Acceleration Of Protons Based On Laser-Plasma Interactions

Energetic ion beams, especially proton beams, have potential applications in many important fields such as fast ignition in the inertial confine fusion, table-top particle accelerator, proton imaging, medical treatment and so on. Generation of high quality proton beams is the goal many scientists are pursuing.In the last three-decades, with the motivation of the rapid development of the chirped-pulse amplification technique, ultra-short and ultra-intense laser pulses have been realized. In the interaction of the laser pulse with I > 10 22 W /cm2 with plasma, the electrostatic field induced by the charge separation is much higher than that in traditional accelerators. As a result, generation of high energy protons in the interaction of ultra-short ultra-intense laser with plasmas has received more and more attention. In this dissertation, we study the physical phenomenon during the interaction by using two dimensional particle-in-cell simulations.Firstly, in order to gain mono-energetic protons, we propose a three-layer target structure. This structure is a combine of the shaped foil target and the sandwich target. The former is proposed to reduce the laser non-uniformity in the transverse direction, and then restrain the distortion of the target. The later is used to reduce the energy spread of the beam caused by the Gauss-temporal distribution. From the simulation results, it is shown that the three–layer target can both restrain the target distortion and improve the mono-energetic of the beams with proper parameters.Secondly, we study the collimation of protons in a target with a capillary attached at the rear side. Contrasting the simulation results with capillary and without capillary, it is shown that the capillary can decrease the emittance of protons. This result can be analyzed from the interaction process. When the electrons enter the capillary, a spontaneous magnetic field is generated in the inner-boundary of the capillary by the recirculating electrons. This spontaneous magnetic field can collimate the electrons. Although the Lorenz force on the protons intends to diffuse protons, the effect is slender because of the small charge- mass-ratio of the proton. Protons are accelerated mainly by the charge separated field; in another word, protons are drawn by the electrons. So, the collimation of electrons certainly causes the proton collimation. The simulation results show well agreement with the theoretical analysis.Finally, the interaction of laser with moderate-density plasmas has been examined. The target is thick and the main acceleration regime is shock-wave acceleration. When the pulse propagates into the target, a magnetic vortex is generated which can induce a strong longitude electronic field to accelerate protons. The study shows that a density gradient in the rear side of the target is beneficial for enhancing proton energy. Then, a two-pulse scheme is examined. It is shown that two shock-waves are generated, and when the two shocks meet with each other, the cut-off beam energy rise greatly.In summary, we set light on the mono-energetic, collimation, and the cut-off energy of the beams. Simulations show that if the parameters are appropriate, the qualities of energetic proton beams are promoted.