Abstract
Accurate characterization of the fluence distribution [J/cm2] in the focus of ultra-high intensity laser beam is of great importance in experiments investigating laser-plasma interactions. We studied the fluence distribution in the laser beam focus utilizing the direct method of ablative imprints.
The method is based on creating cross-sections at varied levels of the fluence profile by imprinting the beam into a thin layer of PbI2 deposited on a slab of poly(methyl methacrylate). By varying the laser pulse energy between the PbI2 ablation threshold and few orders of magnitude above the threshold, the fluence distribution is reconstructed from the cross-sections and effective area is calculated.
The effective area gives a relation between the pulse energy and the maximum of the fluence distribution. We measured focal spots at two ultra-high intensity laser facilities.
Focal spots with effective areas of less than 1 μm2 were observed at the European XFEL which produces femtosecond pulses in the X-ray spectral region. At sub-kilojoule near-infrared Prague Asterix Laser System providing sub-nanosecond pulses, we measured larger focal spots with recognizable details smaller than 1 μm2 in individual cross-sections.
In this Paper, we present experimental results obtained at these two distinct laser sources and discuss use of ablative imprints method in characterization of ultra-intense laser foci.