Laser Interaction With Solid Targets In The Collision Impact On The Generation And Transport Of Fast Electrons

In the fast ignition scheme (FI) of inertial confinement fusion, electron fast ignition has attracted the most attention, i.e. using fast electrons as the energy source to ignite the fuel. In this case, the production and transportation of fast electrons are the critical problems in FI. The fast electrons are produced near the critical density through laser plasma interaction. Then the fast electrons will penetrate dozens of microns to deposit their energy into the compressed dense core (with density-300g/cm3). For DT (Deuterium and Tritium) core, the density corresponds to a number density of7.3x1025cm-3. That is to say, the number density of electrons changes by3-4orders of magnitude in the production and transportation processes. According to the time scales of plasma wave and collision, collisional effects are not important in underdense plasma, where the fast electrons are produced. But it should be considered in the transportation of fast electrons, where the plasma density as high as over hundreds of critical density. It will influence the fast electron transportation. In the fast ignition scheme, the ultra-intensity short laser pulse used to produce fast electrons is assumed to have intensity of1019-1020W/cm2. Plasma will be formed in the interaction between the laser prepulse and matter with such high laser intensity, i.e. the preplasma is produced. Studies have shown that preplasma has a great influence on the production of fast electron beam.In this paper, the effects of collision on the generation and transportation of fast electrons are investigated by one dimensional collisional PIC code. It is found that interacting process between ultra-intense laser and high density plasma ex-ists two stages obviously. The developing stage comes first, where collisional effects have little influence on fast electron production. And the accelerating mechanisms of fast electrons changes according to the interface density in laser plasma interaction region. A relatively stable state of fast electron energy flux exists in the second stage, where collision contributes to increasing the produc-tion of fast electrons. Detail analysis indicates that the energy exchange between fast electrons and bulk plasma increases in the collisional case. It leads to the efficient local heating of the thermal electrons, which results in higher thermal pressure. And then the steepening process of interface in laser plasma interaction region will be reduced. Therefore, fast electrons can be effectively accelerated through2ω oscillation from J x B force in the collisional case, while it is sup-pressed in the collisionless case because of the highly steepened plasma density. The production of fast electrons influenced by preplasma is also studied in this paper. And the collisional effects on the transportation of fast electrons in the solid target are also discussed.