## 3 Results and Discussion
Besides the issue of imperfections of the sample design, the reproducibility of the experiments is additionally subjected to fluctuations in the shape of the keyhole. Especially the fluctuations of the keyhole depth as depicted in Figure 11 are important to the interpretation of the measured temperatures. These fluctuations are not predictable by now and cannot be controlled by the experimental boundary conditions. Although the absolute measuring depth as seen from the sample's surface is controlled by the sample design, the relative measuring depth at the keyhole wall is also determined by the total keyhole depth itself. As it is not possible to predict or even control the fluctuations during each measurement performed under equal experimental conditions, the relative measuring depth is arbitrary, as the keyhole depth in the valid RoM remains unknown. In Figure 11, it is shown schematically that, at \(t_{2}\), the keyhole depth takes the value \(l_{\text{key}}(t_{2})\). As \(l_{\text{key}}(t)\) is also sensitive to boundary conditions that are beyond the control of the experimenters, \(l_{\text{key}}(t_{2})\) might differ for each measurement. Figure 11: Schematic of the relative position between the probe and the melt pool during different phases of the ongoing measurements. Figure 10: The measured temperature profile with corresponding positions inside the melt pool. RoM: region of measurement. Throughout the experiments, eight measurements were conducted successfully. The temperature profile obtained from the valid RoM, as distinguished in the previous section, is chosen as the example for discussion (Figure 12). The temperature starts at values slightly below the melting point of aluminum. In the beginning, the temperature curve is rather smooth and, then, becomes increasingly noisy. In this early stage of the interaction between the melt pool and the refractory probe, damage to the probe is unlikely, so the noise is not ascribed to scattered radiation or reflections of any kind but, rather, to potential vibrations of the melt pool that are transferred to the probe. Thereby, the angle between the temperature screen and the beam path of the pyrometer might change in short time scales, influencing the detected intensity of the thermal radiation. The pronounced temperature drop at the end of the depicted region is related to the blow-out that occurs when the keyhole reaches the tantalum foil. It is possible that the fact that the temperature drops within a finite time interval and not abruptly is due to the cooling of the tantalum foil as a consequence of the exposure to gaseous convection by shielding and ambient gas entering the crater left behind by the blow-out. Strong fluctuations of the temperature at time scales of 2 ms up to 6 ms can be seen at the beginning at \(t\approx-26\) ms and in the middle of the displayed curve at \(t\approx-15\) ms. To the best of our knowledge, it remains unclear whether these deviations emerge from changes in the probe structure, e.g., from the thermal expansion of the tantalum foil, thereby changing its curvature or its whole position on the tube. This could happen as the force-fit connection between the tantalum foil and the tube may no longer exist when the foil starts to expand at high temperatures. The exposition to the flow velocity of the prerunning melt could cause a tilting of the temperature screen, which could account for the strong and abrupt temperature changes on time scales too long, as they could be ascribed to vibrations. The latter ones, as they are labeled in Figure 12, exhibit frequencies of \(f_{\rm vib}=2\) kHz to 5 kHz and, therefore, possibly emerge from the keyhole oscillations, according to [14].