Dynamic nuclear polarization (DNP) is a signal enhancement technique becoming more and more important in NMR studies of biological samples and materials. Enhancement of NMR signals is accomplished by doping samples with stable free radicals. The trapped free radicals in the cooled solid sample are irradiated continuously at the EPR microwave frequency. Microwaves are generated by a gyrotron (which requires iits own superconducting magnet in addition to the superconducting NMR magnet). The microwave radiation is introduced into the NMR probe by way of a wave guide. While the unpaired electrons are irradiated, the population distribution of the Zeeman states of NMR active nuclei are modified providing more polarization and therefore a large NMR sensitivity enhancement. Typically the polarized protons in the sample are used as a cross polarizatuion source for less abundant nuclides. The data are typically acquired at low temperature with magic angle spinning. The overall NMR enhancement is typically one or two orders of magnitude when one compares NMR spectra acquired with the microwave source on vs off. Commercial DNP-NMR instruments are now available.
Thorsten Maly authors a very informative BLOG on all things DNP-NMR. I encourage you to take a look at it.