A proper spatial characterization of a laser beam profile is indisputably important for any laser-mater experiment as well as for protection of beamline optical elements. Method of ablation and desorption imprints provides thorough beam profile analysis applicable to a broad range of photon energies. This method, however, often requires up to thousands of shots which must be then manually analyzed. Here we present method based on deep learning image segmentation model which is able to substitute human element currently indispensable in this time-consuming ex situ post processing. It is a part of AbloCAM project – an universal device for semi-automatic beam profile analysis.
We report on ion emission from plasma produced on thick targets irradiated with nanosecond and femtosecond pulses delivered by mid-ultraviolet and soft x-ray lasers, respectively. To distinguish between different ion acceleration mechanisms, the maximum kinetic energy of ions produced under different interaction conditions is plotted versus laser fluence. The transformation of the time-of-flight detector signal into ion charge density distance-of-flight spectra makes it possible to determine the mean kinetic energy of the fastest ion groups based on the influence of the acoustic velocity of ion expansion. This allows obtaining additional characteristics of the ion production. The final energy of the group of fast ions determined using the ion sound velocity model is an order of magnitude larger in the fs-XFEL interaction than in the ns-UV one. On the contrary, the ablation yield of ions in our experiment is seven orders of magnitude greater when applying ns-UV laser pulses, not only due to higher energies of UV laser pulses, but also due to a significant difference in interaction and ion formation mechanisms.
For quick, efficient and accurate alignment and characterization of focused short-wavelength (i.e., extreme ultraviolet, soft x-ray, and x-ray) laser beams directly in the vacuum interaction chambers, an instrument has to be developed and implemented. AbloCAM should represent such a handy tool looking at ablation imprints of the beam in a suitable material without breaking vacuum and need for a liberation of exposed samples from the chamber to analyse them ex situ. First steps we made in this direction can be found in ref. [1] The technique of the fluence scan (F-scan method; for details see [2,3]), proven at several FEL facilities, e.g., FLASH (Free-electron LASer in Hamburg) and LCLS (Linac Coherent Light Source), makes possible to characterize the beam utilizing just an outer contour of the damage pattern. It is not necessary to measure a crater profile for the beam reconstruction. Not only lateral, but also a longitudinal distribution of irradiance can be determined in the focused beam by its imprinting (z-scan method [4]). Technically, the AbloCAM tool consists of a vacuum compatible motorized positioning system executing a series of well-defined irradiations of a chosen slab target according to algorithms fulfilling requirements of the combined F(z)-scan procedure. Damage patterns formed in that way should then be visualized in situ by means of Nomarski (DIC – Differential Interference Contrast) microscope equipped with the software which indicates and processes pattern outer contours. There is a feedback established between positioning and inspecting components and functions of the tool. The software helps to align and characterize any focused beam in the interaction chamber semi-automatically in a reasonable time.
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