Abstract
An analytical model is presented, which allows estimating the expected dose rates resulting from X-ray emission from ultra-short-pulse laser-produced plasma under industrial conditions. The model is based on the calculation of the Bremsstrahlung spectrum in the X-ray region between about 5 keV and 50 keV, which is created by the hot electrons in the plasma. The model was calibrated with both spectral and dose rate measurements. The scaling of the hot-electron temperature and the fraction of hot electrons in the plasma served as calibration values. The agreement between experiments and model for the investigated irradiances in range from 1012 to 1015 W/cm2 is excellent. The expected dot{H}(0.07) and dot{H}(10) dose rates at a distance of 20 cm from the process in air were calculated for upcoming lasers with 1 kW of average power. Although the dose rates close to the plasma significantly exceed the allowed dose of 50 mSv per year for an irradiance exceeding about 2·1015 W/cm2, the calculations show that shielding with a 2-mm sheet of iron already at a distance of 20 cm attenuates the radiation to a safe value below 0.4 µSv/h.
Highlights
Materials processing with so-called ultrafast lasers with pulse durations below about 10 ps has been proven to allow very precise processing of a wide range of materials with mechanical and thermal accuracies in the micrometer range [1,2,3,4,5,6,7]
The fraction of radiation above 5 keV from these plasmas is low, the X-ray emission might become an issue with increasing average power of the ultrafast lasers, because the dose rate increases proportionally with the average power
For this work it is assumed that the predominant part of the Bremsstrahlung emission in the part of the spectrum of interest is produced by the hot electrons, during the laser pulse, and at a constant hot-electron temperature in Expected X‐ray dose rates resulting from industrial ultrafast laser applications
Summary
Materials processing with so-called ultrafast lasers with pulse durations below about 10 ps has been proven to allow very precise processing of a wide range of materials with mechanical and thermal accuracies in the micrometer range [1,2,3,4,5,6,7]. Materials processing with ultrafast lasers has gained attention for industrial applications due to the increase of the average power up to the 100-W region, allowing reasonable productivity. Only little work was published for the X-ray emission from plasma under industrial processing conditions, i.e., in ambient air at atmospheric pressure, at rather moderate irradiances of 1010–1014 W/cm, and for average laser powers up to a few 100 W. The fraction of radiation above 5 keV from these plasmas is low, the X-ray emission might become an issue with increasing average power of the ultrafast lasers, because the dose rate increases proportionally with the average power. Legall et al published extensive measurements of the X-ray emission from laser-produced plasma under industrial processing conditions [11]. The model allows one to estimate the dose rates created during processing with upcoming kilowatt-level lasers, and to determine the required shielding of the laser processing cell for safe operation
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