Photons leak undisciplined through each crack, but they may also "laser" in a perfect harmony - thanks to induced emission found by Einstein in 1916: An electron bound within the atom is stimulated by a photon to emit another, identical photon.

A laser that rests upon free electrons (FEL) exploits as acceleration - that bears photons spontaneously - a slalom route enforced by magnets (figure). The emitted light ray may be amplified between two mirrors, as with the 'normal' laser.

The advantage: In the FEL the laser frequency can be continuously changed. The problem: There are no suitable mirrors for Roentgen light. The trick: Magnetic structure, velocity and wavelength are tuned in a way that the self-amplification builds up already en route - the 'electron disks' follow one another with precisely a light wavelength distance and do permanently emit further such photons ... SASE ("Self Amplified Spontaneous Emission") is a completely new laser principle.