Research On The Generation Of γ-Rays And Positrons By Relativistic Femtosecond Laser Solid Target Interaction

With the application of Chirped Pulse Amplification,ultra-intense and ultra-short lasers have emerged and are continuously developed,which provide an unprecedented extreme physical conditions for mankinds.Nowadays,with the construction of petawatt（PW）laser devices all around the world,laser intensity can reach 10²²W/cm².Under these conditions,the laser-plasma interaction enters the realm of relativistic non-linearity,and many physical phenomena emerge,such as relativistic electron acceleration,ultra-short and ultra-brightγ-ray,positron-electron pair generation,et al.γ-ray sources and positron sources are widely applied in scientific research for medical imaging,cancer treatment and materials diagnosis.This thesis focuses on theoretical and numerical simulations of the interaction between relativistic femtosecond laser and solid target,with particular attention to the generation of isolated,polarized,brilliant and attosecondγ-rays,and dense Ge V positron.This thesis mainly involves the following two parts.Firstly,we propose a physical scheme for the generation of isolated polarizedγ-ray pulses from a relativistic femtosecond laser-driven nanofoil.First,a circularly polarized Gaussian laser irradiates a nanofoil,a sub-femtosecond（～800 as）relativistic electron sheet（RES）is generated.Then a counter-propagating laser pulse collides with the RES,resulting in the generation of an isolated ultra-shortγ-ray pulse with Me V energy.Finally,the effects of laser polarization on the polarization degree ofγ-ray pulse are investigated by using proof-of-principle calculation under local constant field approximation conditions.The acceleration of RES andγ-ray radiation are theoretically analyzed.3D-PIC simulations show that theγ-ray pulse generated is characterized by a high polarization degree of 92%,a high energy with Me V and an isolated structure in the sub-femtosecond range.Thisγ-ray pulse is an important pump light source for nuclear resonance fluorescence excitation and dark matter characterization.Secondly,we investigate an all-optical scheme for brilliant attosecondγ-ray and high-yield positron generation,which is accomplished with a relativistic femtosecond laser pulse incident upon a Nano-Micro-array（NMA）.In the ultra-intense laser-NMA interaction,laser energy absorption,electron acceleration and guidance,high-energy radiation generation and positron-electron pair generation are theoretically analyzed.Under consideration of laser focusing,we perform large-scale simulations,and obtain angular distribution and peak brightness ofγ-ray and energy spectrum and density distribution of positron.We study the effects of laser and NMA parameters onγ-ray and positron,and obtain scaling of the energy conversion efficiency ofγ-ray and positron with different laser intensity.The simulation results show that the peak brightness ofγ-ray with attosecond（～440 as）up to～10²⁴photons s^-1mm^-2mrad^-2per 0.1%BW at 15 Me V.Positron yield and energy conversion efficiency up to 1.48×10¹¹and 0.7%,respectively.Such highly-brilliant attosecondγ-ray would provide potential opportunities for attosecond physics and ultrafast science.The scheme would a physical scheme for dense positrons source generation experiments based on 100 PW laser devices.