Abstrakt
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- matter interactions and non-linear phenomena. We studied the fluence distribu- tion 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 CVD diamond.
By varying the laser pulse energy between the PbI2 ablation threshold and few or- ders 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 scientific instruments, namely, HED (hard X-ray) and SQS (soft X-ray) at the European XFEL. At the HED instrument focal spots with effective areas of less than 1 µm2 were observed. At the SQS instrument the focus measurement showed an astigmatic aberration of the focusing adaptive KB optics.
In this Paper, we present experimental results obtained at these two scientific in- struments.