# Investigation of Pre-Pulse Effects on Ultrashort-Pulse Laser Interaction with Structured Targets
## 3 Experiment Results  
Figure 2a-c show typical optical spectrum plots of the plasma emission from the laser target interaction at APP delay timings of \(-\)2, \(-\)5, and \(-\)14 ns, respectively, with APP fluence ranges of (I) no APP, (II) 0.1-0.2, (III) 0.4-0.6 and (IV) 2-5 J/cm\({}^{2}\). It is observed that at no-APP or low-APP fluence (conditions I and II), a strong narrow peak at \(\approx\)402 \(\pm\) 2 nm is dominant at all time delays. At the highest APP fluence (condition IV), multiple broad peaks with wavelengths (\(>\)650 nm) start to dominate while the peak at 402 nm is attenuated. When the pre-pulse delay \(\Delta\)t is set to \(-\)5 ns, the 402 nm peak remains dominant, but a distinct peak at wavelength 719 nm is also visible at moderate (III) and high fluences (IV) (Figure 2b). The spectral profile in case of \(\Delta\)t 5 ns and moderate APP fluence (III) is shown in detail in Figure 2d. Apart from the broadened signal at 402 nm and signal close to 719 nm (wavelengths between 725 and 925 nm are filtered out), there is a distinct signal observed at \(\approx\)493 nm. CR39 solid-state nuclear track detector plates were used as a proton energy (PE) diagnostic tool in the experiment due to their ability to provide both the energy and the spatial distribution of the protons. The use of overlapping aluminum-net filters enables proton energy discrimination, albeit coarse, to a certain extent. Figure 3a shows a typical aluminum-net filter attached to the surface of a 5 cm \(\times\) 5 cm CR39 plate of 850 \(\upmu\)m thickness. The filter set consists of a base filter covering the entire surface of the CR39 and a grid net structure consisting of aluminum strips of 3 mm width. By choosing different combinations of aluminum foil thickness, both the range and resolution of proton energy measurement can be controlled. In a typical experiment, the delay between the APP and the MP is kept fixed, with the MP energy at \(\approx\)25 \(\pm\) 2 mJ. Six CR39 plates are mounted on a rotating motor, and only the intended plate is exposed to the proton beam path, which is drawn schematically in Figure 1b, with the others being shielded. The APP fluence is varied between 0(no APP) and \(\approx\)15 J/cm\({}^{2}\), centered around 0.4 J/cm\({}^{2}\)--the spikes optical damage threshold was found to be close to the snow optical damage threshold obtained by [PERSON] et al. [43]. The procedure is repeated at different time delays. Post-experiment, the CR39 plates are developed by etching. Figure 3b shows the result of an experiment at \(\Delta t\approx-\)5 ns and APP fluence of 0.5 J/cm\({}^{2}\), where protons with energies in the range 2.5 MeV \(\leq\) PE \(<\) 2.8 MeV were obtained, the highest in this work with MP energy of 25 mJ. The adjacent SEM image in Figure 3b shows typical holes created by the protons in detail (post-development). A counting algorithm implemented in MATLAB estimated the average flux of protons in this energy range to be \(\sim\)10\({}^{4}\) cm\({}^{-2}\) per shot. Proton energy ranges at various APP to MP time delays and APP fluence conditions were measured, and the lower bound value (e.g., 2.5 MeV for protons in the range 2.5 MeV \(\leq\) PE \(<\) 2.8 MeV) was considered for statistics. Figure 3c shows the variation in proton energy with APP fluence (between 0.1 and 15 J/cm\({}^{2}\)) at selected time delays: (i) \(-\)2 ns (red square with dashed line), (ii) \(-\)5 ns (blue circle with dash-dot line), (iii) \(-\)8 ns (green diamond with dash-dot line). The data points are connected by a Bezier curve to aid in discerning the trend. It is observed that in the absence of APP, the proton energy is always below 250 keV, and is excluded from the semi-log plot. Each data point in the plot is the mean of five experiments repeated under similar experimental conditions.