Frequency-chirped dynamic nuclear polarization with magic angle spinning using a frequency-agile gyrotron #DNPNMR

Gao, Chukun, Nicholas Alaniva, Edward P. Saliba, Erika L. Sesti, Patrick T. Judge, Faith J. Scott, Thomas Halbritter, Snorri Th. Sigurdsson, and Alexander B. Barnes. “Frequency-Chirped Dynamic Nuclear Polarization with Magic Angle Spinning Using a Frequency-Agile Gyrotron.” Journal of Magnetic Resonance 308 (November 2019): 106586.

We demonstrate that frequency-chirped dynamic nuclear polarization (DNP) with magic angle spinning (MAS) improves the enhancement of nuclear magnetic resonance (NMR) signal beyond that of continuous-wave (CW) DNP. Using a custom, frequency-agile gyrotron we implemented frequencychirped DNP using the TEMTriPol-1 biradical, with MAS NMR at 7 Tesla. Frequency-chirped microwaves yielded a DNP enhancement of 137, an increase of 19% compared to 115 recorded with CW. The chirps were 120 MHz-wide and centered over the trityl resonance, with 7 W microwave power incident on the sample (estimated 0.4 MHz electron spin Rabi frequency). We describe in detail the design and fabrication of the frequency-agile gyrotron used for frequency-chirped MAS DNP. Improvements to the interaction cavity and internal mode converter yielded efficient microwave generation and mode conversion, achieving >10 W output power over a 335 MHz bandwidth with >110 W peak power. Frequency-chirped DNP with MAS is expected to have a significant impact on the future of magnetic resonance.

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