10.6: Operation of a tunable second-harmonic 330 GHz CW gyrotron

We report the operation of a frequency tunable continuous-wave (CW) 330 GHz gyrotron oscillator operating at the second harmonic of the electron cyclotron frequency. The gyrotron has generated 18 W of power from a 10.1-kV 190-mA electron beam working in a backward-wave TE_-4,3 cylindrical mode. The minimum start oscillation current of the device was measured to be 33 mA. A continuous tuning range of 1.2 GHz was achieved experimentally via a combination of magnetic, voltage, and thermal tuning. The gyrotron output power and frequency stabilities were assessed to be ±0.4% and ±003 ppm, respectively, during a 110-hour uninterrupted CW run. Evaluation of the gyrotron microwave output beam using a pyroelectric camera indicated a Gaussian-like mode content of 92% with an ellipticity of 28%. The gyrotron will be used as a source for 500 MHz nuclear magnetic resonance (NMR) experiments with sensitivity enhanced by dynamic nuclear polarization (DNP).