Gyrotron traveling-wave tubes (gyro-TWTs) hold significant potential for applications in millimeter-wave radar, communications, electronic countermeasures, and deep-space exploration.

This paper investigates the high-frequency interaction circuit of a gyro-TWT operating in the Q-band under third-harmonic conditions. With an operational magnetic field of approximately 0.6 T, achievable using conventional solenoid magnets, this design overcomes the limitations associated with superconducting magnets. Furthermore, the adoption of a large-orbit electron beam for interaction addresses the low efficiency inherent in small-orbit electron beams under high-harmonic operation. The interaction structure employs a five-fold helical corrugated waveguide, which not only enhances interaction bandwidth but also effectively suppresses mode competition.

The impedance perturbation method and coupled-wave equations are used.

The transmission coupling characteristics of the five-fold Q-band helical waveguide have been derived.

The mode coupling mechanisms have been analyzed, and the dispersion equation has been formulated, yielding the dispersion curve of the waveguide. Analysis of the dispersion properties reveals the existence of three eigenmodes. Mode 1 is largely decoupled from Modes 2 and 3. Mode 1 has been selected as the operational mode, as it exhibits broad tangential interaction with the electron beam mode within the 42–47 GHz frequency range. This feature significantly extends the interaction bandwidth while simultaneously suppressing mode competition.