Three-dimensional particle-in-cell simulations of energetic electron generation and transport with relativistic laser pulses in overdense plasmas

摘要

In recent years, the advent of multi-terawatt lasers capable of producing focused laser intensities over 10{sup}19 W/cm{sup}2 and driving electron motion into the relativistic regime, has opened up many new interesting fields of research such as fast ignition for inertial fusion[1]. In the fast ignitor scheme, the intense laser pulse propagates through a coronal plasma up to several times the critical density and delivers energy to relativistic particles; these highly energetic particles then transport the energy through the overdense plasma to the center of the compressed core and ignite the fuel there. Considerable amount of experimental and numerical simulation works has been done on the physics of intense laser pulse propagation and the acceleration of electrons and ions in high density plasmas. However, the related problem of the propagation of intense electron bunches into the core, has not received as much attention. It has only recently been realized that collective phenomena play a crucial role in this energy transport. This paper is devoted to a three dimensional (3D) particle-in-cell (PIC) simulations of these collective phenomena. Typically, the electron currents generated near the vacuum plasma interface exceed the Alfven critical current[2] which should produce intense self-consistent B fields bending the electron trajectories backwards and preventing their penetration into the overdense plasma. However, in a dense plasma important shielding effects arise and the high energy electron current is neutralized by a cold electron return current. This allows the high energy electrons to propagate unimpeded into the overdense plasma. However, this system is unstable to a relativistic electromagnetic two stream instability (the so-called Weibel instability[3]) which breaks up the fast electron current into filaments. The work of our group [4] and the more detailed work of Honda et al. [5] have shown that the current filaments are self-organized to a state in which each filament carries a net current less than the Alfven limit. In this article, we extend these works to the three dimensional case with the help of 3D particle simulation.