Following the construction of the Diamond synchrotron in Oxfordshire, a high intensity X-ray source, it's been announced that XEFL has received the green light in Hamburg. They're billing it as an 'X-ray laser'. I had my doubts that it was actually a laser, that there really was stimulated emission, the block diagram on the BBC site looked like it was just electrons being 'wiggled', and hence made to radiate, however according to the XFEL site "In February 2000, for the first time worldwide, laser light in the far-ultraviolet range was created using the SASE effect, which is also the basis for the XFEL." SASE is a new one on me, and so I've done a little research. Brookhaven refers to the SASE effect in 2002. This Japanese paper (in English) goes into some more detail .
From what I gather, the electron beam is undulated. There is an interplay between the undulating beam and the light produced by virtue of the undulation. This means that the light produced becomes coherent. The electrons are then stripped from the beamline. The beam produced is spatially coherent, but not temporally coherent. In other words, there is a fixed phase relationship between parts of the beam as one moves across the beam - however knowing the phase now tells you nothing about the phase a short time later.