A compact Ku -band coaxial relativistic klystron amplifier (RKA) with permanent magnets is proposed for the sake of producing repetitive high-power microwaves (HPMs) in this article. The improved input and output mode converters are designed to load the input signal at the end of this device to ensure the integrity of permanent magnets. Besides, the intense relativistic electron beam (IREB) is changed to be collected on the outer collector, which makes collector cooling feasible in the proposed device. The typical experimental results show that the Ku -band HPM with a power of 540 MW is generated, corresponding to a gain of 48 dB and an efficiency of 29%, when the guiding magnetic field is around 0.45 T produced by the permanent magnets. The result of continuous operation for 30 s at a repetition rate of 35 Hz indicates that the proposed coaxial RKA has the capability of operating at a high repetition rate.

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.

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%.

An $X$ -band coaxial relativistic klystron oscillator (RKO) is proposed to improve the beam–wave interaction efficiency in the low magnetic field and low impedance. The device adopts a four-gap modulation cavity to increase the fundamental current, and a three-gap extraction cavity is used to improve the microwave extraction capability. The simulation results show that when the guiding magnetic field and impedance are 0.55 T and 40 $\Omega $ , respectively, the output microwave power is 3.65 GW and the corresponding efficiency is 40%. Subsequently, a preliminary experimental investigation was carried out to verify the feasibility of this technical route. In the experiment, the output microwave with a power of 2.3 GW, a pulsewidth of 69 ns, and a center frequency of 9.39 GHz was generated when the magnetic field and impedance are 0.62 T and 41 $\Omega $ , respectively. Moreover, the explosive electron emission model of the extraction cavity is established, and the simulation results show that the explosive electron emission generated by the radio frequency (RF) breakdown at the extraction cavity is the main reason for the difference between the experimental and simulated microwave power.

In order to reduce the volume and energy consumption of high-power microwave (HPM) systems, HPM sources with low guiding magnetic fields have become one of the most important trends. However, the beam-wave conversion efficiency of such devices is typically low due to the divergence of the electron beam, which lacks the constraint of a high magnetic field. Here, we investigate a coaxial dual-mode relativistic Cherenkov oscillator (RCO) that exhibits efficient beam-wave interaction at a low guiding magnetic field. On the one hand, the space charge effect of electrons can be reduced by increasing the radius of the device; on the other hand, dual-mode operation can be achieved for efficient beam-wave conversion, while the competition of higher order modes from large radii can be avoided by introducing a coaxial inner conductor to the slow wave structure (SWS). In particle simulations, a guiding magnetic field of only 0.33 T has been demonstrated to confine the electron beam, resulting in an output microwave power of 1.98 GW and a beam-wave conversion efficiency of approximately 43% when the diode voltage and diode current are 510 kV and 9 kA, respectively. Moreover, preliminary validation experiments were conducted, wherein microwaves with a power of 1.71 GW were generated with a beam-wave conversion efficiency of 36%. The experimental results obtained under low magnetic fields provide a high degree of confidence that the size and weight of HPM systems utilizing this device will be significantly reduced.

The disk-beam relativistic klystron oscillator (DB-RKO) with the radial-line high frequency structure is one of the attractive high power microwave (HPM) sources due to its specific virtues of weak space-charge effect, high power handling capacity and strong electron collection ability. However, such a device generally exhibits a bulky volume in the radial direction, retarding its actual applications in some compact occasions. Besides, it usually saturates slowly because of the insufficient beam-wave coupling strength. For this purpose, a compact DB-RKO with a rapid saturation process is proposed and physically designed in this paper. By compressing the axial width of the electron beam drift tube suitably, the beam-wave coupling strength is enhanced clearly and the saturation process is accelerated significantly. The whole beam-wave interaction structure is shortened with a radial length of about 5 cm. Besides, in order to increase the device compactness further, an improved magnetic-excited method is presented to enhance the magnetic field in the diode area by introducing a pair of soft-magnets. Furthermore, with the guiding of a low magnetic field of 0.4 T, the compact DB-RKO is experimentally demonstrated to be capable of generating Ku-band HPM radiations with a peak power of 810 MW, a power conversion efficiency of 28%, a center frequency of 14.19 GHz and a pulse duration of about 27 ns. The proposed DB-RKO is promising for the package of the permanent magnets which are preliminarily designed in this paper.