High Energy Density Positrons Generation Via Ultra Intense Laser Irradiating A Target

Ever since its discovery in 1930s,the investigation of positrons has attracted great attention.Novel positron sources come into sight based on the quickly development of laser technology.Positrons induced by ultra intense lasers have unique properties,such as large flux,high density,and high energy,all of which greatly expand the applications and investigations.The properties of positrons mentioned above are of crucial impor-tance in laboratory astrophysics,particle physics,fundamental physics,etc.Considering the widely applications of positrons and its complicated generation processes induced by ultra intense lasers,we accomplished the following investigations in this dissertation nu-merically and theoretically:Firstly,the generation of high quality electron beams induced by ultra intense lasers is investigated,as the electron beam can directly induce positron generation via the trident process and the BH process.In the study of ultra intense laser irradiating a solid target with a large angle,an electron beam with high charge?nC?,small divergence??5^??,and quasi-monoenergetic??6MeV?is detected in the laser reflection direction.The generation and acceleration processes are investigated via numerical simulation,which agrees well with the experiments.When the ultra intense laser irradiates near-critical-density plasmas with high-Z elements,a bubble is generated in the plasmas due to the laser field,radiation fraction,and the charge separation field.However,there is no bubble in near-critical-density plasmas with low-Z elements?H,He?.The electron filament generated in high-Z elements plasmas has a very high density?>100n_c?and energy density(?10¹⁹Jm^-3),which is analyzed detailedly in the dissertation.Secondly,an enhanced positron generation scheme via ultra intense laser irradiating a compound target is proposed.When irradiating the gas-solid compound target,the laser firstly accelerates electrons in the gas layer,and then it is reflected by the solid target.High energy gamma photons are generated via the accelerated high energy electrons Compton back-scattered in the reflected laser field.The generated photons further interact with the laser photons to generate high density(6.02×10²¹cm^-3)positrons via the multi-photon BW process.Compared with the only gas or the only solid target scheme,the compound target takes advantages of the gas target in electron acceleration and the solid target in the laser reflection.The generation of positrons is greatly enhanced in the compound target,with the positron yield two orders of magnitude higher than that in the only solid target scheme.Thirdly,different methods based on two laser pulses are explored to enhance the positron generation.In pulses collision scheme,the radiation and positron generation probabilities are increased,and the peak laser intensity required in the BW process de-clines.When two laser pulses collide in a cylinder channel,electrons are dragged into the channel and accelerated by the laser.These electrons will further run into the opposite laser pulse to radiate plenty of gamma photons via the nonlinear Compton back-scattering process.The positron generation process is initiated as soon as these photons interact with laser photons.It is found that the positron yield and its density are tunable by adjusting the cylinder radius and the laser spot size.When two pulses collide in thicker near-critical-density plasmas,electrons can be accelerated sufficiently before they run into the opposite laser.When the two pulses overlap with each other,a standing wave is formed,the nodes of which can collect the generated positrons and increase the positron density.In the study of two sequent pulses in the same direction,it is also found that the positron generation is enhanced when compared with a higher intensity laser pulse.Detailed analysis shows the concrete process,and an optimal time interval between the two pulses is provided,which is?t?2T₀.Fourthly,the influences of tapped hollow target on the collimation and the energy of positrons are studied.The electromagnetic field generated at the wall of both the cone and the columnar hollow is in favor of positron collimation and acceleration.In the tapered hollow target in the back,the divergence is even smaller and the energy is higher than that in the columnar hollow target.In the former,the divergence of positrons is only?15^?,while the cutoff energy is?3.5GeV.However,the columnar hollow target is more favorite in high density positron generation.When the laser intensity is 4×10²³Wcm^-2,positron yield in the columnar hollow target is?3.03×10¹⁰,with a maximal density of4.77×10²¹cm^-3.During the comparison of tapered hollow target and columnar hollow target,it is found that increasing the thickness between the hollow and the target front can enhance the positron generation and its density.The collimation and energy of positrons are tunable by varying the taper angle.Finally,a target with density graded distribution in transverse is investigated to gen-erate and accelerate positrons.When an ultra intense laser irradiates onto this target,it is found that both the positron generation process and the acceleration process are different from the laser interaction with near-critical-density homogenous plasmas.On one hand,the target with a graded density distribution can induce the pulse self-focusing,which is useful for positron generation.On the other hand,the positron generation and acceleration are accomplished in the same target,which makes the scheme more compact.Both the positron generation process and acceleration process are discussed.High energy positron beam with a cutoff energy>20GeV is obtained in a few tens of micron.The evolution of positron energy as a function of the normalized laser intensity agrees well with the results given by Pukhov,et al.