CW operation of a tunable 330/460 GHz gyrotron for enhanced nuclear magnetic resonance

In this work we describe the operation of a continuous-wave 330/460 GHz gyrotron oscillator operating at the second harmonic of the electron cyclotron frequency. The oscillator is intended to be used in 700 MHz ¹H nuclear magnetic resonance (NMR) experiments enhanced by dynamic nuclear polarization (DNP). At 460 GHz, the gyrotron operates in the TE_11,2 mode and is able to generate 16 W of power continuously and stably for a beam current of 100 mA and beam voltage of 13 kV. Power and frequency tuning measurements as a function of beam voltage and magnetic field were carried out. The results show continuous frequency tunability over a range of 700 MHz with a minimum power level of 5 W for a fixed beam current of 100 mA. Extended frequency tuning due to the interaction of the electron beam with the backward-wave component of successive high order axial modes is observed. Power and frequency tuning measurements are also reported for the TE_4,3 mode at 330 GHz, a frequency suitable for 500 MHz DNP/NMR spectrometers.