Origin of microstructures in picosecond X-ray laser beams

Beam sections of transient collisional excitation (TCE) soft X-ray lasers exhibit highly contrasted intensity modulations forming complex patterns. We present model calculations that reproduce these experimental observations both qualitatively and quantitatively. The low spatial coherence, high longitudinal coherence and short pulse duration of the source are shown to control the visibility of the beam structures. Comparison with other types of laser-based soft X-ray lasers is discussed. We derive simple relationships that relate the spatial and temporal characteristics of the emitting source to the main features of the beam small-scale structures.