Design Of XFEL Beamline Focusing System And Research On Interaction Between Laser Composite Beam And Plasma

The rapid development of laser has greatly promoted the study of light-matter interaction physics.Many related physics,such as inertial confinement nuclear fusion,laser-driven particle source,vacuum QED effect and other fields have received extensive attention and research.With the construction of X-ray free electron laser(XFEL)facilities and ultra-intense and ultrafast lasers worldwide,now it becomes possible to observe the vacuum birefringence in the laboratory.On the other hand,the energy obtained by protons in the laser-driven proton acceleration strongly depends on the intensity of the laser.Laser beam combining technology is one of the methods to improve laser output power.The light field of the laser combined beam has unique properties,and the interaction with the plasma will have different properties.This latter includes researches related to vacuum birefringence experiments and the acceleration of the back sheath,as follows:1.The station of extreme light(SEL)at Shanghai High repetition rate XFEL and Extreme light facility(SHINE)combines 100 PW laser pulse and an X-ray free electron laser(XFEL)beam,which provides possible experimental conditions for observing the effect of vacuum birefringence.We have designed the XFEL beamline of SEL with X-ray energy of 12.914 ke V aiming for vacuum birefringence experiment.Ultra-high polarization purity of the X-ray beam at the level of 10^-10 is required before colliding with a 100 PW laser pulse.An X-ray polarizer is set at the end of the designed beamline to avoid the potential risk of depolarization induced by any other X-ray component.The designed beamline provides a focal X-ray beam FWHM size of 6.35?m and total transmission of 3.5%.Further calculation indicates that 333 shots are needed to reach5?confidence level for the vacuum birefringence experiment under optimal conditions with present design parameters of 100 PW laser pulse and XFEL beam in SEL.Moreover,potential radiation damage,and proposed engineering stability requirements and system's adjustment capabilities of X-ray beamline have been discussed2.Increased proton energy in laser-driven proton acceleration is strongly correlated with laser peak power density.laser coherent beam combining technology enables laser output power to break through the threshold limit of single-channel lasers.Laser acceleration research using coherent beam combining(CBC)laser is a trend of the future.The effect of the light field of the CBC laser on the proton acceleration due to its different distribution must also be taken into account.The 3D PIC simulation found that compared with the perfect Gaussian light,the acceleration effect of the coherent composite beam in the target behind-the-sheath acceleration(TNSA)mechanism will not weaken,but can increase part of the acceleration energy instead.In the case of perfect beam combining,the effect of proton acceleration is better as the number of combined beams increases.However,for coherent combined light,random phase control between sub-beams is always an important factor affecting the efficiency of the composite light field and the distribution of the light field.Considering the effect of random phase,it is found that the beam of a combined beam with completely random phase has a wide range of acceleration effect in TNSA,but it is still better than a single perfect Gaussian beam.The acceleration effect of protons depends on the average intensity and stability of the light field peaks caused by multiple random phases.By controlling the random phase,a stable and high-energy proton acceleration can be obtained.