Construction of an experimental second-harmonic CUSP GUN GRYO-TWT amplifier

Abstract

Summary form only given. 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 has been significant achievement in fundamental cyclotron frequency mode gyro-TWTs; however, they require superconducting magnets which increases the size, weight and complexity of operating 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 more suited to interact in a harmonic device. In the case of the MIG, electrons are constrained to a fixed azimuth so some electrons experience a weak RF field, while others over saturate in a strong field. In the axis-encircling beam, electrons rotate through both the peaks and nulls of the mode yielding a more efficient harmonic interaction. A second harmonic TE21 gyro-TWT is being constructed 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 a 70 kV, 3.5 A axis-encircling electron beam from a Northrop Grumman cusp gun. The magnetic circuit consists of water cooled copper magnet coils and iron pole pieces. The magnetic circuit is designed to provide both the magnetic field reversal required in the cusp gun region to produce the axis-encircling beam and at the same time a constant axial magnetic field of 5.48 kG in the interaction circuit region over the 42 cm length of the circuit. Recently the magnetic circuit was reoptimized to provide a better field profile match to the field profile required by the cusp gun in the field reversal region. In addition, the axial magnetic field is designed to ensure that each section of the device is free from oscillation