Ion Acceleration by High Intensity Pulsed Laser: Transport, Diagnostics and Theoretical Modelling

Laser at intensity above of 10^10 W/cm^2 that interacts with a solid target generate photons, electrons, and ion beams, emitted from a plasma which expands in vacuum. The electron and ion energies depend strongly on the laser parameters, on the irradiation conditions, and on the target properties. This work was performed at the University of Messina, using a Nd:YAG laser, with 3 ns pulse duration, and 1064 nm wavelength, at an intensity of 10^10 W/cm^2 to generate a plasma by means of different targets. Ion emissions occur mainly along the normal to the target surface and can be detected using the Time-of-Flight technique, through Ion Collector, when the ion current is high enough, or Secondary Electron Multiplier, when the current is less than 10 µA. To increase the current, magnetic fields with cylindrical symmetry can be applied along the axis of ion emission, to obtain a focusing effect for the charged particle beam emerging out of the plasma. The formation of electronic traps, due to the magnetic field’s force lines, drives the ions’ acceleration by improving their kinetic energy. The application of a magnetic field generated by a coil, or an electric field generated by semi-cylindrical electrodes, directed orthogonally to the ions beam produces a deflections of charged particles, according to their mass-to-charge ratio and their velocity or energy, respectively. Ion accelerations of the order of hundreds of eV per charge state, plasma temperatures of the order of tens of eV, and Boltzmann energy distributions have been obtained for the different irradiated targets. At higher intensities, such as those investigated at the INFN-LNS in Catania (10^12 W/cm^2 with a post-acceleration system up to 30 kV), and at the PALS Laboratory in Prague (10^16 W/cm^2), a compact Thomson Parabola Spectrometer, designed at the University of Messina, was employed. It allows to detect particles emitted by hot plasmas and do fast analysis of the charge state, kinetic energy and mass-to-charge ratio. The spectrometer consists of a double input pinhole, for alignment, a permanent magnet (0.004 ÷ 4 kG) and an electric field (0.05 ÷ 5 kV/cm) parallel to each other and orthogonal to the direction of the beam. It can be equipped with different types of planar detectors such as phosphor screen, Gafchromic, PM–355 and others. Further measurements were conducted at the IPPLM of Warsaw, using a Ti:Sapphire laser, with 45 fs pulse duration and intensity of about ~10^19 W/cm^2, to irradiate an advanced target based on a thin film of Graphene oxide covered with metal layers, in order to investigate the acceleration in forward direction, in the Target Normal Sheath Acceleration regime. The Time-of-Flight technique was employed, using semiconductor detectors based on silicon carbide. By optimizing the focusing conditions, a maximum energy for protons of 2.85 MeV was measured, using a gold metallization of 200 nm. Finally, the experimental data obtained are compared with the simulations performed using the Particlein-Cell (PIC) method. PIC provides the electronic densities as a function of time and space, and allows to evaluate the electric field developed in the rear surface of the irradiated foil. The simulation indicates that carbon ions are subject to a lower acceleration than protons, depending on the charge-to-mass ratio. Thus, carbon ions are not affected by the maximum electric field due to its fast time decay. Considering the angular emission distribution of protons and the six charge states of carbon, and their Boltzmann energy with a fixed cut-off, the data obtained are in agreement with the experimental measurements. These measurements, analyses and simulations, collected and performed during my PhD years, are discussed and presented in the following chapters of this thesis.