Laser Plasma

 Introduction

 Fast Ignition

 I C F

 Experiments

 PIC Modelling

PIC Modelling

Hybrid PIC modeling of energetic electron beam transport

By M. Wei ( mwei@ucsd.edu )

In the concept of fast ignition for ICF energy study, relativistic electron beam produced from the interaction of a high intensity short pulse laser with a solid target has to traverse a huge density gradient over several tens to hundreds of microns and then deposit its energy to initiate the fusion spark. We have used the 3D hybrid fluid-kinetic LSP code to model the electron beam transport in solid density target. LSP is a 3-D electromagnetic Particle In Cell (PIC) code for large scale plasma simulations. The background plasma is treated as fluid particles. Fast electrons are treated as kinetic particles, which can be either promoted from the background cold electrons using heuristic scaling laws or produced directly from the interaction of the high intensity laser pulse with the preformed plasmas which are treated as kinetic particles. In latter scheme, laser plasma interaction and energetic electron beam production and propagation can be studied self-consistently.

Figure 1 shows the integrated LSP simulation results of the density contour of the kinetic electrons in a thin titanium wire target. The laser pulse is launched from the left boundary. It then propagates in the vacuum and interacts with a preformed plasma. The critical surface was pushed in by the intense laser pressure. Electrons produced from LPI has a two-temperature energy distributions. The slope temperature in the hot tail is comparable to the laser ponderomotive energy, while the not-so-hot component evolves with time to ~ 1 MeV at a laser intensity of 7.4?1019 W/cm2. With the presence of the preformed plasma, most of the fast electrons are bottlenecked near the critical surface region by the local B-fields. The overall propagation distance is on the order of ~ 100 µm. However, longer simulation times, a few picosecond compared to 0.5 – 1 ps laser pulse duration, are required for direct comparison with the experiments as relativistic electrons have long life (> 5 ps) time and fluorescence signals are time-integrated measurements in experiments.

Figure 1 LPI generated fast electron propagation in the solid wire target. a) fast electron density contour at 1.3 ps; b) on-axis lineout shows the time evolution of fast electron propagation along the wire