Study on X-ray Emission Using Ultrashort Pulsed Lasers in Materials Processing.

Joerg Schille,Sebastian Kraft,Maurice Clair,Alexander Horn,Udo Loeschner,Theo Pflug,Christian Scholz

doi:10.3390/ma14164537

Joerg Schille, Sebastian Kraft + Show 5 more

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https://doi.org/10.3390/ma14164537

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Abstract

The interaction of ultrashort pulsed laser radiation with intensities of 1013 W cm−2 and above with materials often results in an unexpected high X-ray photon flux. It has been shown so far, on the one hand, that X-ray photon emissions increase proportionally with higher laser power and the accumulated X-ray dose rates can cause serious health risks for the laser operators. On the other hand, there is clear evidence that little variations of the operational conditions can considerably affect the spectral X-ray photon flux and X-ray emissions dose. In order to enhance the knowledge in this field, four ultrashort pulse laser systems for providing different complementary beam characteristics were employed in this study on laser-induced X-ray emissions, including peak intensities between 8 × 1012 W∙cm−2 < I0 < 5.2 × 1016 W∙cm−2, up to 72.2 W average laser power as well as burst/bi-burst processing mode. By the example of AISI 304 stainless steel, it was verified that X-ray emission dose rates as high as > 45 mSv h−1 can be produced when low-intensity ultrashort pulses irradiate at a small 1 µm intra-line pulse distance during laser beam scanning and megahertz pulse repetition frequencies. For burst and bi-burst pulses, the second intra-burst pulse was found to significantly enhance the X-ray emission potentially induced by laser pulse and plasma interaction.

Highlights

Ultrashort pulse lasers feature excellent performances for high-efficient and precision machining that has attracted increasing attention for innovations in micro fabrication and surface engineering
A maximum X-ray dose rate level was reported at a 30◦ detection angle, as investigated in the range between
The correct detector–substrate distance is of relevance because, firstly, the X-ray photons are absorbed in air, and secondly, to ensure that the active detector area is homogeneously exposed by the laser-induced X-ray emissions

Summary

Introduction

Ultrashort pulse lasers feature excellent performances for high-efficient and precision machining that has attracted increasing attention for innovations in micro fabrication and surface engineering. The recent power scaling of femtosecond (fs) and picosecond (ps) lasers reaching multi-kW class level [1,2] will pave the way of the ultrashort pulse laser technology from the laboratory into industrial production. In typical material processing scenarios, ultrashort laser pulses of micro to milli joules optical energy and hundreds of femtoseconds to picoseconds pulse durations are employed, for example, in laser micro drilling [3,4], laser engraving [5,6], or laser surface texturing and modifications [7,8]. When using laser pulses of such high intensity in material ablation, high-intense electron plasmas will be produced, itself emitting X-ray photons at a several keV level. The X-ray photons result from Bremsstrahlung and characteristic line emissions when accelerated electrons lose their kinetic energy by scattering, acceleration and recombination with other charged particles, in free–free, free–bound, and bound–bound processes

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