Modeling of laser induced plasma expansion in the presence of non-Maxwellian electrons

Abstract

The one-dimensional expansion into vacuum of ion-electron plasma produced by laser ablation is investigated. The ions considered as an ideal fluid are governed by a fluid model where charge quasineutrality is assumed to prevail, while electron density follows a non-Maxwellian distribution. Showing that the expansion can be described by a self-similar solution, the resulting nonlinear Euler equations are solved numerically. It is found that the deviation of the electrons from Maxwellian distribution gives rise to new asymptotic solutions of physical interest affecting the density and velocity of plasma expansion.Plasma expansion into vacuum is a basic physical problem with a variety of applications, ranging from space to laboratory scales. 1-4 Caused generally by electron pressure, it serves as an energy transfer mechanism from electrons to ions. The expansion process is often described under the assumption of Maxwellian electrons with velocities in local thermal equilibrium ͑LTE͒, known to be isotropically distributed around the average velocity. This assumption easily fails with a lack of collisions. Indeed, in many cases of astrophysical and laboratory plasmas expansion, the electron distribution functions ͑EDFs͒ are non-Maxwellian and exhibit more complex shapes showing high-energy tails, as in the weakly collisional corona and solar wind acceleration region. 5 The fundamental reason is that fast electrons collide much less frequently than slow ones. Indeed their free path is very large since proportional to v e 4 , where v e is the electron velocity, and cannot relax to a Maxwellian. The energetic electrons could have a significant effect on ionization and expansion of the plasma. As reported by many authors, 6-8 in an expanding plasma produced by laser ablation experiments, the high mobility light electrons escape faster into vacuum compared to heavier particles, thus generating a selfconsistent ambipolar electric field that accelerates the ions and slows electrons.Another phenomenon observed in plasma expansion is that the heat exchange between electrons and heavy particles such as ions is inefficient as only a fraction of the thermal energy of the order of m e / m i is transferred per collision, where m e and m i are the electron and ion mass, respectively. This causes the electron temperature T e to differ from the ion temperature T i . T e in non-LTE plasmas is higher than T i because of insufficient electron-electron collisions which provide the essential mechanism to reach thermal equilibrium. 9 This phenomenon is also crucial in pulsed laser deposition ͑PLD͒ experiments, where the efficiency of the deposited films depends on the parameters of the laser induced plasma that expands into vacuum or in an ambient environment. A very simple view of the PLD divides the process into two main stages: first, the incident laser rapidly heats the target, then a dense, warm plasma is created near the target surface, leading the plume to expand adiabatically. Finally, the plume deposits onto a substrate with nonequ...

Combined effects of electronic trapping and non-thermal electrons on the expansion of laser produced plasma into vacuum

“…The energetic electrons could have a significant effect on ionization and expansion of the plasma (Beilis, 2012). As reported by many authors (Gurevich et al, 1979;Bennaceur-Doumaz & Djebli, 2010;Kiefer et al, 2013), in expanding plasma produced by laser plasma experiments, the high mobility light electrons escape faster into vacuum compared to heavier particles, thus generating a self-consistent ambipolar electric field that accelerates the ions and slows electrons. Ions are accelerated by non-Maxwellian electron distribution enriched with fast particles.…”

Effect of Space Fractional Parameter on Nonlinear Ion Acoustic Shock Wave Excitation in an Unmagnetized Relativistic Plasma

“…It is well established that the relativistic electron-positron-ion (REPI) plasmas are not only existed in the evolution of the early Universe [1,2] but also in many astrophysical and space environments (ASEs), especially in the relativistic wind of pulsar magnetosphere [3], polar regions of neutron stars [4], pulsar magnetospheres [5], inner regions of the accretion disks surrounding the central black holes [6], active galactic nuclei [7], the center of the Milky Way galaxy [8], laser-plasma interaction [9,10], and so on. The production of electron-positron pair along with massive ions is confirmed in peculiar ASEs by Advanced Satellite for Cosmology and Astrophysics.…”

Kadomtsev-Petviashvili solitons propagation in a plasma system with superthermal and weakly relativistic effects

“…22 Non-thermal electrons and positrons are also predicted to exist in the expansion phenomenon of laser induced plasmas. 23 The mathematical and physical properties of the kappa velocity distribution are discussed in detail by authors. 24,25 Chuang et al 26 studied the ion acoustic solitons in the presence of kappa distributed electrons and predicted that the solitary structures with depleted density are unlikely to exist in the non-Maxwellian plasmas.…”