Hydrodynamic impacts of short laser pulses on plasmas

TL;DR

This paper establishes conditions under which the interaction of short, intense laser pulses with cold plasma can be accurately modeled using hydrodynamics, simplifying the analysis of electron acceleration mechanisms.

Contribution

It introduces a relativistic plane model with new estimates that determine when plasma evolution remains hydrodynamic during laser pulse interaction.

Findings

Hydrodynamic evolution is valid under specific density and pulse conditions.

The model reduces complex PDEs to nonlinear Hamilton equations for analysis.

Results apply to finite laser spots with cylindrical symmetry.

Abstract

We determine conditions allowing to simplify the description of the impact of a short and arbitrarily intense laser pulse onto a cold plasma at rest. If both the initial plasma density and pulse profile have plane simmetry, then suitable matched upper bounds on the maximum and the relative variations of the initial density, as well as the intensity and duration of the pulse, ensure a strictly hydrodynamic evolution of the electron fluid (without wave-breaking or vacuum-heating) during its whole interaction with the pulse, while ions can be regarded as immobile. We use a recently developed fully relativistic plane model whereby the system of the (Lorentz-Maxwell and continuity) PDEs is reduced into a family of highly nonlinear but decoupled systems of non-autonomous Hamilton equations with one degree of freedom, with the light-like coordinate $\xi=ct\!-\!z$ instead of time $t$ as an independent variable, and new apriori estimates (eased by use of a Liapunov function) of the solutions in terms of the input data (initial density and pulse profile). If the laser spot radius $R$ is finite but not too small the same conclusions hold for the part of the plasma close to the axis $\vec{z}$ of cylindrical symmetry. These results may help in drastically simplifying the study of extreme acceleration mechanisms of electrons.