Mode Distortion and Performance Degradation Caused by Electron-Beam Misalignment in W-Band Gyro-TWTs

Mode distortion and performance degradation caused by an OFF-axis gyrating electron beam in our W-band gyrotron traveling wave tube (gyro-TWT) were studied by the particle-in-cell (PIC) simulation and verified with the hot test experiments. The simulated electron beam misalignment may lead to a mixed pattern, mainly composed of TE01, TE02, TE21, and TE22 modes. They will further deteriorate the performance by disturbing the bunched electron beam. When the misalignment is over 0.1 mm, the operating mode shows a significant distortion, and the output power is reduced to 66% of that without misalignment at 93 GHz. For the prototype gyro-TWT with an estimated beam misalignment of about 0.1 mm, the measured saturated output power drops by more than 57% below 93.5 GHz compared with the normal one, whose estimated misalignment is less than 0.05 mm. The downward trend proves that the beam misalignment is the main factor causing the performance degradation at the W-band and above.