The electron beam is pulsed at 30 Hz, and the electrons are ON for 8 microseconds. In fact, using a two-photon absorption autocorrelator, the laser micropulses have been measured from 0.75 to 1 picoseconds. See Figure 2 for a representative schematic of the pulse train. However, narrower line widths have been observed, and wider widths are typical. The electron beam emits enhanced spontaneous radiation at the odd harmonics of the lasing wavelength.
Filters are available to remove the harmonics.
The stability of the FEL is pretty good, but not as good as the best lasers. This consideration must be built into any experiment, normalizing to a picked off reference for instance. Due to FEL physics, the higher the energy per macropulse the worse the stability. So, if you do not need very high energy be sure to inform your operator. Almost always between shifts the FEL will have been re-steered for another lab.