Abstract
Ultrashort pulse laser processing of materials allows for precise machining with high accuracy. By increasing the repetition rate to several 100 kHz, laser machining becomes quick and cost-effective. Ultrafast laser processing at high repetition rates and peak intensities above 1013 W/cm2 can cause a potential hazard by generation of unwanted x-ray radiation. Therefore, radiation protection must be considered. For 925 fs pulse duration at a center wavelength of 1030 nm, the x-ray emission in air at a repetition rate of 400 kHz was investigated up to a peak intensity of 2.6 × 1014 W/cm2. Based on the presented measurements, the properties of potential shielding materials will be discussed. By extending our previous works, a scaling of the x-ray radiation emission to higher peak intensities up to 1015 W/cm2 is described, and emitted x-ray doses are predicted.
Highlights
Ultrashort pulse laser processing of solids allows for precise machining with high accuracy because of a weak thermal interaction between the laser beam and the bulk material
It was shown that ultrafast laser processing can be accompanied by safety relevant emission of x-ray radiation at such high repetition rates if the peak intensity exceeds 1013 W/cm2.1 x-ray radiation protection in ultrashort laser material processing became a focus of scientific interest.[2,3,4,5]
It could be shown that the “resonance absorption” leads to a Maxwellian tail of hot electrons, and the electron temperature scales with (Iλ2)1/3.6–10 these results cannot be transferred to the conditions in laser material processing with higher repetition rates at lower peak intensities, since both topographical scitation.org/journal/jla changes during laser processing and a heat accumulation in the sample may significantly influence the x-ray generation4—the latter by reducing the laser pulse energy consumed in the initial plasma formation stage
Summary
Ultrashort pulse laser processing of solids allows for precise machining with high accuracy because of a weak thermal interaction between the laser beam and the bulk material. The incident laser field causes free electrons in the solid to oscillate, which subsequently collide with the atoms of the solid, knock electrons out of their shells, which in turn are excited to oscillate in the laser field, until a hot mixture of free electrons and ions is present and a laser-induced plasma is born This laser plasma is usually created by the leading edge of the incoming laser pulse. A known mechanism leading to x-ray generation in the keV range is the so-called “resonance absorption.”[6] By this collision-less absorption mechanism, plasma electron waves are resonantly excited The efficiency of this process strongly depends on the polarization state of the laser beam, the pulse duration, and the angle of incidence of the laser pulse.
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