Study On Plasma Block Acceleration Driven By Intense Laser Pulse

Due to rapid technological development of lasers in past decades,especial-ly the appearance of the chirped pulse amplification?CPA?method and the op-tic parametric chirped pulse amplification?OPCPA?method,the intensity of laser pulse has largely increased from 10¹⁵W·cm^-2to 10²²W·cm^-2,providing many new physics views and opportunity for new physical research.Among them,the high field physics as a new research subject has received significant progress in many fields,including the particle acceleration based upon laser-plasma interaction,fast ignition of the inertial confinement fusion,quantum electrodynamics and so on.In terms of laser-plasma interaction,plenty work have been carried out both experi-mentally and theoretically,among which particle acceleration based on the target normal sheath acceleration?TNSA?and the radiation pressure acceleration?RPA?mechanisms have attracted lots of interest and widely studied.Nowadays,together with the generation of high energy particle beams,im-provement of beam quality has been an important and intriguing research topic,which has many potential applications in fields of medical therapy,proton imag-ing,fast ignition of inertial confinement fusion and so on.Generally,due to large mass of ions,it is difficult for ions to be accelerated together with electrons.Mean-while,the quality of particle beams generated via laser acceleration is influenced by both the laser-plasma interaction mechanisms and space charge effects,which may leads to spatial dispersion,energy dissipation and decreasing brightness of the particle beams.And the higher the current density is,the greater the space charge effects become.In this paper,we focus on the achieving of effective plasma block acceleration via laser-plasma interaction,so that the electrons and ions can be spatially accel-erated in sync and propagate together to have the space charge effects effectively suppressed.Such a collective phenomenon as the plasma block interaction is sug-gested to avoid the stopping power effect caused by electromagnetic interaction in many fields,as well as the proton-boron 11(p-¹¹B)fusion reaction,so that fusion target can be effectively compressed and the energy conversion efficiency can be increased.The main study of present paper is carried out as fellows:Firstly,a new scheme is proposed to improve the quality of proton beams via circularly polarized ultra-intense laser pulse interacting with double plasma targets,which consist of a pre-target with relatively low density and a main target with high density.Both one-and two-dimensional Particle-in-Cell?PIC?simulations show that,an appropriately selected pre-target can help to greatly enhance the charge separation field in the main target,which then leads to generation of a strongly ac-celerated and well collimated plasma block with proton energy in GeVs.In contrast to the case of using only the single main target,the proton beam with lower mo-mentum divergence,better monochromaticity and collimation,as well as higher current density is generated.Moreover,due to the strengthened coupling between the laser pulse and targets,the energy conversion from laser pulse to the main tar-get has also been increased.Secondly,the scheme has been extended to the study of the interaction be-tween plasma targets and an ultra-intense linearly polarized laser pulse with in-tensity of I=5×10²²W·cm²via PIC simulations.Through investigation of factors affecting the acceleration process,including the polarization of laser pulse,the are-al density and components of pre-target plasma,highly efficient and stable plasma block acceleration with a monochromatic proton beam peaked at GeVs has been achieved,providing an alternative and efficient scheme of plasma block accelera-tion.Both the simulations based on the linearly and circularly polarized laser pulses show the advantage of using pre-target owes mainly to its electrons,which results in the enhanced longitudinal bipolar field and better confinement of protons of the main-target,yet there is difference between s-and p-polarizations of laser pulse for distinguished transverse effects.By contrast to the circularly polarized laser pulse,higher areal density ratio of pre-target to main-target is needed to achieve efficient plasma block acceleration driven by the linearly polarized one with identical inten-sity.Thirdly,the plasma block acceleration based upon the interaction of several picoseconds laser pulse with plasma target has been studied via PIC simulations with an extended range of laser intensities.For the linearly polarized laser pulse interacting with plasma target,the interacting process can be divided into four stages:generating of hot electrons in a short interacting period?0-0.1ps?;trans-ferring of pulse energy to target ions via electrostatic field from electrons?0.1-3ps?;plasma implosion driven by rapid heating of plasma targets?3-5.3ps?,which lead-s to plasma block acceleration at two different directions;and unloading of laser pulse which results in plasma diffusion to vacuum?5.3-9ps?.It should be noted that plasma block acceleration driven by‘implosion'at the third stage is the main cause of ultrahigh pressure,of which the‘nonlinear force'plays significant roles.For the circularly polarized laser pulse interacting with plasma target,on one hand the plasma implosion has been effectively suppressed,on the other hand,there generates highly effective,stable and neutral plasma block acceleration.Ba-sically,the ponderomotive forces exerting on charged particles are different for lin-early and circularly polarized laser pulse,that is the reason why electron heating is largely suppressed during the interaction of circularly polarized laser pulse with plasma target,resulting in the suppression of plasma‘implosion'.Meanwhile,the plasma block acceleration driven by a circularly polarized laser pulse is more stable and collimated spatially.Furthermore,simulating results show that the accelerat-ed plasma block can propagate within the plasma at stable and undisturbed pace,having the electromagnetic stopping power and space charge effect during propa-gation extremely suppressed,which provides significant testification of the advan-tages of‘block ignition'for laser driven fusion reaction proposed by Hora et.al.from the view of numerical simulation.