Schematic of the laser-plasma interaction at the material surface during ultrashort pulse laser material processing
Source: BAM
The development of the formerly exotic ultrashort pulse lasers (USPL) has made enormous progress in recent years. Today, commercial USPL emit average powers up to the kilowatt level at pulse repetition rates reaching the MHz range. Stable laser pulses with durations in the picosecond and femtosecond domain are routinely generated. This modern laser technology is the key to a wide range of applications in industrial laser material processing on the micro- and nanometer scale. The advantages of low heat load into the material are exploited. Additionally, ultrashort laser pulses can be used universally in materials processing, almost regardless of the material. Thus, the processing of steel to glass as well as the modification of complex composite materials up to fragile/sensitive biological objects is possible. Of course, laser material processing should also be safe for the operator when using USPL. The primary hazard from the laser radiation itself is well known and appropriate protective measures, such as the use of laser safety cabins and protective goggles, are being taken and are already regulated by standards. However, secondary hazard potentials, such as the emission of X-rays, must increasingly be taken into account. For example, when high laser intensities are used on the workpiece, X-rays can also be generated undesirably by generating a highly excited laser-induced plasma at the material surface during the machining process. This plasma is reheated by the laser pulse itself and eventually causes X-ray generation through interaction with the machined target (see figure). The nowadays available high laser pulse repetition rates can thus lead to radiation accumulation and to X-ray doses exceeding the permitted regulatory limits for human exposure. Within the framework of the project Determination of radiation protection scenarios as a prerequisite for a sustainable guarantee of radiation protection when using ultra-short pulse lasers, inter alia to support uniform enforcement, which is funded by the German Federal Office for Radiation Protection (BfS), the Physikalisch-Technische Bundesanstalt (PTB) and BAM have jointly carried out investigations on high-power laser systems of Schott AG. The results of the dosimetric and spectrometric measurements of laser-induced X-ray emissions in a realistic industrial working environment show that the detection of worst-case X-ray emissions under reference conditions is a complex process. The results jointly developed and published by PTB, Schott AG and BAM will receive immediate regulatory attention.
X-ray emission hazards from ultrashort pulsed laser material processing in an industrial setting
U. Stolzenberg, M. Schmitt Rahner, B. Pullner, Herbert Legall, Jörn Bonse, M. Kluge, A. Ortner, B. Hoppe, Jörg Krüger
published in Materials, Vol. 14, No. 7163, pages 1 - 17, 2021
BAM division Interfacial Processes and Corrosion