Continuous-Wave Operation of a Frequency-Tunable 460-GHz Second-Harmonic Gyrotron for Enhanced Nuclear Magnetic Resonance

Abstract

The design, operation, and characterization of a continuous-wave (CW) tunable second-harmonic 460-GHz gyrotron are reported. The gyrotron is intended to be used as a submillimeter-wave source for 700-MHz nuclear magnetic resonance experiments with sensitivity enhanced by dynamic nuclear polarization. The gyrotron operates in the whispering-gallery mode TE 11,2 and has generated 16 W of output power with a 13-kV 100-mA electron beam. The start oscillation © 2010 IEEE Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Long-term CW operation was evaluated during an uninterrupted period of 48 h, where the gyrotron output power and frequency were kept stable to within ±0.7% and ±6 ppm, respectively, by a computerized control system. Proper operation of an internal quasi-optical mode converter implemented to transform the operating whispering-gallery mode to a Gaussian-like beam was also verified. Based on the images of the gyrotron output beam taken with a pyroelectric camera, the Gaussian-like mode content of the output beam was computed to be 92% with an ellipticity of 12%. NIH Public Access

Operation of a Continuously Frequency-Tunable Second-Harmonic CW 330-GHz Gyrotron for Dynamic Nuclear Polarization

“…In addition, the measured frequency range for the start current between 329.88 GHz and 331. 13 GHz is in reasonable agreement with the range for high-order axial modes presented in Table I. Frequency detuning may exist at the lower edge of the excitation zone for B 0 < 6.01 T since the measured power at these start current values is greater than 1.4 W. The neighboring fundamental mode TE 5,1 is excited for magnetic field values B 0 > 6.035 T.…”

“…As in gyrotron resonators the axial wavenumber is typically much smaller than the transverse wavenumber, cavity modes that only differ by their number of axial variations on their electric field profile may lead to continuous tunability. This tuning scheme was demonstrated in a millimeter-wave gyrotron operating in the first harmonic of the electron cyclotron frequency, both in pulsed mode [10] and continuous-wave mode [11], and later in a second-harmonic continuous-wave THz gyrotron [12], [13]. The smooth tuning range obtained in these experiments was equal to or greater than 1 GHz.…”

“…The output power of the oscillator was measured using a Scientech laser disc calorimeter, model 36-0401, whereas the frequency was measured using a heterodyne system detailed in [13]. Table II summarizes the main operating parameters of the 330-GHz gyrotron oscillator.…”

“…1. This gyrotron employs a diode magnetron injection gun described in [13]. The electron gun is connected to a CW power supply able to provide a maximum voltage of 25 kV and a maximum current of 200 mA, restricted to a maximum total power of 4 kW.…”

“…The fabricated cavity was cold tested following the procedure outlined in [13], and the cavity radius was estimated to be r cav = 1.834 mm, which is within manufacturing tolerance.…”