Purea, Armin, Christian Reiter, Alexandros I. Dimitriadis, Emile de Rijk, Fabien Aussenac, Ivan Sergeyev, Melanie Rosay, and Frank Engelke. “Improved Waveguide Coupling for 1.3 Mm MAS DNP Probes at 263 GHz.” Journal of Magnetic Resonance 302 (May 2019): 43–49.
We consider the geometry of a radially irradiated microwave beam in MAS DNP NMR probes and its impact on DNP enhancement. Two related characteristic features are found to be relevant: (i) the focus of the microwave beam on the DNP MAS sample and (ii) the microwave magnetic field magnitude in the sample. We present a waveguide coupler setup that enables us to significantly improve beam focus and field magnitude in 1.3 mm MAS DNP probes at a microwave frequency of 263 GHz, which results in an increase of the DNP enhancement by a factor of 2 compared to previous standard hardware setups. We discuss the implications of improved coupling and its potential to enable cutting-edge applications, such as pulsed high-field DNP and the use of low-power solid-state microwave sources.