Abstract:
Background Gyrotron traveling-wave tubes (gyro-TWTs), based on the electron cyclotron maser mechanism, are extensively utilized in critical military domains such as high-resolution millimeter-wave imaging radar, communications, and electronic countermeasures. Experimental observations indicate that when the cathode magnetic field exceeds a specific range, the electron beam bombardment of the tube wall occurs.
Purpose This study aims to reduce damage risks to the electron gun during experiments, provide guidance for identifying optimal operating points in experimental testing of Ka-band second-harmonic large-orbit gyrotron traveling wave tube (gyro-TWT).
Methods This paper introduces the formation theory of large-orbit electron guns and analyzes the motion of electron beams in non-ideal CUSP magnetic fields. Using CST Particle Studio and E-Gun software modeled and simulated the electron gun. The effects of magnetic fields, operating voltage, and beam current on the quality and trajectories of large-orbit electron beams were investigated.
Results As the absolute value of the cathode magnetic field increases, both the velocity ratio and the Larmor radius increase, while the velocity spread decreases. With an increase in voltage, the velocity ratio decreases, and the Larmor radius drops to a minimum at a certain point before rising again. Variations in current have limited impact on the Larmor radius and the transverse-to-longitudinal velocity ratio; however, the electron-wave interaction efficiency reaches its maximum at the optimal operating current.
Conclusions The study demonstrates that excessively low operating voltage leads to high transverse-to-longitudinal velocity ratios and electron back-bombardment phenomena, which detrimentally affect the cathode. Therefore, within this voltage range (20–40 kV), the power supply voltage should be increased promptly. Conversely, excessively high reverse magnetic fields at the cathode result in an oversized electron cyclotron radius, causing beam-wall bombardment and gun damage. To prevent electron beam bombardment of the tube wall, the cathode magnetic field should not exceed 0.0085 T.
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