Damage thresholds of various materials irradiated by 100-ps pulses of 21.2-nm laser radiation

Irradiation experiments were conducted at Prague Asterix Laser System (PALS) with the Ne-like zinc soft x-ray laser (SXRL) at 21.2 nm (58.5 eV) delivering up to 4 mJ (~4 x 1014 photons), 100-ps pulses in a narrowly collimated beam. The SXRL beam was focused using a 1 inch diameter off-axis parabolic mirror (f = 253 mm at 14 degrees) with a Mo:Si multilayer coating (R = 30% at 21 nm) placed 2825 mm from the SXRL. The diameter of the SXRL beam incident on the mirror was about 11 mm. Ablation experiments with a gradually attenuated beam were performed to determine the single-shot damage threshold of various materials. In this case, the sample was positioned at the tightest focus of the SXRL whose pulse energy was attenuated by aluminum filters of various thickness to adjust the fluence. Both the focal spot area and single-shot damage threshold were determined from the plot of damaged surface areas as a function of a pulse energy logarithm to dete. For PMMA, the focal spot area and the ablation threshold inferred from the data are Sfoc = (1172±230) μm2 and Fth = (1.25±0.4) J/cm2, respectively. Inorganic materials have thresholds significantly higher than organic polymers, e.g., amorphous and monocrystalline silicon gave values 2.5 J/cm2 and 4.2 J/cm2, respectively. For prospective SASE FEL optical elements, the SiC coating is of great interest. Its damage threshold is of 20 J/cm2, i.e., slightly lower than that of monocrystalline silicon. The thresholds determined with the 100-ps pulses from plasma-based, quasi-steady state SXRL are significantly higher than the thresholds obtained for 20-fs pulses provided by the SXR freeelectron laser in Hamburg. There is a difference in PMMA thresholds of two orders of magnitude for these two sources.

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