Design and Experiment of High-Frequency Circuit for High-Power 0.65 THz TWT

A high-power sawtooth waveguide (STW) high-frequency circuit operating in a 0.65 terahertz (THz) traveling wave tube (TWT) is designed, fabricated, and cold-tested in this article. To address the challenge of the THz electromagnetic (EM) wave being amplified through beam-wave interaction while experiencing significant attenuation on the structure walls of the slow wave structure (SWS). The SWS design process emphasizes achieving a balance between interaction impedance and transmission loss to maximize output power. The optimized STW configuration achieves an interaction impedance of 0.76 Ω with a transmission loss of -1.41 dB/mm. Driven by a 19.5 kV, 15 mA pencil beam (PB), particle-in-cell (PIC) simulations confirm the superiority of this balanced design, demonstrating a saturation output power of 2.75 W. This performance surpasses reference high-impedance (2.52 W) and low-loss (2.35 W) designs by 9.1% and 17.0%, respectively. Iterative fabrication refinements via nano-computer numerical control (CNC) and deep reactive ion etching (DRIE) techniques were implemented to further reduce EM wave attenuation. Cold-test results showed an insertion loss of -10.50 dB at 650 GHz for the 6.6 mm-long nano-CNC fabricated circuit and -25.46 dB for the 25.74 mm DRIE circuit, fitting to equivalent conductivities of 3.0 × 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><roman>7</roman></sup> S/m and 3.6 × 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><roman>7</roman></sup> S/m, respectively, exceeding the conventional benchmark 2.0 × 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><roman>7</roman></sup> S/m over the target frequency range.