Weili Xu

To advance compact Ku-band high-power microwave (HPM) sources, this letter proposes a novel miniaturized transit time oscillator (TTO) with a permanent magnet system. The design widens the anode entrance to prevent electron beam interception and employs a compact beam-wave interaction structure to minimize magnet weight. Further, a TM₀₁/TM₀₂ hybrid extractor suppresses surface electric fields. Experiments using an 80 kg permanent magnet providing a 0.28 T field over a 140 mm uniform region demonstrate 410 MW output at 12.6 GHz with a 41 ns pulse duration. These results validate the feasibility of the designed Ku-band TTO with permanent magnet packaging as a practical, compact HPM source for future applications.

This manuscript presents a 2 + 2 metamaterial (MTM) transit-time oscillator (TTO) employing coaxial extraction, engineered to leverage the advantages of MTM and TTO. Cold cavity testing was conducted to examine the electromagnetic properties of the MTM resonator, revealing that at least 1/3 of the radial size can be reduced compared to traditional designs. Comparison of various energy extraction techniques has also been conducted, highlighting the effectiveness of coaxial method over other axial schemes. Computational simulations with electron beams, conducted under parameters of 650 kV, 5.36 kA, and 1.2 T magnetic field, yields a 1 GW microwave output at 1.54 GHz, exhibiting a stable, single mode without competition. This MTM integration significantly reduces the diode impedance in high-power microwave devices, resulting in a shorter axial length and achieving GW-level power output. This design suggests a potential step forward for MTM-based high-power microwave sources, offering the possibility of greater power density and compactness, and may contribute to the advancement of MTM applications in electron vacuum technology.

Since its invention in 1921 by Hull, magnetrons have undergone rapid development, transitioning from low-power devices to high-power relativistic magnetrons (RMs). However, research on RMs has been predominantly confined to L-X bands until now. To explore the potential of RM in higher frequency bands and align with the trend toward miniaturization of high-power microwave sources, a Ku-band RM has been investigated in this paper. Theoretical analysis dictates the selection of 24 cavities for optimal performance at a low magnetic field. Utilizing a three-dimensional particle-in-cell simulation platform, we demonstrate that the RM can generate a microwave power of 116 MW at a resonant frequency of 13.717 GHz with an electron beam of 126 kV and 2.0 kA, under a magnetic field of 0.22 T, corresponding to a power conversion efficiency of 46%. The integration of a modified all-cavity axial extraction structure in the Ku-band RM allows for a minimized inner radius of the magnetic field system to approximately 25 mm, while maintaining a high-power microwave output.

This study is an experimental investigation aimed at addressing the limitation of pulsewidth in traditional Ku -band transit-time oscillator (TTO) devices within a long-pulse accelerator as highlighted in a previous study. By controlling the number of shots and analyzing the variations in breakdown phenomena within the device, we identified the junction between the extraction cavity and drift section as a susceptible area for breakdown discharges. Through an increase in the number of shots, we confirmed the validity of the “anode plasma impact” model. Building upon adjustments made to the internal structure of the drift section, we successfully optimized the output pulsewidth of the TTO device from 68 to 82 ns, effectively maximizing the energy efficiency of the pulse. This research holds significant implications for the design optimization of devices in long-pulse accelerators.

Improving compactness is essential for high-power microwave (HPM) sources. In this paper, a novel reflective modulation cavity is proposed and investigated in a V-band relativistic coaxial transit-time oscillator (RCTTO). The cold cavity analyses and particle-in-cell simulations show that the reflective modulation cavity has larger reflection coefficients of TEM mode and stronger electron beam modulation capability when compared with a uniform modulation cavity. When the input diode voltage is 391 kV, the beam current is 4.91 kA, and when the guiding magnetic field is 0.6 T, the compact V-band RCTTO produces an output microwave power of 518 MW (conversion efficiency of 27.0%). Compared with the original RCTTO, the compact V-band RCTTO featuring a reflective modulation cavity exhibits a 24.8% increase in output power and a 5.4% improvement in efficiency, and the axial length of the magnetic field uniform region is reduced by 24.2%. The compact V-band RCTTO also demonstrates a broad operation voltage range, indicating potential for stable operation with voltage fluctuations in experiments. Furthermore, the reflective modulation cavity can be integrated into other high-frequency O-type HPM devices to enhance compactness, thereby diminishing the demands on the magnetic field region, which is advantageous for the future permanent packaging of HPM sources.

