Theoretical Study of Particle Motion Under High Intensity Laser–Plasma Interaction Aiming for High Energy Density Science

Abstract

In recent years, high power short pulse lasers in the range of 10^18–22 W/cm² have been developed and explored new science and applications. One of them is the fast ignition-based laser fusion, which is expected as one of clean and abundant energy sources. In determining the interaction between such high intensity lasers and plasmas, the ponderomotive force (light pressure) plays an essential role due to the strong non-uniformity of the laser field strength originated from tight focusing of the laser light. The force has been expressed as that proportional to the gradient of the laser field amplitude at the oscillation center of the particle. However, under the tight focusing, not only the gradient, which corresponds to the first-order perturbation to the uniform field, but also the higher order structures, e.g. field curvature to the second order, becomes important in determining the particle orbit. In order to precisely describe the relativistic ponderomotive force including such effects, here, we introduce the noncanonical Lie perturbation theory. We successfully derived the oscillation-center equation of motion up to the second order keeping the Hamiltonian structure rigorously. The resulting equation is found to be same as that of the first order indicating that no additional force appears up to the second order due to the symmetric nature of the field curvature.