Experimental demonstration of a 527 GHz gyrotron for dynamic nuclear polarization

Summary form only given. The experimental demonstration of a 527 GHz, continuous wave (CW) gyrotron operating at the second cyclotron harmonic in a 9.7 T magnetic field is described. The gyrotron is currently being used in Dynamic Nuclear Polarization (DNP) experiments at the University of Utrecht in the Netherlands. DNP is a technique used to enhance the sensitivity of nuclear magnetic resonance (NMR) experiments by transferring the higher polarization of unpaired electrons spins to nuclear spins. The gyrotron is designed to operate in the second harmonic TE_15,3,1 mode. The interaction is driven, nominally, by a 17.5 kV, 140 mA electron beam produced by a single-anode magnetron injection gun. A Bruker cryogen-free superconducting magnet provides the 9.7 T magnetic field required for the interaction. An internal mode converter, consisting of a rippled-wall launcher and four steering and shaping mirrors, is used to convert the TE_15,3 interaction mode to a high-quality Gaussian beam. The resultant Gaussian beam exits the gyrotron through a vacuum window and is injected into a corrugated waveguide and transported to the NMR spectrometer. During gyrotron tests, CW output power levels up to 25 W were achieved with an 18 kV, 97 mA electron beam. Infrared images of the output beam showed excellent Gaussian characteristics which were nearly identical to those of the theoretical design beam. A mathematical modal decomposition of the measured beam images showed a 98.5% overlap with the design Gaussian beam. The gyrotron operated stably for prolonged periods of time with frequency variations of less than 1MHz and power variations of less than 0.3 W. The gyrotron has been in use for DNP experiments since November 2012.