In order to achieve the objectives of compactness and miniaturization in high-power microwave systems, along with reducing system energy consumption, this paper presents a design of a compact coaxial distributed intense relativistic electron beam (IREB) focusing system based on the transit time oscillator. The design methodology integrates theoretical analysis with a 2.5-dimensional particle simulation method. The entire IREB focusing system is composed of three sets of distributed magnetic rings and front-end focusing structures. The layout of this magnetic system is optimized based on periodic permanent magnets, leading to the formation of a quasi-trapezoidal wave magnetic field configuration. This optimization reduces the required number of magnetic rings while ensuring the efficient transmission of the IREB. To prevent the breakdown of the loaded magnetic rings and to optimize the radial electric field in the diode region, an additional front-end focusing structure was added. Based on the aforementioned structural configuration, a 100% electron beam transmission rate was achieved within a 150 mm range.

This paper addresses the challenge of extending the pulse duration of high-power microwave (HPM) sources, a critical focus in HPM technology. Transit-time oscillators (TTOs) typically modulate electron beams by forming standing waves through the cutoff of the TM01 mode in the modulation cavity. This study proposes enhancing the power capacity of TTOs by widening the drift channel. To enhance modulation capability without compromising power capacity, front and rear reflectors are strategically introduced. In particle-in-cell simulations, the device exhibits an output power of 1.65 GW at a frequency of 12.6 GHz, with an efficiency of 33.6%. The saturation time for the output microwave is 30 ns, and the maximum internal electric field strength is 860 kV/cm. The presented TTO, with design improvements, offers a promising solution for applications requiring prolonged pulse durations in high-power microwave sources.

This article addresses the challenge of achieving extended pulse widths in high-power microwave sources operating in long-pulse accelerators. Through modifications and improvements to the transit-time oscillator (TTO) structure, including a high- ${Q}$ modulation cavity, angled corners, a four-cavity extraction structure, and an internally extracted microwave output, the device’s pulsewidth was successfully increased from 42 to 68 ns. Experimental results confirmed the effectiveness of these enhancements, enabling the generation of a 1.03 GW microwave output at a frequency of 12.4 GHz. This study provides valuable experimental results for the development of high-power microwave sources and serves as a reference to advance research and applications in related fields.

In this paper, a novel high-efficiency L-band Metamaterial Relativistic Oscillator (MRO) with high input and output power under relatively low guiding magnetic field has been proposed. In order to increase the power level while decreasing the guiding magnetic field, through our theoretical and simulation investigations on previously designed metamaterial (MTM) structure, we found that the thickness of the ring and the length of the drift tube are the two key factors that can improve the power handling capacity of the MTM structure and field uniformity. Through transmission analysis and electron conductance calculation of the novel MRO based on the improved MTM structure, preliminary MRO design is done. Within CST particle simulation of the proposed MRO and after optimizations, we computationally got a pure 718 MW TE₁₀ output signal at 1.49 GHz using a pulsed electron beam with energy of 700 keV and current of 2 kA under 1 T guiding magnetic field, and the corresponding conversion efficiency is 51.3%. Relevant results of particle simulation verified the previous high frequency analysis. A comparison between our proposed MRO and the previous one reveals that while keeping the high efficiency and miniaturization merits of MRO, the power handling capacity has been increased and the novel MRO works more stable under low guiding magnetic field.

A Ku-band relativistic transit-time oscillator with a three-cavity extractor and a distance-tunable reflector was proposed in our previous work [Ling et al., Phys. Plasmas 24(1), 013103 (2017)]. Recently, a new experiment was carried out on this device with an intense relativistic electron beam generator that produces electron beams with a duration of 55 ns and voltage of 0.4–1 MV. In preliminary experiments, mode competition was found with three main frequencies of which the reasons are analyzed theoretically and experimentally. The results show that plasmas generated from the collector's surface are most likely to cause this problem. To solve this problem, the collector structure is extended axially. With an input voltage of 600 kV and a current of 11.7 kA, a microwave output power 1.9 GW with pulse width 26 ns and frequency 12.42 GHz is obtained under a 0.7 T guiding magnetic field. These experimental results effectively confirm the potential of high power capacity of this proposed Ku band transit-time oscillator.