Design and Measurement of a Mode-Suppression Circuit for a G-Band Gyrotron Traveling Wave Tube

In the hot test of our G-band (0.21–0.23 THz) TE₀₁-mode gyrotron traveling wave tube (gyro-TWT), the beam current was increased from 3.0 to 3.9 A to obtain a higher gain and output power. However, a strong TE₂₁-mode backward-wave oscillation occurred in the nonlinear interaction region. To solve this problem, a novel mode-suppression circuit (MSC) is proposed, designed, and measured in this article. The MSC contains six transverse slot-coupled loops surrounded by an external lossy dielectric ring, which can provide a sufficient coupling loss for the parasitic TE₂₁ mode without affecting the transmission of the operating TE₀₁ mode. Based on the MSC, a simulated loss of 8.09 dB/cm is provided for the TE₂₁ mode near the oscillation frequency. Considering the engineering realization, the beam voltage and pitch factor are set to 50 kV and 1.0, respectively. Particle-in-cell simulation shows the MSC-based gyro-TWT has excellent zero-driven stability and can achieve a saturated output power over 20 kW in the range of 210–228 GHz with a beam velocity spread of 3.01%. For verification, an MSC was manufactured and cold tested. The cold-tested results agree well with our analysis.