Dynamic nuclear polarization with a cyclotron resonance maser at 5 T

DNP (dynamic nuclear polarization) experiments at 5 T are reported, in which a cycoltron resonance maser (gyrotron) is utilized as a 20 W, 140 GHz microwave source to perform the polarization. MAS (magic angle spinning) NMR spectroscopy with DNP has been performed on samples of polystyrene doped with the free radical BDPA (\ensuremath{\alpha},\ensuremath{\gamma}-bisdiphenylene-\ensuremath{\beta}-phenylallyl) at room temperature. Maximal DNP enhancements of \ensuremath{\sim}10 for $^{1}\mathrm{H}$ and \ensuremath{\sim}40 for $^{13}\mathrm{C}$ are observed and are considerably larger than expected. The DNP and spin relaxation mechanisms that lead to these enhancements at 5 T are discussed.