TE21 Second-Harmonic Gyro-TWT Amplifier Experiment

Summary form only given. A second-harmonic TE₂₁ gyro-TWT has been constructed and is ready for hot testing at UCD that is predicted to produce 50 kW in Ka-band with 20% efficiency, 30 dB saturated gain and 3% bandwidth. The device is driven by an axis-encircling electron beam from a Northrop Grumman cusp gun. There is considerable interest in millimeter-wave amplifiers to be used in high-resolution advanced radar and communications applications. The gyro-TWT is an excellent candidate for these applications because of its potential to produce high power at millimeter-wave frequencies with broad bandwidth. There have been significant advancements in fundamental cyclotron frequency mode gyro-TWTs; however, they require superconducting magnets which increases the size, weight and complexity of the gyro-TWT. The required magnetic field for a gyro-TWT for a given frequency of operation is reduced by the number of the cyclotron harmonic; thus, with harmonic operation the gyro-TWT can potentially operate at high frequencies with conventional magnets or even permanent magnets. Unfortunately, gyro-TWTs operating at harmonics with a magnetron injection gun (MIG) suffer a significant loss in efficiency especially at higher harmonics. An axis-encircling beam is better suited to interact in a harmonic device. The gyro-TWT was carefully designed for stability. The device employs a sliced mode-selective circuit to suppress odd-order azimuthal modes by interrupting their wall currents. Surrounding the interaction circuit is a lossy cylinder to absorb the radiated power. Furthermore, the gyro-TWT uses the concept of distributed loss to suppress the TE₄₁ ⁽⁴⁾ gyro-BWO mode, the strongest oscillatory threat for this gyro-TWT. The first 30.5 cm of the interaction circuit has a wall resistivity 2300 times that of copper to yield a stable interaction length of 220r_w to suppress the TE₄₁ ⁽⁴⁾ mode gyro-BWO. The last 11.5 cm of the 42 cm circuit long does not have loss added so that the high power wave is not attenuated. The cusp gun is currently being used for the UCD peniotron experiment. Hot testing of the second-harmonic TE₂₁ gyro-TWT will begin upon completion of the peniotron experiment.