Studies On Electron-positron Pair Production During Ultraintense Laser And Solid Target Interaction

The invention and development of laser technology provide important tools and methods to lots of scientific research areas.When intense laser interacts with matter,molecules and atoms in the matter will be ionized and plasma state is generated,thus opening up a new area of laser plasma.In recent years,with the development of laser technology,laser plasma field includes inertial confinement fusion,high harmonic generation,electron and ion acceleration,etc.Its potential application includes energy,medical treatment,accelerator and diagnosis.Due to the abundance of new physics and wide applications,more and more reseachers devote into this area.When the laser power is smaller than PW and focused intensity is smaller than 10²²W/cm²,the main theories in laser plasma include many-body problem,classic eletrodynamics and ionization theory.With the construction and application of 10PW laser facilities,the quantumn field theory now is required in the area of laser plasma.Quantumn electrodynamics describes the fundamental electromagnetic interaction.It is one of the most mature and precise thoeries.The introduction of quantumn eletrodynamics in laser plasma field fixes the problem of electron radiation and new particle generation.By using the particle-in-cell program which combines classic eletrodynamics and quantumn eletrodynamics,this disseration studies the generation of electron-positron pairs in the interaction of ultra-intense laser with solid target.The main content includes three parts:1.We studied electron-positron pair generation and factors affecting the generation processes in the interaction of single 10PW laser with solid target.Firstly,we use linearly polarized laser with different duration time interacting with targets with different transverse size.We find when the laser pulse duration is short,the target with transverse size close to laser focal spot size produces highest number of pairs;when the laser pulse duration is long,the target with large transverse size produces highest number of pairs.Meanwhile the angular distribution of positrons is not affected by laser duration,positrons generated in the target with transverse size of 2?m always have the best collimation.Secondly,we studied circularly polarized laser with same energy as the linearly polarized one interacting with high density solid target.Simulation results show that the time of pair production in circular polarization is later than the one in the linear polarization laser case.When the laser duation time is short,the number of pair produciton in circular polarization is smaller than the one in the linear polarization case;when the laser duration time is long,the numbers of pair production is roughly the same for different laser polarization cases.Lastly,target transverse size has little affect on pair production.Finally,we come up with the idea of effective electron radation tagging and use it to explore how laser polarization causes different pair production by studying electron mean energy distribution and effective electron radiation information.We designed different target structrue for different laser polarization to achieve highest number of pair production.2.We studied pair generation and cascading process in the interaction of ultra-intense laser with solid target.Firstly,we added QED parameters and particle generation orders in the code.Secondly,we used related particles'?distribution to analyze laser polarization's influence on photon and positron number density distibution and secondary particles'influence on QED cascading qualitatively.Meanwhile,because of QED cascading effect,laser polarization affect the hole-boring process,resulting in different hole-boring depth and shape.Thirdly,we draw generation order spectrum according to the generation order and distinguish strong and weak cascade processes by generation order spectrum shape,define the cascade depth.We distinguished target electrons and secondary particles'contribution to QED cascade qualitatively and studied laser polarization effect on QED cascade accordingly.Finally,we studied the effects of simulation dimension on pair production for different laser polarizations.3.We studied pair production in the interaction between two counter-propagating lasers and thin solid target.We first studied the effect of initial target density on pair production.Through simulations,we found when the initial target density is in the range of 200?280n_c,the number of pair production is the highest.Under this condition,the laser can penetrate through the target and form stable standing wave.Then we use the optimal target density to compare the effect of different laser durations and frequencies for pair production.When laser duration time is short,double frequency lasers can penetrate through the target and form stable standing wave earlier resulting in highest pair number;when laser duration time is long,although fundmental lasers penetrate through the target laterly,larger laser vector potential can accelerate electrons to higher energy,thus leading to highest number of pairs.