Frequency-tunable second-harmonic submillimeter-wave gyrotron oscillators

Abstract

This thesis reports the design and experimental demonstration of frequency-tunable submillimeter-wave gyrotrons operating in continuous wave (CW) at the second harmonic of the electron cyclotron frequency. An unprecedented continuous frequency tuning range of more than 1 GHz has been achieved in both a 330- and a 460-GHz gyrotron via magnetic field tuning or voltage tuning. The 330-GHz gyrotron has generated 19 W of power in a cylindrical TE4,3,q mode from a 13-kV 190-mA electron beam. The minimum start current was measured to be 21 mA, where good agreement was verified between the measured start current values and the calculation from linear theory for the first six axial modes, q = 1 through 6. A continuous tuning range of 1.2 GHz with a minimum output power of 1 W has been achieved experimentally via magnetic or beam voltage tuning. The output stability of the gyrotron running under a computerized control system was assessed to be ±0.4% in power and ±3 ppm in frequency during a 110-hour uninterrupted CW test. Evaluation of the gyrotron microwave output beam using a pyroelectric camera indicated a Gaussian-like mode content of 91%. Measurements were also carried out in microsecond pulse operation at a higher beam current (610 mA), yielding a minimum output power of 20 W over a tuning range of 1.2 GHz obtained by means of cyclotron frequency tuning and thermal tuning. The 330-GHz gyrotron will be used as a source for 500 MHz nuclear magnetic resonance (NMR) experiments with sensitivity enhanced by dynamic nuclear polarization (DNP). In addition to the 330-GHz gyrotron, the design and CW operation of a tunable second-harmonic 460-GHz gyrotron are described. The 460-GHz gyrotron operates in the whispering gallery mode TE1 1 ,2 and has generated 16 W of output power with a 13-kV 100-mA electron beam. The start oscillation current measured over a range of magnetic field values is in good agreement with theoretical start currents obtained from linear theory for successive high order axial modes TE1,2,q. The minimum start current is 27 mA. Power and frequency tuning measurements as a function of the electron cyclotron frequency have also been carried out. A smooth frequency tuning range of 1 GHz with a minimum output power of 2 W has been obtained for the operating second-harmonic mode either by magnetic field tuning or beam voltage tuning. Long-term CW operation was evaluated during an uninterrupted period of 48 hours, where the gyrotron output power and frequency were kept stable to within ±0.7% and ±6 ppm, respectively, by a computerized control system. Proper operation of an internal quasi-optical mode converter implemented to transform the operating whispering gallery mode to a Gaussian-like beam was also verified. Based on images of the gyrotron output beam taken with a pyroelectric camera, the Gaussian-like mode content of the output beam was computed to be 92% with an ellipticity of 12%. The 460-GHz gyrotron is intended to be used as a submillimeter-wave source in a 700-MHz DNP/NMR spectrometer.