Design and characterization of a W-band system for modulated DNP experiments

摘要

Magnetic-field and microwave-frequency modulated DNP experiments have beenshown to yield improved enhancements over conventional DNP techniques, and evento shorten polarization build-up times. The resulting increase insignal-to-noise ratios can lead to significantly shorter acquisition times insignal-limited multi-dimensional NMR experiments and pave the way to the studyof even smaller sample volumes. In this paper we describe the design andperformance of a broadband system for microwave frequency- andamplitude-modulated DNP that has been engineered to minimize both microwave andthermal losses during operation at liquid helium temperatures. The systemincorporates a flexible source that can generate arbitrary waveforms at 94 GHzwith a bandwidth greater than 1 GHz, as well as a probe that efficientlytransmits the millimeter waves from room temperature outside the magnet to acryogenic environment inside the magnet. Using a thin-walled brass tube as anovermoded waveguide to transmit a hybrid HE11 mode, it is possible to limit thelosses to 1 dB across a 2 GHz bandwidth. The loss is dominated by the presenceof a quartz window used to isolate the waveguide pipe. This performance iscomparable to systems with corrugated waveguide or quasi-optical components.The overall excitation bandwidth of the probe is seen to be primarilydetermined by the final antenna or resonator used to excite the sample and itscoupling to the NMR RF coil. Understanding the instrumental limitations imposedon any modulation scheme is key to understanding the observed DNP results andpotentially identifying the underlying mechanisms. We demonstrate the utilityof our design with a set of triangular frequency-modulated DNP experiments.