High-Yield Positron Beams Generated By Laser Wakefield Electronics And The Study On Positron Beam Characteristics

In this thesis,the process of positron beam generated by indirect method based on ultra-intense laser was studied.The main research contents are as follows:Firstly,the shortcomings of the traditional inversion algorithm of electron energy spectrum are analyzed.When the electron receiving position of the IP plate deviates from the theoretical position by 1 cm,for this electron spectrometer,the electron energy error solved by the conventional method is between 5%and 20%.In order to effectively eliminate the influence of electron beam divergence angle on energy spectrum measurement,an algorithm model based on the maximum entropy method is established.The results show that this method can greatly improve the accuracy and reliability of electron energy spectrum.Secondly,the experiment based on ultra-intense laser to generate positrons by indirect method is designed and carried out.The average positron number measured by IP plate is 6.81×10~5/shot,the standard deviation is 2.92×10~5/shot,and the total positron yield is about 4.4×10~7/shot,which is obviously higher than that measured by Sarri.In addition,both of the energy spectra of the positrons and electrons behind the target satisfy the quasi-Maxwell distribution.The effective temperatures are 38.5 Me V and50.5 Me V,respectively.Compared with the positron and electron temperature of2.8Me V and 4.8Me V measured by Chen,the indirect method produces positron and electron with higher slope temperature.Thirdly,in order to increase positron production,a multilayer target scheme is proposed.The multilayer target can make the positron escape from the solid target with lower energy through the interval of the multilayer target.The multilayer target can also reduce the annihilation of positron and make the energetic electrons and photons trapped in the target and continue to interact with the secondary target to produce more positrons.The calculation results show that the positron yield by using the multilayer target is increased by 137.1%compared with that using the single layer target,and the energy utilization of the wakefield electrons is 7.3%higher.In addition,when the positron yield is achieved the highest,the distance L,the thickness D and the radius R of the target satisfy the matching relation RL d?C,where C is constant.