Research On Sub-Terawatt Laser Wakefield Acceleration Effected By Radially Polarized Laser Pulse

Laser wakefield acceleration(LWFA)is an emerging electron acceleration technology,which has developed rapidly in recent decades.Compared with traditional particle accelerators,this acceleration scheme can obtain higher acceleration gradients in a smaller space.The tabletop of the particle accelerator can be realized.In experiments,a hundred terawatt or even petawatt laser device are usually used to obtain high-energy(GeV)particles by LWFA,but the low electron beam repetition frequency(10 Hz).Terawatt and sub-terawatt laser are easy to obtain high repetition rates laser,and then the electron beam with high repetition rate(kHz)can be obtained by LWFA.These electrons can obtain femtosecond time resolution in ultrafast electron diffraction experiments.In recent years,terawatt lasers have been used for LWFA studies,and the electrons had a large electron beam energy dispersion and continuous distribution of energy spectrum,which limits its application scenarios.Therefore,it is of great research value and significance to use sub-terawatt laser to obtain electron beam with quasimonoenergetic by LWFA.This paper proposes a method of using a radially polarized pulse to control the electron injection during the LWFA.This method is suitable for plasma wakefield acceleration driven by terawatt laser pulse.The radially polarized pulse has a strong longitudinal electric field component under tight focus conditions,which can control the electron injection and optimize the performance of the electron beam.Through the study of the radially polarized pulse driving the plasma wake field,it is found that the scheme of using the radially polarized pulse as the driving pulse has instability,and the periodic injection of electrons causes poor energy dispersion.When the radially polarized pulse is used as an auxiliary pulse and the linearly polarized terawatt laser interacts with the plasma,a quasi-monoenergetic electron beam can be obtained.We use the PIC(particle-in-cell)simulation code Osiris for three-dimensional particle simulation,using a 30 fs,30 mJ linearly polarized pulse as the main pulse to drive the plasma wake field,and another 30 fs,～ 1 mJ radially polarized pulse as the secondary pulse to control the injection of electron,which produce quasi-monoenergetic electron beam of 20 MeV,～ 10 pC,ΔE/E = 6%.Through the analysis of the electron injection process,it is found that the electron injection comes from the modulation of the longitudinal electric field on the sheath layer,and the injection position occurs near the focal plane of the radially polarized pulse.By analyzing the influence factors of electron injection.It is found that the change of the delay of the two pulses will have a periodic effect on the electron injection.The delay of the plasma period time scale(～10 fs)will periodically change the energy and energy spread of the injected electrons;the delay of the pulse period time scale(～1 fs)will periodically change the charge of injected electrons.The change of the position of the focal plane of the radially polarized pulse will affect the distance of electron acceleration,so that the electron injection can be tunable by changing the position of the focal plane.In conclusion,we control the electron injection process through radially polarized pulse,and obtain a quasi-monoenergetic electron beam with tunable charge and energy.This is a light-controlled electron injection mechanism suitable for sub-terawatt laser pulses.