Second Harmonic Operation at 460 GHz and Broadband Continuous Frequency Tuning of a Gyrotron Oscillator

Abstract

We report the short pulse operation of a 460 GHz gyrotron oscillator both at the fundamental (near 230 GHz) and second harmonic (near 460 GHz) of electron cyclotron resonance. During operation in a microsecond pulse length regime with 13 kV beam voltage and 110 mA beam current, the instrument generates several watts of power in two second harmonic modes, the TE2,6,1 at 456.15 GHz and the TE0,6,1 at 458.56 GHz. Operation in the fundamental modes, including the TE0,3,1 mode at 237.91 GHz and the TE2,3,1 at 233.15 GHz, is observed at output powers up to 70 W. Further, we demonstrate broadband continuous frequency tuning of the fundamental modes of the oscillator over a range of more than 2 GHz through variation of the magnetic field alone. We interpret these results in terms of smooth transitions between higher order axial modes of the resonator. The 460 GHz gyrotron is currently being processed for continuous duty operation, where it will serve as a microwave source for sensitivity-enhanced nuclear magnetic resonance (dynamic nuclear polarization) studies at 16 T (700 MHz 1 H), a field strength which is twofold higher than has been accessible with previous technology.

250GHz CW gyrotron oscillator for dynamic nuclear polarization in biological solid state NMR

“…As has been reported previously for the fundamental modes of a 460 GHz second harmonic gyrotron oscillator [46], we observe broadband continuous frequency tuning with variation of the magnetic field alone. Figure 24 summarizes the experimental frequency tuning as a function of magnetic field recorded near the starting current for resonant cavity modes from 5.8 to 9 T (open symbols denote fundamental modes and filled symbols denote second harmonic modes).…”

“…First, it is important to have the frequency stable in the CW DNP experiments discussed here since the enhancement in solid effect, thermal mixing and cross effect experiments [38] is strongly dependent on the position of irradiation in the EPR spectrum. Second, it would be useful to be able to tune a gyrotron oscillator across the breadth of the EPR spectrum, a feature that is discussed elsewhere [46] and mentioned in Section 3. Here we discuss relatively small variations of the output frequency, which we term frequency pulling, that describe the variation of frequency as a function of beam voltage, cathode parameters, and magnetic field while the gyroton is operating in the TE 0,3,2 mode.…”

“…Table 5 summarizes the frequency tuning of the modes shown in Figure 24, where, notably, 1.8 GHz tuning has been observed in the TE 2,2 mode and 1.4 GHz in the TE 8,1 , and only tens of megahertz of tuning in the second harmonic modes. The magnetic frequency tuning of the fundamental modes has been previously analyzed in detail [46].…”

“…There are several possible explanations for this phenomenon. First, the performance of the diode-type electron gun used in these experiments, which has been previously analyzed in detail [46], is characterized by large changes in the beam pitch factor and velocity spread as a function of beam voltage and magnetic field. However, linear theory and non-linear simulations using MAGY, shown in Figure 25, suggest that the effects of velocity spread alone cannot account for the lower starting currents.…”

“…A cursory description of the 250 GHz oscillator appeared elsewhere [44] and the design and initial operation of the 460 GHz oscillator is described in detail in other publications [45][46][47][48]. However, since the appearance of the brief description of the 250 GHz gyrotron, we have implemented many important changes to the system.…”