Controlled Injection For The Generation Of High-quality Electron Bunch In Laser-driven Electron Acceleration

The laser wakefield accelerator(LWFA)is a promising,compact next-generation accelerator because of its ultrahigh acceleration gradient(?GV/cm).Nevertheless,there are still several critical outstanding issues to be solved,such as how to enhance the quality,charge and energy of the electron beams,while devising methods for enhancing the stability of the LWFA,to make it suitable for applications.Effective spatio-temporal control of the electron injection process is the key to solving these issues.On the other hand,there is an increasing interest in producing electron bunches with shorter duration down to the sub-femtosecond or attosecond regime,for ultrafast applications.In contrast to conventional accelerators,LWFAs have the advantages of both ultra-high acceleration gradient and compact acceleration structure.As a result,electron bunches produced from LWFAs are typically as short as ?10 femtoseconds.Nevertheless,it is still an outstanding and unresolved challenge to generate sub-femtosecond electron bunches in the LWFA,since it crucially demands a highly localized electron injection.Based on the above two aspects,we propose some schemes for the control of electron injection in the LWFA using an external transverse or longitudinal magnetic field,density-profiletailored plasma,radial polarized laser or their combinations.These schemes enable the start and end of electron injection to be well controlled,which allow the generation of high-charge high-quality electron bunches or sub-femtosecond electron bunches for broad applications.The major contents of this thesis are divided into three parts.In the first part,we propose a magnetic-controlled ionization injection scheme,which can provide both spatial and temporal control of the injection process.A theoretical analysis and numerical simulations verify that this scheme is suitable for the generation of high-charge beams with narrow energy spread.The ionization injection stands out from other injection mechanisms of the LWFA because it requires relatively low laser intensity.However,to reduce the energy spread in the ionization injection,electron injection must be limited to a short distance,which is usually accompanied by a reduction of the beam charge.Fortunately,we find that a nonlinear ionization injection process occurs when the plasma is subject to an appropriate magnetic field,which postpones the electron injection while enhancing the peak injection rate.As a consequence,the energy spread can be reduced simultaneously with an increase in the beam charge by using this magneticcontrolled ionization injection scheme.Moreover,a trapezoidal-shaped longitudinal charge profile can be formed under a moderate large magnetic field.Such a charge profile flattens the accelerating field so that it is nearly constant along the propagation direction.This means that the beam loading is automatically compensated for.As a result,the magnetic-controlled ionization injection scheme can increase the electron energy while simultaneously reducing the energy spread.To our knowledge,no other injection scheme can provide such control of beam charge profiles.The synergy of the nonlinear injection process and optimized beam loading simultaneously lead to a reduced energy spread,enhanced beam charge,and enhanced beam energy in the LWFA.Note that the required external magnetic field is only of the order of 10 T,which may be produced relatively easily by a compact pulsed solenoid or superconducting magnet.Our studies also show the robustness of our scheme with changing acceleration distance,magnetic field strength,all of which are important for practical applications.In the second part,we propose a novel scheme to achieve well-controlled highly localized electron injection for the generation of sub-femtosecond electron bunches via the three-dimensional manipulation of laser-driven plasma bubble.We find that the plasma bubble and electron injection can be flexibly manipulated in three dimensions by combining a plasma density gradient and an external magnetic field.On the one hand,the down-ramp of a density-profile-tailored plasma will increase the wavelength of the laser-plasma wake and therefore reduce its phase velocity,which can be used to trigger electron injection.On the other hand,a longitudinal magnetic field will induce an expanding hole in the rear of the wake bubble,which tends to reduce the peak electron velocity.Electron injection is terminated when the peak electron velocity is reduced to less than the phase velocity of the bubble.As a consequence,the start and end of electron injection can be flexibly controlled,leading to the formation of sub-femtosecond/attosecond electron bunches.This three-dimensional manipulation of plasma bubbles may enable the realization of sub-femtosecond electron bunches with readily accessible parameters for both density profiles and magnetic field strengths.The required magnetic field can be on the order of 10 Tesla,which is relatively easy to achieve in the laboratory.The generated sub-femtosecond electron bunches have a high peak current and low emittance,which are suitable for broad applications.In the third part,we propose a novel scheme to produce attosecond electron bunch using radial polarized laser(RPL).In the interaction of a radial polarized laser pulse with a sharp near-critical plasma bump,a laser-driven plasma bubble can be formed which has the similar shape and size as the laser envelope.The wave breaking will occur near the top of plasma bump,and then a ringlike electron bunch is injected into the tail of laser pulse which is also the rear of plasma bubble.Therefore,this electron bunch will be affected by the hybrid field of the laser and the bubble.As the plasma bubble evolves dramatically along the sharp down-ramp of the plasma bump,most of electrons in the injected bunch will be decelerated by the hybrid axial electric field that is positive when the ringlike bunch is focused on the axis.After the radial polarized laser pulse leaves the plasma region,only a small part of electrons originated from the ringlike bunch can remain in the accelerating phase of the radial polarized laser and be re-accelerated to a speed close to the speed of light.Meanwhile,these electrons will be further modulated by the radial field of radial polarized laser.Finally,a high-quality atteosecond electron bunch can be generated and quickly separate from the laser field as the latter is tightly focused and hence will be divergent sharply.