and the basic operation of the amplifier are presented. Generally, the rod structure has great impacts on the space-charge potential depression and the resonant frequency of the cavities. Nevertheless, if only the resonant frequency is tuned to the desired operation frequency, effects of the rod size on the basic operation of the amplifier are expected to be very weak. The 3-dimension (3-D) PIC simulation results show an output power of 0.98 GW corresponding to an efficiency of 33% for the WKA, with a 594 keV, 5 kA electron beam guided by an external magnetic field of 1.5 Tesla. Moreover, if a conductive plane is placed near the output gap, such as the electron collector, the beam potential energy can be further released, and the RF power can be increased to about 1.07 GW with the conversion efficiency of about 36%.

[en] The diode spurious oscillation in wide-gap klystron amplifier had been studied by using three-dimension high frequency program and particle-in-cell code. A cut-off mode TM₃₁ determined by the space between cathode and diode was found in the diode region, then the diode and diode together form a resonant cavity and a strong electric field was built in the anode-cathode gap, which result in an intense electron beam modulation. According to the simulation results, some improvements were brought forward to optimize the diode structure, and the diode spurious was eliminated ultimately. (authors)

[en] Using external longitudinal magnetic-field to guide dual-beam out of the dual-shift tubes is a key step for the practicality of synchronizing dual-beam produced by a single accelerator. On the basis of the simulation of the confining magnetic-field for the solid dual-beam, the experiment of magnetic-field guiding annular dual-beam was presented. When the diode voltage was 380 kV, dual-beam currents of 5.10 kA and 4.92 kA were obtained. The experimental results indicate that the designed magnetic-field system could confine the annular dual-beam effectively, and the critical confining magnetic-field is about 0.5 T. (authors)

[en] Theoretical analyses and preliminary experiments on the phase-locking characteristics of an inductively loaded 2-cavity wide-gap klystron amplifier (WKA) with high power injection driven by a GW-class relativistic backward wave oscillator (RBWO) are presented. Electric power of the amplifier and oscillator is supplied by a single accelerator being capable of producing dual electron beams. The well phase-locking effect of the RBWO-WKA system requires the oscillator have good frequency reproducibility and stability from pulse to pulse. Thus, the main switch of the accelerator is externally triggered to stabilize the diode voltage and then the working frequency. In the experiment, frequency of the WKA is linearly locked by the RBWO. With a diode voltage of 530 kV and an input power of ∼22 MW, an output power of ∼230 MW with the power gain of ∼10.2 dB is obtained from the WKA. As the main switch is triggered, the relative phase difference between the RBWO and the WKA is less than ±15° in a single shot, and phase jitter of ±11° is obtained within a series of shots with duration of about 40 ns.

[en] A high power accelerator which can simultaneously produce three electron beams was developed. It is made up of primary energy storage system, spiral-strip transformer, pulse forming line using deionized water as insulation and diode with three cathodes. The three cathodes are stretched into three separate drift tubes, which can isolate the electromagnetic force between the produced electron beams. When a single cathode is connected to the diode, the accelerator can produce over 50 GW electric power; when three cathodes are connected to the diode, three electron beams each with over 10 GW electric power are produced. When the accelerator is used to drive three microwave tubes, three microwaves of 1.0 GW (L-band MILO), 1.0 GW (S-band MILO) and 300 MW (C-band VCO) can be obtained. (authors)

[en] This paper provides a fresh insight into the effect of non-uniform slow wave structure (SWS) used in a relativistic backward wave oscillator (RBWO) with a resonant reflector. Compared with the uniform SWS, the reflection coefficient of the non-uniform SWS is higher, leading to a lower modulating electric field in the resonant reflector and a larger distance to maximize the modulation current. Moreover, for both types of RBWOs, stronger standing-wave field takes place at the rear part of the SWS. In addition, besides Cerenkov effects, the energy conversion process in the RBWO strongly depends on transit time effects. Thus, the matching condition between the distributions of harmonic current and standing wave field provides a profound influence on the beam-wave interaction. In the non-uniform RBWO, the region with a stronger standing wave field corresponds to a higher fundamental harmonic current distribution. Particle-in-cell simulations show that with a diode voltage of 1.02 MV and beam current of 13.2 kA, a microwave power of 4 GW has been obtained, compared to that of 3 GW in the uniform RBWO

[en] An X-band overmoded relativistic klystron is proposed, the operation mode of which is the TM₀₂ mode. The drift tube could not cut off the TM₀₁ mode; isolating the buncher cavity from the input cavity is achieved by introducing a sectional RF lossy material. Microwaves are extracted from the modulated electron beam using a cylindrical waveguide, rather than a coaxial waveguide; thereby, the output structure is significantly simplified. Particle-in-cell simulations show that microwaves with power of 1.28 GW and frequency of 9.30 GHz can be obtained, corresponding to an efficiency of 32% and relative bandwidth of about 8%

[en] A unified theory of multipactor discharge on a dielectric is proposed by utilizing the similarity of the energy balance between multipactor and collisional gas discharge. In this theory, the electron energy gain in multipactor is predicted using the classical formulas for the electric field powers in gas discharge, and the electron energy loss is caused by electron bombardment to the dielectric surface. As with kinetic effects in collisional gas discharge, the electron energy distribution function is important in saturated multipactor. Due to the complex energy distribution functions in saturated multipactor, the average bombardment electron energies are varied and much larger than the traditional predicted value indicated by the first crossover of unity in the secondary electron yield. Using the proposed energy-balance model, multipactor in envelope microwaves and multipactor under an extra magnetic field are both theoretically predicted. The theoretical results for the surface-normal accumulative electric field and the deposited power in saturated multipactor are consistent with the results from electromagnetic particle-in-cell (PIC) simulations. (paper)

[en] A dual-mode operation mechanism in an overmoded relativistic backward wave oscillator is presented. The electron beam interacts with the −1st space harmonic of TM₀₁ mode synchronously in the slow wave structure. Then the backward propagating TM₀₁ mode is converted to the forward propagating TM₀₂ mode. As the phase velocity of the volume harmonic of TM₀₂ mode is about twice that of the surface harmonic of TM₀₁ mode, the TM₀₂ mode also plays an important role in the high-power microwave generation. Particle-in-cell simulation shows that an efficiency of 48% and a significant improvement of the power capacity have been obtained

[en] A dual-cavity TM₀₂–TM₀₁ mode converter is designed for a dual-mode operation over-moded relativistic backward-wave oscillator. With the converter, the fundamental mode output is achieved. Particle-in-cell simulation shows that the efficiency of beam-wave conversion was over 46% and a pureTM₀₁ mode output was obtained. Effects of end reflection provided by the mode converter were studied. Adequate TM₀₁ mode feedback provided by the converter enhances conversion efficiency. The distance between the mode converter and extraction cavity critically affect the generation of microwaves depending on the reflection phase of TM₀₁ mode feedback