High Frequency Gyrotron for DNP/NMR Research.

Massachusetts Institute Of Technology, Cambridge MA

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Abstract

DESCRIPTION (provided by applicant): The proposed research program is focused on the research and development of a widely tunable, high frequency (527 GHz) gyrotron oscillator for application to dynamic nuclear polarization solid-state nuclear magnetic resonance spectroscopy (DNP/SSNMR). In NMR, signal intensities are intrinsically low due to the small gyromagnetic ratios of the observed nuclei. DNP/NMR experiments can provide signal enhancements of 20 to 400, making DNP/NMR an important technique for elucidating the structure, function, and dynamic properties of biological systems. The full implementation of these techniques at high magnetic fields has been limited by two problems: 1.) the paucity of high power microwave sources that generate microwaves in the region 140 - 600 GHz and 2.) the fact that essentially all NMR magnets operate at fixed field in persistent mode, making it difficult to match the microwave frequency to the correct frequency in the EPR spectrum to optimize DNP. This proposal requests funding to develop a high-stability gyrotron oscillator that provides a solution to these two problems. The 10 to 50 Watt, 527 GHz gyrotron with a tunable bandwidth of 2 GHz will be used in conjunction with an 800 MHz NMR spectrometer, making it the highest magnetic field DNP/NMR spectrometer in the world. This spectrometer should provide dramatic signal enhancement at the very high magnetic fields where contemporary NMR research is currently being performed. The development of the 527 GHz gyrotron presents unique scientific and engineering challenges, including: greatly reduced gyrotron gain when operating at a low voltage and at the second harmonic gyro-frequency;high ohmic loss at high frequency;and the limited bore size of the superconducting magnet. Variation of beam parameters in a conventional gyrotron oscillator with a high Q cavity gives a frequency tuning range of less than 0.1 %, inadequate for this application. We propose to build the 527 GHz gyrotron with a novel tuning approach: namely, by varying the magnetic field and by utilizing a gyrotron cavity that couples a series of high order axial modes. The proposed 527 GHz gyrotron oscillator will benefit greatly from the highly successful results of our research on a tunable 330 GHz gyrotron oscillator. We have recently demonstrated more than 21 Watts of output power from a 330 GHz gyrotron over a tuning range of 1.2 GHz. Tuning was accomplished by varying the magnetic field, the voltage and the cavity temperature. The tuning was thus accomplished without having to provide mechanical tuning of the resonator, assuring more stable, simple, and convenient operating conditions. The proposal presents a complete design of a 527 GHz gyrotron showing that the predicted power level and tuning range are feasible. During year one of the proposed research, we will complete wide-range testing, optimization and commissioning of the tunable 330 GHz gyrotron while designing and ordering long lead time items for the 527 GHz gyrotron. Funding of this continuation proposal is crucial to maintaining progress in this important field of NMR spectroscopy of biomolecules. PUBLIC HEALTH RELEVANCE: The proposed research is directed at building a high frequency microwave source that will greatly enhance the sensitivity of Nuclear Magnetic Resonance (NMR) spectrometers and will therefore dramatically speed up spectral acquisition in NMR experiments on biological solids. The novel gyrotron microwave source will be the highest frequency source in the world for use in enhanced NMR research and, when applied to an 800 MHz NMR spectrometer, should reveal unique molecular structure information. The improved techniques will lead to increased understanding of the structure of amyloid and membrane proteins which are key to understanding their role in biological systems.