Kinematics of femtosecond laser-generated plasma expansion: Determination of sub-micron density gradient and collisionality evolution of over-critical laser plasmas

G G Scott,L Scaife,T Ma,F Beg,U Teubner,D R Symes,D Neely,P Forestier-Colleoni,P Mckenna,A A Andreev,G F H Indorf,C Thornton,S J Hawkes,M A Ennen,M Sedov

doi:10.1063/5.0038549

Abstract

An optical diagnostic based on resonant absorption of laser light in a plasma is introduced and is used for the determination of density scale lengths in the range of 10 nm to &gt;1 μm at the critical surface of an overdense plasma. This diagnostic is also used to extract the plasma collisional frequency, allowing inference of the temporally evolving plasma composition on the tens of femtosecond timescale. This is found to be characterized by two eras: the early time and short scale length expansion (L &lt; 0.1λ), where the interaction is highly collisional and target material dependent, followed by a period of material independent plasma expansion for longer scale lengths (L &gt; 0.1λ); this is consistent with a hydrogen plasma decoupling from the bulk target material. Density gradients and plasma parameters on this scale are of importance to plasma mirror optical performance and comment is made on this theme.

Highlights

A renewed interest in the plasma mirror (PM)1–3 is evident from a wealth of recent investigations emerging from the literature
This diagnostic is used to extract the plasma collisional frequency, allowing inference of the temporally evolving plasma composition on the tens of femtosecond timescale. This is found to be characterized by two eras: the early time and short scale length expansion (L < 0.1k), where the interaction is highly collisional and target material dependent, followed by a period of material independent plasma expansion for longer scale lengths (L > 0.1k); this is consistent with a hydrogen plasma decoupling from the bulk target material
The experimental technique in itself is simple; we have shown it to be powerful for its sensitivity in extracting plasma parameters that are in good agreement with the predictions of models in the literature

Summary

Introduction

A renewed interest in the plasma mirror (PM) is evident from a wealth of recent investigations emerging from the literature. As the PM’s repertoire evolves, the matter of their efficiency becomes increasingly important as they have traditionally been considered a lossy component with a reflectivity commonly measured to be around 70%. It was recently shown this can be significantly increased to 96%, when a 1015 W cmÀ2, 1.054 lm, s-polarized. Scitation.org/journal/php laser pulse interacted with a finite plasma density gradient of (0.1–0.3)k on the surface.. Scitation.org/journal/php laser pulse interacted with a finite plasma density gradient of (0.1–0.3)k on the surface.4 To further investigate this efficiency control mechanism, we have devised a technique to experimentally determine the plasma sub-micron density scale length, collisionality, and temperature, such that they can be experimentally correlated with PM performance. Measuring sub-micron density gradients is a useful measurement to make in itself for diagnosing overdense laser-plasma interactions, as many phenomena have been shown to be sensitively dependent on the laser interaction with short density scale lengths, such as harmonic generation, ion acceleration from surface monolayers, soft or keV33 x-ray generation, and instability growth.

Methods

Results

Discussion

Conclusion