Characterization of focused free-electron laser beams using their imprints in solid materials

An unflagging development of novel X-ray laser sources has stimulated a continuous growth in the X-ray scientific community and in new types of scientific endeavours. Unprecedented progress in X-ray lasing and improvement of all observed laser beam parameters brings ever higher requirements on laser field diagnostics. One of the most challenging issues is a rigorous spatial characterization of focused X-ray laser beams. Owing to excessive radiation intensities and (sub)micron focus sizes, radiation damage, linear response and sufficient resolution power of the sensing element are important problems to be addressed. Advanced methods of ablation and desorption imprints represent a promising way towards solution of these issues since the radiation damage is no longer considered as an unwanted effect but rather as a means of measurement. Up to now, a versatile ensemble of tools, exploiting ablation and desorption in various materials, was developed. The method of fluence scan makes it possible to characterize the transverse intensity profile and energy density distribution of a focused non-Gaussian beam from its ablative imprints in a suitable material. This is of very high importance for laser-matter interaction experiments since the beam represents an inseparable part of the interaction process. Especially in high-energy density physics the measurements of nonlinear phenomena can be significantly distorted by the beam profile. Moreover, recent developments in desorption imprinting brought a possibility to retrieve the wavefront and transverse coherence of a focused X-ray laser beam. In this talk, an overview of imprinting techniques and their practical use in real experiments will be given. A possible way towards direct, in-situ and nearly real-time application of imprinting methods will be introduced and described.