Spectral Characteristics of a 140-GHz Long-Pulsed Gyrotron

Abstract

Gyrotrons operating in the millimeter and submillimeter wavelength ranges are the promising sources for applications that are requiring good spectral characteristics and a wide range of output power. We report the precise measurement results of gyrotron spectra. Experiments were conducted using a 140-GHz long-pulse gyrotron that is developed for the dynamic nuclear polarization/nuclear-magnetic-resonance spectroscopy at the Massachusetts Institute of Technology. Transient downshift of the frequency by 12 MHz with a time constant of 3 s was observed. After reaching equilibrium, the frequency was maintained within 1 ppm for over 20 s. The coefficient of the frequency change with cavity temperature was -2.0 MHz/K, which shows that fine tuning of the gyrotron frequency is plausible by cavity-temperature control. Frequency pulling by the beam current was observed, but it was shown to be masked by the downward shift of the gyrotron frequency with temperature. The linewidth was measured to be much less than 1 MHz at 60 dB relative to the carrier power [in decibels relative to carrier (dBc)] and 4.3 MHz at 75 dBc, which is the largest dynamic range to date for the measurement of gyrotron linewidth to our knowledge.

Fig. 1

Schematic diagram of the gyrotron used for this experiment. The basic components of the gyrotron include a magnet, an electron gun, and a vacuum tube which consists of a beam tunnel, a resonator, and a mode converter and collector.

Fig. 2

Block diagram of the heterodyne system that is used to measure the spectral characteristics of the pulsed gyrotron. A trigger signal with a repetition rate of 0.01 Hz is generated by the computer and sent to the pulse generator, which is used to control the pulse of the high-voltage power supply and to gate the SA. A segmented sweep over the duration of the pulse was made by changing the delay of the time gate in the SA.

Fig. 3

Frequency shift over the duration of a pulse at 12.9 kV, 30 mA, and repetition rate of 0.01 Hz. Solid dots represent the measured data, and the solid line is a curve fitted to an exponential function. The SA is set to a 14-ms sweep time and 30-kHz resolution bandwidth.

Fig. 4

(a) Frequency variation over the duration of the pulse with respect to the cavity temperature (the temperatures are set values in the recirculating chiller). (b) Frequency change as a function of the temperature, where the frequencies are averaged values after reaching equilibrium (from 10 to 20 s).

Fig. 5

(a) Frequency variation over the duration of the pulse with respect to the beam current. The beam current was maintained by controlling the heater current. (b) Frequency change as a function of the beam current, where the frequencies are averaged values after reaching equilibrium.

Fig. 6

Spectral linewidth of the 140-GHz gyrotron in the IF domain, in which the LO frequency was locked at 19.99400 GHz, and its seventh harmonic was mixed with the gyrotron frequency. For this measurement, the SA was set to 10 kHz of resolution bandwidth and 100 ms of sweep time. Sweeping was delayed by 10 s to wait until the frequency was stabilized after the onset of radiation.