Holland Julian, Weber Rudolf, Sailer Marc, Graf Thomas
Institut für Strahlwerkzeuge, Universität Stuttgart, Pfaffenwaldring 43, 70569 Stuttgart, Germany.
TRUMPF Laser GmbH, Aichhalder Straße 39, 78713 Schramberg, Germany.
Materials (Basel). 2022 Mar 18;15(6):2257. doi: 10.3390/ma15062257.
Soft X-ray emissions during the processing of industrial materials with ultrafast lasers are of major interest, especially against the background of legal regulations. Potentially hazardous soft X-rays, with photon energies of >5 keV, originate from the fraction of hot electrons in plasma, the temperature of which depends on laser irradiance. The interaction of a laser with the plasma intensifies with growing plasma expansion during the laser pulse, and the fraction of hot electrons is therefore enhanced with increasing pulse duration. Hence, pulse duration is one of the dominant laser parameters that determines the soft X-ray emission. An existing analytical model, in which the fraction of hot electrons was treated as a constant, was therefore extended to include the influence of the duration of laser pulses on the fraction of hot electrons in the generated plasma. This extended model was validated with measurements of H (0.07) dose rates as a function of the pulse duration for a constant irradiance of about 3.5 × 1014 W/cm2, a laser wavelength of 800 nm, and a pulse repetition rate of 1 kHz, as well as for varying irradiance at the laser wavelength of 1030 nm and pulse repetition rates of 50 kHz and 200 kHz. The experimental data clearly verified the predictions of the model and confirmed that significantly decreased dose rates are generated with a decreasing pulse duration when the irradiance is kept constant.
在用超快激光加工工业材料的过程中，软X射线发射备受关注，尤其是在法律法规的背景下。光子能量大于5 keV的潜在危险软X射线源自等离子体中的热电子部分，其温度取决于激光辐照度。在激光脉冲期间，随着等离子体膨胀的增加，激光与等离子体的相互作用增强，因此热电子的比例随着脉冲持续时间的增加而提高。因此，脉冲持续时间是决定软X射线发射的主要激光参数之一。因此，一个现有的分析模型（其中热电子比例被视为常数）被扩展，以纳入激光脉冲持续时间对所产生等离子体中热电子比例的影响。该扩展模型通过测量H(0.07)剂量率进行了验证，H(0.07)剂量率是在约3.5×10^14 W/cm^2的恒定辐照度、800 nm的激光波长和1 kHz的脉冲重复率下，以及在1030 nm的激光波长、50 kHz和200 kHz的脉冲重复率下变化的辐照度时，作为脉冲持续时间的函数。实验数据清楚地验证了模型的预测，并证实当辐照度保持恒定时，随着脉冲持续时间的减少，剂量率会显著降低。
