Second Harmonic 527-GHz Gyrotron for DNP-NMR: Design and Experimental Results.

We report the design and experimental demonstration of a frequency tunable terahertz gyrotron at 527 GHz built for an 800 MHz Dynamic Nuclear Polarization enhanced Nuclear Magnetic Resonance (DNP-NMR) spectrometer. The gyrotron is designed at the second harmonic (ω = 2ω c) of the electron cyclotron frequency. It produces up to 9.3 W continuous microwave (CW) power at 527.2 GHz frequency using a diode type electron gun operating at V = 16.65 kV, Ib = 110 mA in a TE11,2,1 mode, corresponding to an efficiency of ~0.5%. The gyrotron is tunable within ~ 0.4 GHz by combining voltage and magnetic field tuning. The gyrotron has an internal mode converter that produces a Gaussian-like beam that couples to the HE11 mode of an internal 12 mm i.d. corrugated waveguide periscope assembly leading up to the output window. An external corrugated waveguide transmission line system is built including a corrugated taper from 12 mm to 16 mm i.d. waveguide followed by 3 m of the 16 mm i.d. waveguide The microwave beam profile is measured using a pyroelectric camera showing ~ 84% HE11 mode content.