Maser experiments were initiated by a short and low power pulse (8µs at 1.23µW ) with a small initial flip angle (0.01 o ), following a polarization buildup period. A high nuclear polarization state was achieved by irradiating the sample with the µw source without signal monitoring so as to avoid perturbation of the 1 H magnetization by radiofrequency pulses, during a period of time determined from preparatory buildup experiments. This buildup time was set to 40 min. Signal acquisition then started and lasted typically for tens to thousands of seconds, during which the sample was under continuous µw irradiation. This experimental procedure led to consistent observations of multiple maser pulses. In the example shown in Figure , the induction signals were obtained on a total duration of 4000 s. In order to handle these large data sets, the signal was acquired in a pseudo-2D fashion as a series of 200 scans of 20 s each, where a single rf pulse only was used at the beginning of first scan to trigger radiation damping. All other induction signals were acquired under µw irradiation, and no further rf pulse was applied.