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.

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Avoiding Pile-Up during spectral X-ray measurements during ultrafast laser processing with a single detector requires long measurement times at large distances from the processing area due to the short pulse duration and high photon fluxes. To enable fast measurements, an algorithm is presented which calculates the underlying Pile-Up free spectrum of any measured spectrum. Therefore, a statistical approach was used to describe the mean number of photons ⟨𝑛⟩ and their corresponding photon energies Eph hitting the detector at each pulse. This number of photons hitting the detector each pulse was assumed to be geometrically distributed, whereas the photon energies follows a modified Maxwell-Boltzmann distribution, mainly defined by the temperature Thot of the hot electrons in the laser plasma. An initial guess of ⟨𝑛⟩ and Thot was used to calculate an expected Pile-Up spectrum at the detectors position. Comparing the calculated Pile-Up spectrum with the measured one and iteratively adjusting ⟨𝑛⟩ and Thot results in the underlying Pile-Up free spectrum.