Journal
JOURNAL OF MAGNETIC RESONANCE
Volume 261, Issue -, Pages 11-18Publisher
ACADEMIC PRESS INC ELSEVIER SCIENCE
DOI: 10.1016/j.jmr.2015.09.011
Keywords
Dynamic nuclear polarization (DNP); Hyper polarization; Frequency modulated DNP
Funding
- National Science Foundation [CHE-1410504]
- Pilot Project Program of the Dartmouth Physically Based Center for Medical Countermeasures Against Radiation
- NIH from the National Institute of Allergy and Infectious Diseases [U19-AI091173]
- Division Of Chemistry
- Direct For Mathematical & Physical Scien [1410504] Funding Source: National Science Foundation
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Magnetic-field and microwave-frequency modulated DNP experiments have been shown to yield improved enhancements over conventional DNP techniques, and even to shorten polarization build-up times. The resulting increase in signal-to-noise ratios can lead to significantly shorter acquisition times in signal-limited multi-dimensional NMR experiments and pave the way to the study of even smaller sample volumes. In this paper we describe the design and performance of a broadband system for microwave frequency- and amplitude-modulated DNP that has been engineered to minimize both microwave and thermal losses during operation at liquid helium temperatures. The system incorporates a flexible source that can generate arbitrary waveforms at 94 GHz with a bandwidth greater than 1 GHz, as well as a probe that efficiently transmits the millimeter waves from room temperature outside the magnet to a cryogenic environment inside the magnet. Using a thin-walled brass tube as an overmoded waveguide to transmit a hybrid HE11 mode, it is possible to limit the losses to 1 dB across a 2 GHz bandwidth. The loss is dominated by the presence of a quartz window used to isolate the waveguide pipe. This per: formance is comparable to systems with corrugated waveguide or quasi-optical components. The overall excitation bandwidth of the probe is seen to be primarily determined by the final antenna or resonator used to excite the sample and its coupling to the NMR RF coil. Understanding the instrumental limitations imposed on any modulation scheme is key to understanding the observed DNP results and potentially identifying the underlying mechanisms. We demonstrate the utility of our design with a set of triangular frequency-modulated DNP experiments. (C) 2015 Elsevier Inc. All rights reserved.
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