Particle simulation study on anisotropic pressure of electrons in laser-produced plasma interaction

Direct-drive inertial confinement fusion (ICF) requires a symmetric compression of the fuel target to achieve physical conditions for the ignition. The fast ignition scheme reduces the symmetry requirements for the target compression and the necessary driving energy, but symmetrically compressed target will certainly help improve the efficiency of the nuclear fuel burning. In this paper, with the particle-in-cell (PIC) simulation method, characteristics of the anisotropic pressure tensor of hot electrons are reported for the ultra intense laser pulse interaction with over dense plasmas, which mimics the scenario of the last stage when hot electrons are utilized to ignite the compressed fuel core in the ICF fast ignition scheme. A large number of hot electrons can stimulate pressure oscillations in the high density plasma. As the component parallel to the electron velocity dominates the pressure tensor, the electron density distribution perturbation propagates rapidly in this direction. In order to keep those hot electrons in the high density fuel plasma core for a period long enough for them to deposit energy and momentum, a magnetic field perpendicular to the electron velocity is used. The PIC simulation results indicate that the hot electrons can be trapped by the magnetic field, and the components of the anisotropic pressure tensor related to the parallel direction are significantly affected, thereby producing a high peak near the incidence surface. Since it is a relatively long process for the energy transfer from electrons to fuel ions and the nuclear interaction to be completed, the fluid effects take their roles in the fuel target evolution. The anisotropic electron pressure will deteriorate the fuel core symmetry, reduce the density, and achieve a lower efficiency of nuclear fuel burning and a lower gain of nuclear reaction than expected. The effects of the hot electron anisotropic pressure tensor in the fast ignition scheme should be considered as a factor in experiments where the nuclear reaction gain is measured to be much lower than the theoretical prediction.