Design, Simulation, and Cold Test of a W-Band Radial Rectangular Concentric Grating Backward Wave Oscillator

This article introduces a radial rectangular concentric grating (RRCG) backward wave oscillator (BWO). The design utilizes a radial sheet electron beam (RSEB). It aims to overcome the limitations of conventional BWOs at W-band frequencies. In conventional BWOs, the compact size at higher frequencies results in reduced interaction space. This limitation affects the device's efficiency. Additionally, it leads to significant heating challenges, particularly in cooling the anode. Moreover, an external magnetic field is required for effective beam focusing due to space charge effects. This requirement complicates the fabrication process. The RRCG slow-wave structure (SWS) is designed with its high-frequency characteristics analyzed through mathematical equations. The particle-in-cell (PIC) simulation is conducted without an external magnetic field at a 24-kV beam voltage and 45-A/cm <tex-math notation="LaTeX">²</tex-math> current density. The RRCG BWO demonstrates enhanced efficiency compared to traditional W-band BWOs, achieving an average power of 15.7 kW and 4.68% efficiency at 88 GHz. A radial-converging electron beam gun is also simulated as a separate component of the system. The presented device, encompassing 16 periods, is fabricated. Cold-test results show an <tex-math notation="LaTeX">S₁₁</tex-math> parameter below <tex-math notation="LaTeX">$-$</tex-math> 10 dB, consistent with simulation. Additionally, a topology for the high-power extraction from the coaxial output of RRCG-BWO is suggested.