Generation Of Monoenergetic Protons From The Interaction Of Intense Laser Pulses With Double-layer Plasma Target

High-quality proton beams have a wide range of important applications,such as cancer therapy,proton radiography,fast ignition fusion,etc.These applications have certain requirements on the quality of proton beams,including peak energy,energy spread,spacial divergence,beam strength,energy conversion efficiency,etc.Traditional particle accelerators occupy large space and high cost,and the resulting beams cannot be applied in situ.However,the development of ultraintense laser pulses offers a new particle acceleration scheme based on the interaction of laser and plasma,which has the advantage of miniaturization and can accelerate particles without a long transport distance that causes divergence and thus attracted much attention,leading to a lot of related achievements.The process and result of the laser-plasma interaction are affected by lots of parameters and have complex nonlinear effects,which are difficult to study by traditional theoretical means.Meanwhile,the extremely high experimental cost also makes it impossible to examine all kinds of parameter combinations.In the face of these difficulties,the development of large-scale numerical simulation provides new research techniques for us and promotes the rapid development of laser-driven proton acceleration research.In this thesis,a self-developed two-dimensional all-relativistic particle-in-cell simulation code"Opic" is introduced for our work.We make necessary improvements for this code and study the effects of different plasma target configurations and laser arrangements on the proton acceleration results.The thesis mainly consists of three parts:1.We first introduce the related backgrounds of laser-driven proton acceleration,including the basic physical properties of laser and plasma,the requirements of proton quality in some common applications,the major acceleration mechanisms of proton acceleration,the basic methods of improving the quality of proton beam,and some progress in experimental results.Then the basic concepts and fundamental physics of particle-in-cell simulation,as well as the main computational procedure of the Opic code are described.To meet the requirements of simulating multiple laser pulses with oblique incidence angle,some necessary modifications of the code are made here,and tested by several examples.2.The effects of target configuration on the proton quality are investigated.Using triple layer target with long preplasma,it is found from 2D simulations that when the laser pulse propagates in the preplasma,electrons from the preplasma inside the laser pulse are spelled outward,leaving a channel of lower density electrons.Those electrons remaining in the channel can be trapped and accelerated by the laser's wave field directly,resulting in an electron bunch of periodic density distribution,and passes through the target together with the laser pulse,driving ahead the ions in the double-layer target.Compared with normal sheath electrons,such directly accelerated electrons are of higher energy and density,which is beneficial for the characteristics of proton beam without the modification of the laser pulse.Moreover,it is found that using preplasma with laterally modified density profile,for example,preplasma with Gaussian density profile,the peak energy of proton beam can be further enhanced.The relationship between proton's peak energy and density distribution is also obtained.3.The effects of pulse arrangement on the proton quality are investigated.By using two intersecting laser pulses irradiating on double-layer target,it is found in 2D simulations that the target is soon penetrated by the pulse and some electrons from the target can be accelerated directly by the laser's fields.Such electrons can affect the distribution of the backside electric field,as well as the configuration of the target.It is seen from the simulation results that two bends including massive protons occurs on the target,and that such two bends tend to merge in time for proper laser distance and incident angle,resulting in a well-collimated proton bunch.Proton beam from this scheme has higher peak energy,larger proton quantity and less spread in space,which is beneficial for applications.Furthermore,the influence of the laser distance on the acceleration results is also investigated.It is found that except for the above-examined laser distance,one can get a monoenergetic proton sheet under another laser distance.