Study Of Generation And Propagation Of Fast Electrons In Ultraintense Laser Pulse Interaction With Plasmas

Generation and propagation of relativistic electron beams during ultraintense laserpulse interaction with plasmas have attracted significant attention because of their poten-tial applications in inertial confinement fusion, laser-driven ion acceleration, new radia-tion sources, etc. Several special generating mechanisms of fast electrons in ultraintenselaser-plasma interaction and transport of the fast electrons through high density plasmasare studied theoretically and numerically. Measurements of the intrinsic divergence angleof the fast electron beams in experiments are also investigated. The main structure of thedissertation is as follows,Firstly, ultraintenselaserinteractingwithasolidtargetinthepresenceofasubcriticalpreplasma with an exponentially increasing density profile is studied by using LAPINE.It is found that hemispherical fast electron layers with wavelength spacings are generated.Here the heating mechanism of the electrons is mainly attributed to the betatron resonanceabsorption of the electrons in the self-generated electromagnetic fields. Then, due to theself-focusing of the laser pulse, the electrons are accelerated by the Lorentz force andappear behind the solid target as hemispherical shell-like layers separated by the laserwavelength. They can propagate stably behind the solid-slab target. The hemisphericallayers reported here have a potential application in the areas of plasma grating, movingmirrors and production of ultrashot and ultrabright radiation.Secondly, the relativistic collision of ulraintense laser-driven electron beams is stud-ied. Electrons are accelerated to relativistic speeds, cross the target and exit at the rearsurface of the target. Most energetic electrons are bound to the surface of the slab by theelectrostatic field and expand along it. Their current is closed by a return current in thetarget, and this current configuration generates strong surface magnetic fields. The twoelectron sheaths collide at the midplane between the laser impact points. The magneticrepulsion between the counterstreaming electron beams separates them along the surfacenormal direction due to the growth of the filamentation instability, before they can ther-malize through other beam instabilities (such as two-stream instability and oblique modeinstability). A parabolic profile of magnetic field is generated behind the target due tothe currents propagating into the vacuum. It is indicated that the filamentation instability, which is thought to be responsible for magnetic field growth in gamma-ray burst (GRB)jets, can be studied with such an experimental setup.Thirdly，we investigated an ultraintense p-polarized laser pulse interaction with agold cone target and observed attosecond electron bunches generation. It is found that thetargetelectronsarepulledoutbytheoscillatingelectricfieldandacceleratedforwardalongthe cone walls by the Lorentz force. Thus a cone-channel target is proposed to guide andconfine the fast electron propagation. It is shown that the balance of the electrostatic fieldand quasistatic magnetic field can result in the guiding and confinement of the electronsin the channel. Such a cone-channel target might be useful in the design of fast ignitionexperiments.Fourthly，the self-generated magnetic field is calculated using a rigid beam modelas a fast electron beam propagates in a solid target, including both Ohmic and Coulombcollisional heating. It is found that, for compressed targets, beam hollowing is suppressedand the magnetic field increases with the growth of the penetration depth of the fast elec-trons, suggestingthatahighdensitybackgroundmayleadtofastelectronself-collimation.Ahighdensitypedestalisbeneficialtotheenergycouplingefficiencyofthefastelectrons,though, such effect is quite weak overall. The radial extent of the compressed core (as-suming a fixed maximum density) can have a strong effect on the coupling efficiency.Fifthly，the divergence of fast electron beam investigated in experiments usuallyresults from the transport effects of the electrons in the target. In other words, the datado not provide a measurement of the intrinsic initial divergence at the source, which isneeded to be confirmed in some applications. A scheme employing an external axialmagnetic field is proposed to diagnose the intrinsic divergence of the fast electrons. It isfound that, under the effect of an axial field of appropriate amplitude, the beam radius ofthe fast electrons increases with the initial divergence and decreases with the amplitude ofthe axial magnetic field. The maximum beam radius is close to the sum of the initial beamradius and the gyroradius determined by the axial field. It is indicated that the intrinsicdivergence of fast electrons can be inferred from measurements of the beam radius atdifferent depth under the axial field.Finally，a strong self-generated magnetic field is generated along the surface of theburied layer as Kαx-ray emission is employed to diagnose the fast electron divergence,which can magnetize the fast electrons. Especially, for the cases with moderate laser intensities,themagneticfieldcaninducethefastelectronspropagatingalongtheinterface.Thus the fast electron divergence measured from the experiments may be larger than theintrinsic divergence of the beam. It is found that utilizing a low Z tracer layer or somenearby elements for the tracer layer and bulk target and a thin tracer layer can mitigate theinfluence of the tracer layer on the fast electron propagation.