Fast electron transport in laser-produced plasmas and the KALOS code for solution of the Vlasov–Fokker–Planck equation

Abstract

In solid targets irradiated by short pulse high intensity lasers, fast electrons have collision times longer than the laser pulse duration and mean free paths much larger than the radius of the laser spot. In these conditions, fast electron transport is dominated by electric and magnetic field. Although the fast electrons are collisionless, collisions of background electrons determine the ability of the background plasma to carry the return current which balances the fast electron current. Hence collisions are important even in this regime. A successful numerical simulation has to be able to model a plasma in which some electrons are collisionless and others are strongly collisional. An expansion of the electron distribution in spherical harmonics in momentum space is well suited to this, and we describe the formulation of the Vlasov-Fokker-Planck equation in terms of spherical harmonics and its solution in our KALOS code. We review the physics that must be modelled in a numerical simulation of fast electron transport and then describe KALOS.

Theory of fast electron transport for fast ignition

“…The effect of this noise is arguably not yet well-explored in the regime of the dense plasmas that arise in fast-ignition research. For this reason a small number of codes based on finite-difference techniques (Bell et al 2006, Robinson & Sherlock 2007, Sherlock 2009) have arisen in recent years. While techniques based on finite-difference in phase space eliminate noise, they do not currently possess most of the aforementioned advantages inherent in the particle approach (and in fact are often deleteriously affected by their converses).…”

X-ray back-lighter characterization for iron opacity measurements using laser-produced aluminium K-alpha emission

“…h2d models in 2D, the ablation, ionisation and expansion of material irradiated by a laser pulse to give the plasma density, temperature and ionisation as a function of distance and time in a Lagrangian coordinate system in r,z cylindrical geometry. The code incorporates multigroup diffusion of thermal radiation and flux limited electron conduction (flux limiter = 0.1 [17]). Laser absorption is by inverse bremsstrahlung with an energy dump of 20% at the critical density.…”

Effects of buried high-Z layers on fast electron propagation

“…Assuming that the fast electron currents can be neutralized completely by the cold return current (i.e., j c = −j h ), which is usually satisfied in the situation of ultraintense laser-driven fast electron propagation in high density plasma [23,24], where j c = −en e v e is the cold electron current and j h is the fast electron current, one can write…”

“…where j 0 = αI/T h [23], α is the laser-fast-electron energy conversion efficiency, I is the laser intensity, r b is the spot radius of the laser pulse, e z is the unit vector in the z direction, and T h is the fast electron temperature given by the ponderomotive scaling [31], i.e.,…”