Second-harmonic axis-encircling-beam TE21 gyro-TWT amplifier

Summary form only given as follows. A second harmonic TE21 gyro-TWT*** amplifier is being constructed at UCD that is predicted by our large-signal code to generate 50 kW in the Ka-band with 20% efficiency, 30 dB saturated gain and 3% bandwidth. The gyro-TWT is to be driven by a 70 kV, 3.5 A, vt/vz=1.2, 7% axial velocity spread, axis-encircling electron beam from a Northrop Grumman Cusp Gun. The device is predicted to double the efficiency of our previous stable second-harmonic MIG gyro-TWT because the new device avoids the loss in efficiency due to the off-axis electrons interacting with a linearly polarized mode. Linear theory has been used to determine the critical current of the absolute instability for the axis-encircling beam of the gyro-TWT. The device has a safety margin of 30% for an operating beam current of 3.5 A. The amplifier has been designed to be stable to harmonic gyro-BWO. The device employs a sliced mode-selective circuit to suppress the odd-order azimuthal mode interactions by interrupting their wall currents. Furthermore, the gyro-BWO interactions are suppressed by adding loss to the circuit. The interaction circuit has a wall resistivity 2300 times that of copper to yield a stable interaction length of 220rw for the strongest remaining gyro-BWO threat, the TE41 mode. The last 11.5 cm of the 42 cm circuit is lossless so that the high power wave is not attenuated. The device employs identical 0 db input and output couplers containing a Chebyshev tapered array of slots connecting the narrow wall of the TE10 rectangular input waveguide to the TE21 circular interaction waveguide. HFSS simulation results predict that the directional coupler has less than 1 dB insertion loss across the band of interest.