Positron Production Via Interaction Between Femtosecond Laser Irradiating Microstructured Surface Target

Since positron was found,it has been widely used in medical diagnosis,industrial dec-tion and other fields.It has been the goal of people to producea suitable positron source.The super intense laser facilities bring us a new method to produce positron.Generally,the laser driven positron source could generate positrons of high beam quality based on Trident mechanism,Bethe-Heitler mechanism or Breit-Wheeler mechanism.In this dis-sertation,a novel positron production method is proposed via the interaction between femtosecond laser and a micro-structed surface target.The main works are as the follow-ing:Firstly,we studied the electromagnetic field distribution and the angular distribu-tion of the accelerated electron during the interaction between laser and Micro-structured Surface Target.It is found that the distribution of longitudinal electric field for differ-ent target parameters is quite different during the interaction between laser and Micro-structured Surface Target.In a Micro-structured Surface Target with smaller plate spac-ing,E_xis excited by space charge.But in the Micro-structured Surface Target with larger plate spacing,it is the guided wave that propagates.Besides,the angular distribution of electron is also quite different for different spacing.The larger spacing Micro-structured Surface Target has a more obvious guiding effect on the motion of electrons,the angu-lar distribution of electrons is about 15°.But for a Micro-structured Surface Target with smaller plate spacing,the angular distribution of electrons is more than 90°.The electron beam generated by the Micro-structured Surface Target has great potential in positron production.Secondly,we studied the electron acceleration and positron generation during the interaction between laser and Micro-structured Surface Target.It is found that the elec-tron acceleration process in a Micro-structured Surface Target with smaller spacing is dominated by pondermotive force.While the process of electron acceleration in a Micro-structured Surface with larger spacing is dominated by pondermotive force and guided wave E_x.The effect of electron acceleration in larger spacing Micro-structured Surface Target is much better than that in smaller spacing Micro-structured Surface Target.In a small range Micro-structured Surface Target,the energy of the electron is smaller than30Me V,while in a larger spacing Micro-structured Surface Target,the energy of the elec-tron is up to 120Me V,which shows that larger spacing Micro-structured Surface Target is more competent to the generation of positrons.In the study of the production process of positrons,we found that plate spacing has a great influence on the positron production.The yield of positron produced by larger spacing target is 7.62×10~9,which is about two orders of magnitude higher than that in the case of smaller spacing.And the cutoff energy could reach up to 60Me V for larger spacing case.The result can provide us the references in designing the target.