Observation of Low-Frequency Oscillations in W-Band Gyro-TWTs Caused by Weak and Strong Plasma

Observation of the low-frequency oscillations (LFOs) in our designed W-band lossy-circuit loaded gyrotron travelling wave tubes (gyro-TWTs) caused by residual gas ionization is reported in this paper. The gas released from the lossy dielectric is ionized by colliding with the high-energy gyrating electrons. Then weak plasma is generated and the vacuum in the tube decreases to 10
^–5
Pa. An LFO spectrum of 13 MHz was observed in this condition. However, when the vacuum is worse further, an LFO spectrum of 50–125 MHz was observed formed by the strong plasma excited near the cathode region. Both the above-mentioned plasma leads the beam current to be unstable and deteriorate the gyro-TWT performance.