The Simulation Research On Proton Acceleration By Laser Plasma Interaction

When an intense short-pulse laser illuminates a thin foil target,the laser pre-pulse ionizes the target front surface into plasma,and the laser main pulse interacts with the plasma.The electrons in the plasma are pushed by the laser ponderomotive force,and then form an electron cloud at the target rear surface.The electric charge separation generates a strong electric field,which accelerates the protons to high energy.Compared with the proton beam produced by the conventional accelerator,the proton beam accelerated by laser have many advantages,such as small beam,small divergence,good direction,and small pulse width.Which make the laser proton accelerator has a wide range of applications in the isotope production,medical imaging,cancer treatment,nuclear fusion and other fields.However,there are still some shortcomings such as low proton energy,poor monotonicity,low efficiency of laser proton energy conversion,and high requirements for laser parameters,which limits its applications.By computer simulation,it is possible to calculate different physical parameters,structures and geometries,and guide the design of the device.Thereby improving the quality of the proton beam and promoting the early application of the laser plasma accelerator.We develop a two-dimensional Particle-in-Cell(PIC)simulation code ZZUPIC2 D to simulate the physical process of laser plasma interaction.The space in which the plasma located is divided into a series of meshes,and simulation particles which have a certain shape are used to replace the real particles in plasma.The electromagnetic field is assigned on the grid nodes,and the simulation particles move in the grids.The electromagnetic field and the simulation particles are promoted in a certain time step,and the basic parameters of the plasma(particle velocity,position,electromagnetic field,etc.)are obtained.By using the ZZUPIC2 D code,we study the influence of hot electrons,sheath electric field,target shape,thickness,laser number,direction on the proton acceleration.We study the improvement of proton beam quality when using a laser beam to irradiate an aluminum target with a funnel-shaped hole,and contrasted with thetarget with cylinder-shaped hole and trumpet-shaped hole.It is found that the protons in the holes are constrained into different shapes by the transverse electric field generated by the side walls of the holes.In the funnel hole target,the structure of the transverse electric field is inward-tilted,and more protons are restricted near the center of the longitudinal accelerating electric field,thus protons experiencing longer accelerating time and distance in the sheath field compared with that in a traditional cylinder hole target.Accordingly,more and higher energy protons are produced from the funnel-shaped hole target,and the maximum proton energy is improved by about4 MeV compared with a cylinder-shaped hole target.In addition,we simulate the influence of the funnel-shaped hole target parameters such as thickness,hole depth and diameter on the proton energy and divergence angle,and the optimal size of the funnel-shaped hole target is obtained.We study the enhancement of proton beam quality when using three laser beams to irradiate the front layer and two side walls of a rear-holed target,and compared with the case of single laser beam.It is found that the hot electrons in the target side walls enter into the hole when three laser beams are used,which improve the electron cloud density in the hole,thereby increasing the strength of the electric field.The transverse electric field generated by the sidewalls is also enhanced,and more protons are restricted near the center of the longitudinal accelerating electric field,which suppresses the proton beam divergence(22.3°)and improves the maximum proton energy(22.9 MeV).In addition,the dependence of the proton beam on the target parameters is investigated in detail and the optimal size is obtained.