## 3 Experimental Results and Discussion  
As mentioned, we used targets made of three different metallic materials (Ag (\(Z\) = 47), Au (\(Z\) = 79), and Al (\(Z\) = 13)) with increasing electrical resistivity \(\rho\) = (1.59, 2.21, and 2.65) \(\times\) 10\({}^{-8}\)\(\Omega\)m, respectively. Experimental energy distribution spectra of the laser-accelerated electrons from five films of different materials and thicknesses are further presented in Figure 5 the plot a-c. The plot in a) presents the electron flux from three Ag targets with different thicknesses \(d\) = 4, 5, and 10 \(\upmu\)m. It can be observed that the accelerated electron maximum energy (AEME) decreases with the Ag target thickness as follows: from 19 MeV at 4 \(\upmu\)m thickness, to 16.5 MeV at 5 \(\upmu\)m thickness, and to 10 MeV for 10 \(\upmu\)m thickness. Figure 5 (a plot) also illustrates how the electron flux (estimated from the area under the curve) is lowest for the 10 \(\upmu\)m Ag target and the highest for the 4 \(\upmu\)m Ag target. Thus, we find that the total number of electrons escaping at the back side of the target, as well as the AEME, increases linearly with the inverse of the target thickness d; see Figure 5d, the  
Figure 4: Target system experimental setup and electron propagation scheme. Figure 3: Specific experimental setup for particle acceleration in laser–solid target interaction. top plot. The (b) and (c) plots in Figure 5 present the electron spectra from targets made of different materials: (b) plot--Ag and Au films with thicknesses of \(d\) = 4 \(\upmu\)m; (c) plot--Ag and Al films with thicknesses of \(d\) = 10 \(\upmu\)m.  
EMP radiation produced by laser pulse interaction with thin metallic films was analyzed using three commercial antennas (with different frequency bands 0.15-0.8 GHz, 0.8-3 GHz, and 0.8-18 GHz) placed inside the interaction chamber, about 1 m away from the target and under angles of about 45 degrees to the accelerated electrons' propagation direction. A 4-channel 3.5 GHz Tektronix oscilloscope was used for recording the antenna signals. A typical signal that describes the temporal dependence of the voltage across the antenna under EMP radiation is presented in Figure 6a. It is similar in terms of shape and duration with other records from high-power laser facilities with pulse duration of ps (e.g., 1 ps from Vulcan [4]) or below (e.g., from ELI ALPS [33]), while the electric field is in the range of tens of MV/m in the interaction chamber. The signal has a fast rise followed by an envelope with an exponential decay, having a total emission time of a few hundred ns. Signals recorded by the three antennas were Fourier processed for a frequency range between hundreds of MHz up to a few GHz and the electric field was evaluated by weighing recorded signals by the antenna sensitivity factors for the overlapping frequency intervals. A 50-point \"moving window averaging\" method (corresponding to about 250 MHz frequency window) was finally employed for \"smoothing\" experimental system resonant frequencies corresponding to the different positions of the antennas within the interaction chamber. Because of the antenna cables' and connectors' band limitation, we have further considered as \"reliable signals\" a more  
Figure 5: Experimental data on the energy spectrum of the laser-accelerated electrons from different metallic films: (**a**) plot—Ag films with different thicknesses \(d\), (**b**) plot—Ag and Au targets with \(d\) = 4 \(\upmu\)m thickness; (**c**) plot—Ag and Al targets with \(d\) = 10 \(\upmu\)m thickness and (**d**) accelerated electron maximum energy (AEME) as a function of (top plot) thickness \(d\) of Ag targets and (bottom plot) \(d\), \(Z\), and \(\rho\). limited range of frequencies between about 200 MHz and 1.8 GHz while the signals outside this range were disregarded, and the resulting graphs of the Fourier transform of antenna signals originating from targets of different materials and thicknesses are presented in Figure 6b.