W-Band Backward Wave Oscillator Utilizing a Diverging Radial Sheet Electron Beam

This paper introduces a W-band Backward Wave Oscillator (BWO) with a radial rectangular concentric grating (RRCG) slow wave structure (SWS) and a diverging radial sheet electron beam (RSEB). This radial BWO design overcomes the limitations of traditional axial BWOs by offering a larger interaction area and reducing space-charge effects, thereby eliminating the need for an external guiding magnetic field and simplifying the electro-optic system. The high-frequency characteristics of the RRCG SWS are numerically analyzed, and Particle-in-Cell (PIC) simulations are conducted for the BWO using a diverging RSEB without an external magnetic field, operating at a beam voltage of 24 kV and a current density of 49.5 A/cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup>. The device achieves a peak power of 9.9 kW, an average power of 4.95 kW, and an efficiency of 5.15%. FFT analysis of the frequency spectrum reveals the generated signal frequency is 91.3 GHz. These findings demonstrate that the proposed radial BWO design operates effectively without an external magnetic field, making it suitable for high-efficiency millimeter wave generation in W-band applications.