Experimental study of the temporal coherence and spectral profile of the 13.9 nm transient X-ray laser

A new Fresnel interferometer is used to investigate the temporal coherence of the 13.9 nm Ni-like Ag X-ray laser. The results obtained show a coherence time of ≈3ps, corresponding to a spectral linewidth Δλ of 0.7 mÅ, which is narrow. The pulse duration, also measured in this experiment, is shown to be close to the Fourier-transform limit. Finally, we discuss the implications of the high temporal coherence of the X-ray laser on the structure of the far-field pattern of the beam.