Spectral Characteristics of a 140 GHz Lo

Gyrotrons operating in the millimeter and submillimeter wavelength ranges are promising sources for applications that require good spectral characteristics and a wide range of output power. This study reports precise measurement results of gyrotron spectra, conducted using a 140-GHz long-pulse gyrotron developed for dynamic nuclear polarization/nuclear-magnetic-resonance spectroscopy at the Massachusetts Institute of Technology. Significant findings include a transient downshift of the frequency by 12 MHz with a time constant of 3 s, and the frequency was maintained within 1 ppm for over 20 s once equilibrium was reached. The coefficient of frequency change with cavity temperature was determined to be -2.0 MHz/K, indicating that fine tuning of the gyrotron frequency via cavity-temperature control is feasible. Although frequency pulling by the beam current was observed, it was overshadowed by the downward shift of the gyrotron frequency with temperature. Notably, the linewidth measurements showed less than 1 MHz at 60 dB relative to carrier power and 4.3 MHz at 75 dBc, representing the largest dynamic range for gyrotron linewidth measurement known to date.