title: 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 TE11,2and 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,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%.
authors.affiliation: Department of Electrical Engineering and Computer Science and the Plasma Science and Fusion Center, Massachusetts Institute of Technology, Cambridge, MA 02139 USA ( torrezan@mit.edu ).
authors.forename: Antonio C
authors.initials: AC
authors.lastname: Torrezan
authors.forename: Seong-Tae
authors.initials: ST
authors.lastname: Han
authors.forename: Ivan
authors.initials: I
authors.lastname: Mastovsky
authors.forename: Michael A
authors.initials: MA
authors.lastname: Shapiro
authors.forename: Jagadishwar R
authors.initials: JR
authors.lastname: Sirigiri
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Temkin
authors.forename: Robert G
authors.initials: RG
authors.lastname: Griffin
authors.forename: Alexander B
authors.initials: AB
authors.lastname: Barnes
keywords: Dynamic nuclear polarization (DNP); nuclear magnetic resonance (NMR); second cyclotron harmonic; submillimeter wave; terahertz; tunable gyrotron