Fig. 4.

a Schematic diagram of the 129-GHz microwave system used for DNP. The components within the dashed box (attenuator-3 port circulator-diode detector) can be bypassed during DNP. Insets photographs of the top section of the microwave system. b Relative ¹³C polarization level of trityl-doped [1-¹³C]pyruvic acid sample (see Table 1) as a function of microwave power at 4.6 T and 1.1 K where the threshold microwave power is around 20 mW. Data were plotted by obtaining the maximum ¹³C NMR intensity after microwave irradiation of the sample at the positive polarization peak. Microwave power was controlled by a variable attenuator and the power was measured with a diode detector. The dashed curve is a guide for the eyes