## EOS Detector
### Scintillator Screen Diagnostics
#### 3.2.3 Laser-Solid Target Interaction Experimental Campaign
time with respect to the main pulse by 2.5 ps steps, meaning increasing the probe path by 750 \(\upmu\)m steps. As the probe laser is delayed relative to the main laser pulse, the EMPs signal shifts to the left and its amplitude diminishes as reported in Ref. [22]. From these results, the dynamics of the target charging process can be reconstructed [22]. From the horizontal size of the detected signals, the velocity spread of the charges can be retrieved. We calculated the velocity spread as (0.94 \(\pm\) 0.03) \(c\), in agreement with the previous literature [69; 70; 71], where \(c\) is light speed in vacuum. Additionally, the thickness of the traces is proportional to the average duration of the radiation pulses, which, for the signals shown in Figure 24, is (6 \(\pm\) 1) ps. The retrieved peak electric field on the EOS crystal position was found to be 0.8 MV/m, which corresponds to approximately 0.6 TV/m at the target one, as confirmed by PIC simulations reported in Ref. [22]. Furthermore, such a finding allows us to confirm that only electrons with kinetic energies greater than 3 MeV can escape, which is consistent with our previous observations [22]. Electron-Proton Beams Detection and CorrelationTOF measurements are largely used in laser-target interactions [72; 73; 74; 75; 76], allowing the characterization of the ion beam parameters, namely kinetic energy, total charge, and shot-to-shot reproducibility [73]. A typical TOF measurement is shown in Figure 25. It usually consists of a long ion peak signal (tens of ns at a detection distance of \(\sim\)1 m) following a short peak representing fast electrons and X-rays. Figure 24: Evolution of the EMPs emitted by the target. The horizontal axis represents the relative arrival time of the probe (\(t_{p}\)) linked to each pixel corresponding to different temporal slices of the EOS time window. The \(\Delta t_{0}\) indicates the delay between the main and probe laser pulses (1—\(\Delta t_{0}\) = 0 ps, 2—\(\Delta t_{0}\) = 12.5 ps, 3—\(\Delta t_{0}\) = 15 ps, 4—\(\Delta t_{0}\) = 20 ps). Adapted from Ref. [22]. Figure 25: TOF measurement by means of a diamond detector showing the proton temporal distribution. Adapted from Ref. [77]. In this context, semiconductor detectors are good candidates, as their response is proportional to the energy of the incident particles. Thus, the initial peak, due to fast electrons and photons, is lower than that of ions [73]. Moreover, diamond detectors are sensitive to particles with energies above the band gap threshold and insensitive to visible light, which constitutes a large background in laser-plasma experiments. EMPs are also present in laser-plasma interactions, as reported in the previous paragraph, which can spoil TOF measurements. However, specific designs must be realized to eliminate EMP coupling with the desired signal [74]. The maximum detectable energy is affected by the thickness of the detector: a thicker diamond allows for a higher maximum detectable energy. In the experimental campaign performed, for each collected EOS signal the proton energy was also measured using a TOF diamond detector [77; 78] as shown in Figure 26. The peak proton energy behaves as a function of the laser energy, in agreement with the previous literature [65; 79; 80]. As the number of photons striking the solid target increases, protons leave it being accelerated to a higher energy [81]. Experimentally, a correlation between fast electrons and protons has been demonstrated. The proton energy increases with the increase in the number of extracted fast electrons following a power law model with an exponent of 0.6. This results directly from a stronger potential developed on the rear surface of the target, which accelerates protons to higher energies [82]. The experimental evidence showing a correlation between proton energy and the charge of the fast electron supports the hypothesis reported in Ref. [19]. According to this hypothesis, shorter and more highly charged fast electron bunches are expected to induce the production of higher energy protons.