High-quality Electron Beams Generated In Laser Wakefield Accelerators

Attributed to the advantages of extremely high accelerating gradients,compact structure,and high economic benefit,laser wakefield acceleration?LWFA?is foreseen as a promising scheme for the next generation electron-positron colliders,X-ray source,and free electron laser.This scenario,which was originally proposed three decades ago by American scientists Tajima and Dawson?1979?,utilizes the plasma acceleration structures created by the ultra-high-energy-density lasers interacting with specific gases to accelerate electron beams to relativistic velocities with ultra-high energy in a millimeter,centimeter,or even meter scale.Science then,the research in this field has experienced several stages of development,from large amplitude wake excitation at early time,then stable electron acceleration,to controlled high-quality acceleration nowadays.For practical applications,the improvement of the quality of electron beams generated by LWFA?including peak energy,energy spread,emittance and charge?remains to be further strengthened.With the goal of improving the electron beam quality,this thesis is dedicated to experimental investigations on stable LWFA driven by the relativistic intense laser and accelerated electron beam diagnose,enhancement of electron beam quality,dynamic diagnosis of ionization injection in LWFA.The main research results and achievements of this work include:1.An experimental platform on laser-plasma electron accelerator at Shanghai Jiao Tong University has been built up.On this platform,we used 17?50TW,30fs,800nm laser pulses interacting with 4-mm-long helium gas jet to generate stable and high-quality quasi-mono-energetic electron beams with peak energy higher than 100MeV at low helium plasma density,based on the self-injection LWFA regime.Then,we studied in detail about the dependency of electron beam parameters,such as divergence angle,pointing angle,energy spectrum,and charge,on the plasma density,and found that:electron beams generated at low density has obviously higher quality than those at high density;at higher densities,the beam quality dropped dramatically and laser-plasma interaction has become unstable.2.By using laser pulses with the same parameters,we generated quasi-mono-energetic electron beams with energies in the range of 40?120MeV and up to ⁴30pC of charge through self-injection LWFA.The dependency of divergence angle,pointing angle,and charge on the Neon plasma density was investigated in the experiment as well.At a relatively high density,we observed multiple electron beamlets which has been interpreted by numerical simulations to be the result of breakup of the laser pulse into multiple filaments in the plasma.Each filament drives its own wakefield and generates its own electron beamlet.3.In the interaction of highly unmatched laser pulses with 4 mm-long helium gas jet mixed with low ratio?0.3%?of nitrogen,we experimentally demonstrated overall enhanced single-stage LWFA of electron beams via ionization induced injection.Upon the interaction of 30-TW,30-fs laser pulses with a gas jet of the above gas mixture,>300MeV electron beams were generated at a helium plasma densities of 3.3?8.5×10¹⁸cm^-3.Compared with the uncontrolled electron self-injection in pure helium gas jet,the ionization injection process due to the presence of ultralow nitrogen concentrations appears to be self-controlled;it has led to the generation of electron beams with higher energies,higher charge,lower density threshold for trapping,and a narrower energy spread without dark current ?low energy electrons?.An immediate possible mechanism to explain the results is the self-truncated ionization injection due to self-focusing effect excited when highly unmatched laser pulses propagate in the plasma.Further optimizing nitrogen concentration,we obtained high-quality electron beam with 454MeV and 3.4%energy spread.4.A single-beam femtosecond probe has been built up for investigating the dynamics of ionization injection in LWFA.This probe beam was split from the main beam delivered by our current 200 TW femtosecond laser system.After passed through a cube beam-splitter,it was split into two separate optical set-ups:one for measuring the distribution and evolution of plasma density by interferometry,another for capturing the shadowgraph of LWFA to analyze interaction process between laser and plasma.Using the probe beam,we observed swordfish-like shadowgraphs,for the first time,during the LWFA driven by femtosecond intense laser pulses shooting on the He-N₂ mixture gas jet,which may characterise the ionization and injection process from the inner-shell electrons of nitrogen atoms into wake.Meanwhile,we performed time-resolved dynamical investigation on this structure.