# A New Line for Laser-Driven Light Ions Acceleration and Related TNSA Studies
## 5 Results: Pilot Experimental Activity  
The pilot experimental activity presented here was carried out using the laser pulse at the output of the front-end. The pulse was compressed in a separate compressor chamber and focused, at an angle of incidence of 15\({}^{\circ}\) or 30\({}^{\circ}\), using an f/4.5 off-axis parabolic mirror (OAP), in a spot size of 6.2 \(\upmu\)m (FWHM), giving a nominal intensity on the target of about 2 \(\times\) 10\({}^{19}\) (a\({}_{0}\) = 3). A schematic view of the pilot experimental setup is given in Figure 4. Special attention was dedicated in the experimental campaign to the target integrity prior to the arrival of the main pulse on the target, which strongly depends on the temporal profile of the laser pulse [22]. To this purpose, we used optical spectroscopy of the light scattered in the specular direction to monitor the generation of second harmonic emission, 2\(\omega_{L}\), and (3/2)\(\omega_{L}\) of the incident laser light. These components of the scattered radiation are associated with the coupling of the laser light at the critical density and at the quarter critical density, respectively [23]. In fact, the formation of even a very small pre-plasma before of the arrival of the main pulse can provide suitable conditions for the growth of stimulated instabilities including the Stimulated Raman Scattering and the Two Plasmon Decay. Electron plasma waves at \(\omega_{L}/2\) generated by the instabilities can couple non-linearly with incident laser light and give rise to (3/2)\(\omega_{L}\) emission. This emission is therefore a signature of the presence of even a small pre-plasma. Second harmonic emission in the specular direction is instead generated by the non-linear interaction of the main laser pulse at the critical density [24]. Therefore, second harmonic emission can be taken as a signature of the presence of a critical density layer in the plasma at the time of interaction of the main pulse, a prerequisite for the interaction with an over-dense target and the occurrence of TNSA. In our experiments, in spite of the increase of the (3/2)\(\omega_{L}\) intensity, the intensity of the 2\(\omega_{L}\) emission remained significant, indicating that the laser contrast in the best focus was sufficient to ensure the survival of the target rear surface and, therefore, the proper onset of the TNSA accelerating field. Another crucial issue in this class of experiments is the control of the irradiation intensity on the target, which is complicated by the typically very short depth of focus of the focusing optics. These circumstances make the irradiation intensity very sensitive to small changes in the position of  
Figure 4: Schematic view of the experimental setup showing the main diagnostics, including the optical spectroscopy of the specular reflection, the rear side optical imaging, and the magnetic electron spectrometer of the forward escaping electrons. A Thomson Parabola ion spectrometer was used in place of the electron spectrometer to detect forward accelerated ions. CCD: Charge Coupled Device; TR: Optical Transition Radiation. the target relative to the best focus. Also, small changes in the thermal lensing in the laser amplifiers may lead to changes in the position of the best focus which, if not compensated for, can change irradiation conditions. To this purpose, as shown in Figure 4, optical imaging of the rear side of the target at 45\({}^{\circ}\) from the target normal was performed to detect Optical Transition Radiation (OTR) generated from electrons crossing the target rear surface as a marker of optimum focusing conditions. OTR intensity is also very sensitive to the sharpness of the density transition across the rear surface [25], and is therefore a useful additional indicator of the integrity of the target rear surface. We also measured fast electron emission from the target rear side in the forward direction. Forward escaping fast electrons were detected and analyzed using a permanent magnet electron spectrometer with a detection range from 1 MeV to 5 MeV.