This article has nothing to do with DNP. However, the article nicely describes an impressive piece of isntrumentation to reach very low temperatures.
Melhuish, S.J., et al., A sub-Kelvin cryogen-free EPR system. J. Magn. Reson., 2017. 282(Supplement C): p. 83-88.
http://www.sciencedirect.com/science/article/pii/S1090780717301829
We present an EPR instrument built for operation at Q band below 1K. Our cryogen-free Dewar integrates with a commercial electro-magnet and bridge. A description of the cryogenic and RF systems is given, along with the adaptations to the standard EPR experiment for operation at sub-Kelvin temperatures. As a first experiment, the EPR spectra of powdered Cr12O9(OH)3(O2CCMe3)15 were measured. The sub-Kelvin EPR spectra agree well with predictions, and the performance of the sub-Kelvin system at 5K is compared to that of a commercial spectrometer.