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A compact Ku-band triaxial klystron amplifier (TKA) with three cascaded single-gap bunching cavities is proposed for the sake of high output efficiency at low seed injection power. The three bunching cavities are specially designed with optimal external quality factors and specific drift tube length to suppress the self-oscillation of these cavities. By decreasing the coupling coefficients and increasing the frequency interval of the three cavities, the asymmetric mode competition can be successfully eliminated in 3D particle-in-cell simulation. The compact TKA is demonstrated to be capable of producing the Ku-band high power radiation with a peak power of 580 MW corresponding to an efficiency of 31 % and a pulse duration of nearly 100 ns in typical experiments. Furthermore, the seed injection power and axial length of the proposed device are reduced to 9 kW and 10 wavelengths, respectively. [ABSTRACT FROM AUTHOR]

In recent years, high-power microwave (HPM) generator with a low working magnetic field has caught more attention as this device has the merit of low energy consumption and the potential of using a permanent magnet package in the HPM system. However, the conversion efficiency of this device is generally low because the electron beam will fluctuate transversely and the modulation depth will decline when the guiding magnetic field is low. To obtain concurrently high-power, high-efficiency, and low operating magnetic field, a transit-time oscillator (TTO) with two bunchers is designed and fabricated. Two bunchers are adopted in the TTO to modulate the electron beam twice. By introducing the second buncher, the modulation depth of the electron beam increases by 23%, and the conversion efficiency increases from 25% to 40.5% in simulations. A microwave with a power of 3.65 GW and a frequency of 4.31 GHz is obtained with a 600-kV, 15-kA electron beam under the guidance of a 0.5-T magnetic field in simulations. Furthermore, the corresponding experiments are carried out to verify the two-buncher TTO, and a microwave with a power of 3.3 GW and a frequency of 4.31 GHz is obtained. The experimental efficiency is 32% with an external magnetic field of 0.48 T. [ABSTRACT FROM AUTHOR]

The power conversion efficiency of relativistic klystron amplifiers (RKAs) is severely limited by the strong space-charge repulsion force caused by the intense electron beam. A radial-line RKA (RL-RKA) driven by the disk beam may provide great potentials to alleviate this issue due to its radially propagating characteristic of the electron beam. However, such potential advantage of high efficiency has not been fully exploited in previous research due to the insufficient beam modulation and relatively low beam-wave coupling strength. In this article, an initial RL-RKA is improved significantly by utilizing a three-stage cascaded bunching link, a four-gap output cavity, and a lengthened drift tube. The three-stage cascaded bunching link plays a role in increasing the fundamental harmonic current and decreasing the velocity spread of the electron beam simultaneously. The four-gap output cavity is designed to improve the coupling impedance and strengthen the beam-wave power conversion. Moreover, a drift tube is lengthened suitably to boost the fundamental harmonic current further. Eventually, the power conversion efficiency of the RL-RKA is enhanced up to 67% by these measures, which indicates a significant advance over the initial state of 38%. [ABSTRACT FROM AUTHOR]

A beam-steerable lens antenna based on the concept of Risley prism is presented in this communication. The antenna is made up of a feed antenna and three transmit-array (T-A) lenses, including a stationary lens and two rotatable lenses. Beam steering is achieved by rotating the two rotatable lenses around the antenna axis independently. An antenna prototype has been successfully designed, simulated, fabricated, and measured in Ku-band. The results show that the beam of the antenna can be steered to any direction within a 91.2° conical region. During beam steering, the measured peak gain of the antenna reaches 38.1 dBi, the maximum aperture efficiency achieves 80.5%, and the gain variation is less than 1.7 dB. Moreover, the simulated results show that the power-handling capacity of this antenna could reach gigawatt level, which indicates application potentials of this antenna in high-power microwave (HPM) field. [ABSTRACT FROM AUTHOR]

The physical mechanism, simulation results, and main experimental results of an S-band long-pulse relativistic backward-wave oscillator (RBWO) with a coaxial extractor are presented. The central ideas of the device include: 1) dual resonant cavities replacing the cutoff neck are used to enhance the efficiency and to decrease the electric field; 2) nonuniform slow-wave structures (SWSs) are introduced in the device to improve the efficiency; 3) a coaxial extractor is well designed, which is beneficial to the extraction of generated microwaves; and 4) the diode and the collector are reasonably designed to reduce the influence of cathode and collector plasmas. In simulation, microwaves centered at 3.77 GHz are generated, with the power of 3.3 GW and efficiency of 43%. However, the pulse duration of the device does not exceed 70 ns owing to the pulse-shortening phenomenon in the initial experiment. It was found that the plasma caused by an intense radio frequency electric field on the surface of the coaxial extractor played a main role in pulse shortening. To suppress the pulse shortening, we focus on two aspects. One was to optimize the electrodynamic structures to reduce the electric field on the surfaces of the coaxial extractor while maintaining relatively high efficiency. The other was to improve the surface roughness of SWSs thereby increasing the breakdown threshold of the RF field. In the experiment after these measures were taken, microwaves centered at 3.752 GHz are generated, with the power of 2.3 GW, efficiency of 35%, repetition rate of 20 Hz, and pulse duration above 110 ns. [ABSTRACT FROM AUTHOR]