Continuous-wave operation of a frequency-tunable 460-GHz second-harmonic gyrotron for enhanced nuclear magnetic resonance

• Main Authors: Torrezan de Sousa, Antonio Carlos (Contributor), Han, Seong-Tae (Contributor), Mastovsky, Ivan (Contributor), Shapiro, Michael (Contributor), Sirigiri, Jagadishwar R. (Contributor), Barnes, Alexander (Contributor), Griffin, Robert Guy (Contributor), Temkin, Richard J (Author)
• Other Authors: Massachusetts Institute of Technology. Department of Chemistry (Contributor), Massachusetts Institute of Technology. Department of Electrical Engineering and Computer Science (Contributor), Massachusetts Institute of Technology. Department of Physics (Contributor), Massachusetts Institute of Technology. Plasma Science and Fusion Center (Contributor), Francis Bitter Magnet Laboratory (Massachusetts Institute of Technology) (Contributor), Temkin, Richard J. (Contributor)
• Format: Article
• Language: English
• Published: Institute of Electrical and Electronics Engineers / IEEE Nuclear and Plasma Sciences Society, 2011-04-22T19:57:56Z.
• Subjects: Article
• Online Access: Get fulltext

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 TE11, 2 [TE subscript 11, 2,] q and has generated 16 W of output power with a 13-kV 100-mA electron beam. The start oscillation current measured over a range of magnetic field values is in good agreement with theoretical start currents obtained from linear theory for successive high-order axial modes TE11, 2,[TE subscript 11, 2,] q. The minimum start current is 27 mA. Power and frequency tuning measurements as a function of the electron cyclotron frequency have also been carried out. A smooth frequency tuning range of 1 GHz was obtained for the operating second-harmonic mode either by magnetic field tuning or beam voltage tuning. 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%.
National Institutes of Health (U.S.) (Grant EB004866)