## 2 Results and Discussion
### Experiments  
A schematic representation of the compact laser-based accelerator realized and tested is reported in Figure 5. The laser system used is the TW laser beamline at the Intense Laser Irradiation Laboratory of the CNR-INO in Pisa [39]. The laser beam is guided by multiple steering mirrors to an off-axis parabolic (AOP) mirror. The intensity in the focus is estimated to be several times \(10^{19}\) W/cm\({}^{2}\) (see Materials and Methods). The TNSA laser target used is a 5-micrometer thick titanium foil, whose position is controlled by a three-axis motorized stage with micrometer resolution. An optical camera is used to image the laser target in order to control the position of the laser focus on the titanium foil. In Figure 6, the picture of the actual compact laser accelerator is reported, showing the laser beam transport line, the OAP mirror, the TNSA laser target, the MBL, and the  
Figure 4: The effect of the magnetic Beamline on the TNSA fast electrons. Figure 3: SRIM calculations of the few MeV proton beam propagation through the Kapton window and the 4-cm path in air: (**a**) Ensemble of the protons trajectories; (**b**) final proton energy distribution assuming an initial flat distribution of 1000 protons per unit energy. Kapton window. The MBL is supported by a motorized linear stage that can be remotely controlled to insert and remove the MBL from the proton beam path. Figure 5: Schematic of the experimental setup, showing the laser-plasma target, the compact magnetic beamline, and the proton beam diagnostics after the Kapton window, alternatively the EBT3 radiochromic film or the Si PIN diode for Time of Flight measurements. Figure 6: Picture of the actual compact accelerator, highlighting the TNSA laser target, the compact magnetic beamline, and the Kapton window to let the proton beam exit in air. In the picture, the MBL has been removed from the proton beam propagation direction using the dedicated motorized stage for better visualization. The energy of the laser-generated proton beam is characterized with ToF measurements with the particle detector placed in air at 1 cm after the Kapton window. In Figure 7, the typical ToF traces measured with and without the MBL are shown. These data represent the maximum cut-off energy which is achieved when optimizing the TNSA process. When the TNSA target is moved out from the optimal position, proton beams with lower cut-off energy are obtained. The actual cut-off energy value is inferred from the ToF data by the onset of the steep rising edge of the proton signal relative to the reference time corresponding to the laser-plasma interaction on the titanium foil target (t = 0). Such a reference time is set at 1.0 ns before the onset of the first ToF peak which is due mainly to the fast electrons with a contribution from X-rays from the laser-plasma (see Material and Methods Experiments section for details). The maximum cut-off proton energy achieved is \(\approx\)3 MeV in agreement with reported scaling law for the TNSA process [34; 40]. The uncertainty on the cut-off proton energy determined by ToF measurements is estimated to be 0.2 MeV (see the Material and Methods Experiments section for details). Importantly, the first peak in the ToF traces is suppressed when using the MBL, confirming that fast electrons are removed from the proton beam path. Radiochromic EBT3 films [41] are used to characterize the spatial distribution of the proton beam in air and to perform dosimetry. The results from EBT3 film irradiation experiments are reported in Figure 8. For direct and easier comparison between simulations and experiments, Figure 8a shows the particle distribution from GEANT4 simulation at 1 cm after the Kapton window considering a beam with a flat initial energy distribution from 1.7 MeV to 3.5 MeV. In Figure 8b, the image of the EBT3 film at 1 cm after the Kapton window irradiated by 8 shots is reported.