Our observations are in line with previous work in many respects, but also show strikingly specific features. In ref. the induction signals consistently exhibited a short series of ∼ 2 -4 ms maser pulses separated by delays of decreasing duration for ∼ 100 ms, followed by monotonously decaying signal for tens of seconds, without further maser pulses. No such signals were observed here, where, in contrast, all the observed sustained maser series were of the kind shown in Figure 2, without damping of the pulse intensities. These discrepancies are quite puzzling, but some significant differences between experimental conditions in which experiments were performed here and in ref. should be pointed out that may help their interpretation. Here, experiments were performed with a probe of moderate Q, so that radiation damping was less efficient. Therefore, in order to achieve a large enough magnetization, larger volume samples were used (800 µL instead of 150 µL used in our dissolution-DNP setup) and only unlabelled (non deuterated) compounds were used for the sample preparation in order to increase the 1 H density. We found that these conditions were necessary to generate MASER pulses. Besides, the complete absence of 2 H nuclei in the sample precluded repolarization of 1 H through a cross-talk mechanism, in contrast with our previous study. Finally, MASER pulses were not observed in the absence of continuous µw irradiation, which was deemed necessary in all cases in the present study. This is explained by the absence of deuterium in the sample, which is not coupled to the probe but serves as a repolarization reservoir for 1 H. 3 Theoretical hypotheses and Numerical model