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

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 H˙′ (0.07) > 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.

X-ray Emission Hazards from Ultrashort Pulsed Laser Material Processing in an Industrial Setting

“…During the surface process with a pulse energy of 1980 µJ, an ambient dose equivalent rate d H *(10)/d t of around 1 µSv/h was measured. These X-ray emission dose rates are much lower compared to the mSv/h reported recently in the literature for the same laser pulse duration and a lower laser pulse energy [ 11 , 12 , 18 , 19 ].…”

“…With the microSHAPE TM USP-laser machine employing a tungsten workpiece, the measured dose rates are much smaller than expected for a laser-induced plasma generated at peak intensities of 3.3 × 10 15 W/cm 2 with tungsten as a target material. In the literature, typically, dose rates in the range of mSv/h are measured for such high peak intensities [ 18 , 19 ]. As can be seen from Table 1 , the measured dose rate decreased with increasing pulse energy, which is unexpected.…”

“…The use of laser peak intensities above 10 13 W/cm 2 in combination with laser pulse repetition rates in the few 100 kHz range can already lead to X-ray dose rates clearly exceeding the permitted limits for members of the public. Tungsten and steel in particular show significant X-ray emission [ 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 ]. In [ 11 ], it was demonstrated that other materials such as aluminum and glass show significantly lower X-ray emissions.…”

“…It has been shown very recently, experimentally, that significantly higher X-ray dose rates can be generated during USP-laser machining when using a laser burst mode instead of the single-pulse mode utilizing the same total laser intensity. The increase in X-ray yield was attributed to the interaction between the ultrafast laser radiation and an ablation cloud or high-density plasma [ 19 , 20 ]. This special regime was not investigated within the framework of this paper.…”

“…The theoretically achievable peak intensity of 3.3 × 10 15 W/cm 2 is comparatively high and normally used for the processing of transparent glasses making use of nonlinear absorption mechanisms. So far, only Schille et al have employed higher laser peak intensities of up to 5.2 × 10 16 W/cm 2 in a recent study [ 19 ]. The aluminum protection housing at the USP-laser machine at SCHOTT AG was an interesting test subject as the protective effect is expected to be much smaller than for steel enclosures.…”