Progress in vircators towards high efficiency: Present state and future prospects

Background: High-power microwave (HPM) sources remain of interest to research due to their diverse range of applications The motivation of this research field is to increase the efficiency of HPM sources. One of the simplest oscillators for HPM generation is a virtual cathode oscillator (vircator), however, it has a poor beam-to-microwave conversion efficiency. Therefore, enhancement of the vircator efficiency has demanded innovative solutions since it first appeared 46 years ago. Aim of review: Finding potential methodologies, strategies, and important aspects that may help vircators for better efficiency. Thus, the progress over the last 46 years to improve vircator efficiency using various strategies was summarized in this review. Furthermore, a key idea was also provided for increasing the number of virtual cathodes (VCs) in the beam simultaneously, which might be a useful future technique to achieve high efficiency. Key scientific concept of review: A VC is a source of HPM generation. Another factor contributing to the low conversion efficiency in the vircator is that only one VC is formed, and the electron density confined within that single VC is quite modest in comparison to the overall electron density in the device. A considerable number of electrons after escaping from a single VC were lost on the conductive drift-tube wall causing electron loss. This review provides an idea of the formation of multi-VCs in the beam simultaneously. In this way, the generated power of HPM from each newborn VCs sums up to attain better efficiency in one vircator device. Vircator efficiency improved to 28.6% in experiments and 50% in simulations with multiple VCs. This review could be useful in providing a platform for discussing the collective progress made during recent years and highlighting key points for achieving high conversion efficiency that need to be addressed in future studies.