A novel wideband 140 GHz gyrotron amplifier

Abstract

The theory, design and experimental results of a wideband 140 GHz, 1 kW pulsed gyro-traveling wave amplifier are presented. The gyro- TWA operates in the HE(0,6) mode of a novel cylindrical confocal waveguide using a gyrating electron beam. The electromagnetic theory, interaction theory, design processes and experimental procedures have been described in detail. The experiment has produced over 820 W peak power, 34 dB linear gain, and a -3 dB bandwidth of over 1.5 GHz (1.1%) from a 37 kV, 2.7 A electron beam having a beam pitch factor of 0.6, radius of 1.9 mm and calculated perpendicular momentum spread of approximately 9%. The gyro-amplifier was nominally operated at a pulse length of 2 microseconds, but was tested to amplify pulses as short as 4 nanoseconds with no noticeable pulse broadening. Internal reflections in the amplifier were identified using these short pulses by time-domain reflectometry. A novel internal mode converter was designed for this device that transforms the confocal HE(0,6) fields into a fundamental Gaussian beam for ultra-low loss transmission of the millimeter wave output power through a corrugated transmission line. The demonstrated performance of this amplifier shows that it can be applied to Dynamic Nuclear Polarization (DNP) and Electron Paramagnetic Resonance (EPR) spectroscopy.