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

Abstract

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 performance 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.

Ramped-amplitude NOVEL

“…Various groups have demonstrated this in the context of CW DNP. [65][66][67][68][69][70] Our results show that it is also important to modulate the microwave power in pulsed DNP experiments. To this end, an AWG is an elegant solution to conveniently manipulate the properties of the microwave pulses in a precise manner.…”

“…71 The implementation of this approach has been performed at both low and high frequencies. 69,71 With the availability of fast (a few GSa/s) and cost-effective AWG, we anticipate that AWG will evolve into a standard component for the next generation of DNP/NMR instrumentation.…”

DNP-NMR of surface hydrogen on silicon microparticles

“…The MW irradiation was produced via a previously described millimeter wave source. [30,62] Powdered silicon particles (nominally 1-5µm poly crystalline silicon) were bought from Alfa Aesar, and were used as is. The powder was stored for many years in the original packaging, under ambient conditions before use.…”

Enhanced dynamic nuclear polarization via swept microwave frequency combs

“…Inset: similar method could be applied to broad line electron radicals like TEMPO, shown here 3.35T with the spectrum centered at 95GHz [9]. Lower panel: time domain implementation through multiple cascaded frequency sweepers, illustrating the ability to maintain the adiabatic rateω while increasing the effective number of sweeps by N .control techniques has been fueled by advances in instrumentation (sources [24,25] and synthesizers [26]) that enable the rapid and coherent manipulation of electrons at high fields [27][28][29]. Particularly attractive amongst them is the use of frequency or field swept techniques, e.g.…”

“…control techniques has been fueled by advances in instrumentation (sources [24,25] and synthesizers [26]) that enable the rapid and coherent manipulation of electrons at high fields [27][28][29]. Particularly attractive amongst them is the use of frequency or field swept techniques, e.g.…”