Efficiency Enhancement in an S-Band Axial Vircator

This paper presents a high-efficiency S-band axial virtual cathode oscillator (Vircator) for high-power microwave (HPM) applications. The design introduces an innovative waveguide geometry obtained by integrating conical and cylindrical metallic structures into a classic circular axial Vircator. The aim of this approach is to enhance the interaction between the relativistic electron beam and the fundamental TM01 mode, while simultaneously suppressing higher-order modes that limit efficiency in classic axial vircator. Fully electromagnetic particle-in-cell (PIC) simulations demonstrate that the proposed geometry leads to a significant improvement in both output power and efficiency compared to a classic single-reflector axial configuration operating under identical voltage and current conditions. At an operating frequency of 2.55 GHz, under an applied voltage of approximately 520 kV, and a beam current of about 21 kA, the device achieves an average output power exceeding 350 MW and efficiency above 3%. In addition, the emitted spectrum exhibits high spectral purity, with strong dominance of the TM01 mode and effective suppression of competing TE modes. The results confirm that a cylindrical–conical waveguide represents an effective and compact solution for efficiency enhancement in magnet-free axial vircator, making the proposed design a promising candidate for next-generation HPM sources.