Figure 5.

As overheating of the sample during DNP, predicted by the model of Figs. 3c, 3d, was reduced after implementing several system modifications, we observed a four-fold increase in the ultimate ¹²⁹Xe polarization (a) compared to trace (b) of Fig. 3a. Part of this increase (from 13.3 to 16 h along the horizontal axis) was achieved by switching the cryostat from continuous to single-shot operation, where the liquid helium flow through the supply capillary was first increased (between 13.3 and 14.2 h) and then shut off completely. As a result, the equilibrium temperature of the superfluid liquid helium was first raised and then lowered by about 0.1 K relative to the continuous mode. The level of helium rose above the probe sensor during the first maneuver, but then progressively dropped below the sensor, leading to a gradual rise in the recorded temperature (c). The sample, however, was located 4 cm below the sensor, thus being exposed to the lowest temperature for a longer time, as evidenced by the gradual further rise in polarization.