A tunable continuous-wave 330 GHz gyrotron for enhanced nuclear magnetic resonance

Summary form only given. Dynamic nuclear polarization (DNP) is a technique used to enhance the signal-to-noise ratio in nuclear magnetic resonance (NMR) experiments and requires a continuous-wave (CW) source able to generate ten or more watts of power in the 140 to 600 GHz range, a task well suited for gyrotron oscillators. Besides these requirements, a frequency tunable gyrotron is also highly desirable since it would permit DNP/NMR experiments to be performed without tuning the NMR magnet or with NMR magnets without a superconducting sweep coil. We have performed preliminary tuning experiments in a CW 460 GHz gyrotron that have demonstrated 1 GHz of magnetic/voltage tuning at 460 GHz, second-harmonic mode TE_11,2, with a maximum power of 16 W When the magnetic field was reduced, we also obtained at 332 GHz, second-harmonic mode TE_4,3, a maximum power of 2.5 W with 1 GHz of tuning. We are now fabricating a CW 330 GHz gyrotron oscillator operating at the second harmonic of the cyclotron frequency that will be integrated to a 500 MHz DNP/NMR spectrometer. The gyrotron is designed to operate in the TE_4,3 mode, with a 35-mm long cylindrical cavity, radius 1.83 mm. The mode interacts with a 9.5 kV, 100 mA electron beam, radius 1.08 mm, with perpendicular velocity spread of less than 5% and pitch factor of 1.8 according to EGUN simulations. The start oscillation current was computed to be as 32 mA. Self- consistent simulations using the code MAGY estimate a maximum output power of 60 W and a magnetic tuning range of 600 MHz by exciting high order axial modes. Extended tuning range is expected to be obtained by a thermal tuning scheme implemented in the tube. The generated radiation is coupled out of the gyrotron by means of a helical cut launcher and a series of mirrors.