Study On Collimation And Energy Deposition Of Fast Electrons Produced By Ultra-Intense Laser Interacting With Solid And Overdense Plasmas

When an intense laser pulse irradiates solid targets, the light energy is deposited at the relativistic critical density and relativistic electrons are produced. These fast electrons are of great importance in applications, such as inertial confinement fusion(ICF), X-ray radiography, as well as in theoretical study, such as energetic charged particle jets in astrophysics. The beam quality is a key issue for these applications and theoretical study, where monoenergetic, well-collimated, and spatially homogeneous beams are preferred.However, the fast electron beams generated during the laser plasma interaction (LPI) are generally divergent, which will greatly affect the beam quality. There exists strong quasistatic self-generated magnetic field during the LPI that can play a great role in controlling the beam divergence of the fast electron beams. Furthermore, the energy deposition of collimated fast electrons in dense plasmas is also very important for some applications, such as in fast ignition.On the basis of the theory on self-generated magnetic field, a target with two layers of different plasma densities is proposed to produce strong interface magnetic fields. A physical estimate for this field strength is presented. The interface magnetic fields are modeled through two dimensional (2D) explicit particle-in-cell(PIC) simulation under different laser and target parameters. It is found that the field strength increases with incident laser intensity, which agrees very well with physical estimate.The two-layer target and the produced interface magnetic field are designed to collimate fast electron beam through a simple analytical model and test particle model. 2D PIC simulations are performed for different laser and target parameters. Simulation results reveal that the target with inner layer density lower than that of the outer one is more favorable for beam collimation. It is also found that using two-layer target is better than using homogeneous. Furthermore, our simulations show that using two consecutive laser pulses irradiating on a two-layer target can generate better collimation energetic electron beams than using one laser pulse incident on a uniform target. The energy deposition of fast electron beams in a super dense plasma, where the collision dominates, is studied in the frame of kinetic theory. Both binary collision effect and plasma collective effect are included in the kinetic equation. It is show that the deduced kinetic equation can physically preserve particle conservation very well, and can reduce to the classical kinetic equation in non-relativistic limit.