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

• DOI: 10.1016/j.jmr.2019.106586
• Publication Date: 2019-08-29T06:06:52Z
• Authors: Chukun Gao; Nicholas Alaniva; Edward P. Saliba; Erika L. Sesti; Patrick T. Judge; Faith J. Scott; Thomas Halbritter; Snorri Th. Sigurd...; Alexander B. Barnes
• Keywords: Gyrotron; Magic angle spinning; Continuous wave

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 frequency-chirped DNP using the TEMTriPol-1 biradical, with MAS NMR at 7 T. 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.