Broadband operation of a Ka-band tapered gyro-traveling wave amplifier

A wideband low-voltage millimeter-wave gyro-traveling wave tube (gyro-TWT) amplifier operating in the TE₁₀ rectangular waveguide mode at the fundamental cyclotron frequency is under investigation, The device incorporates precise axial tapering of both the magnetic field and the interaction circuit for broadband operation. Experimental results of a wide (33%) instantaneous bandwidth with a small signal gain in excess of 20 dB and saturated efficiency of ~10% were achieved and shown to be in good agreement with the theory. Reflective instability due to multi-pass effects by mismatches was observed and characterized. Gain and efficiency have been limited by this reflective instability rather than by absolute instabilities which limit the performance of gyro-TWT´s with uniform cross-section. The start-oscillation current in terms of the relevant experimental parameters such as the beam velocity ratio (α), magnetic field detuning and reflection coefficient has been measured and compared with theory. Measurements of the phase variation in terms of the RF frequency have shown that the phase varies ±30° from fitted linear phase line