Integrated kinetic simulation of laser-plasma interactions, fast-electron generation, and transport in fast ignition

Abstract

We present new results on the physics of short-pulse laser-matter interaction of kilojoulepicosecond pulses at full spatial and temporal scale, using a new approach that combines a 3D collisional electromagnetic Particle-in-Cell code with an MHD-hybrid model of high-density plasma. In the latter, collisions damp out plasma waves, and an Ohm's law with electron inertia effects neglected determines the electric field. In addition to yielding orders of magnitude in speed-up while avoiding numerical instabilities, this allows us to model the whole problem in a single unified framework: the laser-plasma interaction at sub-critical densities, energy deposition at relativistic critical densities, and fast-electron transport in solid densities. Key questions such as the multi-picosecond temporal evolution of the laser energy conversion into hot electrons, the impact of return currents on the laser-plasma interaction, and the effect of self-generated electric and magnetic fields on electron transport will be addressed. We will report applications to current experiments.

A PIC-Fluid Hybrid Algorithm for Multiscale Simulations of Laser-Plasma Interactions

“…The simulation of the fast ignition approach to laser fusion presents extreme challenges to the traditional explicit algorithm because of the multiscale nature of the plasmathe plasma density ranges from vacuum to ~100× solid density in the compressed core [2]. Here we describe a PIC-fluid hybrid algorithm [3] that accommodates the multiscale nature of the physics of fast ignition and has proven successful in modeling laser-plasma interactions in fast-ignition laser fusion [4,5].…”

“…1. Implementation of the two-region hybrid algorithm in the PSC [2,3] and OSIRIS [4] electromagnetic PIC codes was relatively straightforward because the reduced Maxwell equations and Ohm's law are solved locally in space with finite-difference methods and with no global elliptic equation solve as in implicit PIC codes. Thus, the speed and excellent scaling with the number of processors of these highly optimized explicit PIC codes are retained.…”

“…The hybrid algorithm has been exploited quite successfully in simulations of fast ignition in Refs. [4,5] with a reduction in simulation costs in one-dimensional simulations exceeding an order of magnitude and two orders of magnitude or more reduction in costs in two-dimensional simulations. Terms in the Ohm's law that derive from collisional effects, e.g., the resistivity term, must be consistent with the microscopic collision operator used for the particles.…”

“…Thus, the speed and excellent scaling with the number of processors of these highly optimized explicit PIC codes are retained. In the implementations and applications of the two-region hybrid algorithm [3,4,5], some digital smoothing of field quantities is typically performed in the transition region between the low and high-density regions.…”

“…J cold,e , the last term on the right side of (2) is a momentum exchange term between the cold background electrons and the fast electrons due to collisions, and in the parameter regimes considered in Refs. [3,4,5] the resistivity is a scalar ! " = 0.51m e / n e e 2 # ei and…”