Method for Measuring the Laser Field and the Opacity of Spectral Lines in Plasmas

Eugene Oks

doi:10.3390/plasma4010003

Abstract

In experimental studies of laser-plasma interactions, the laser radiation can exist inside plasma regions where the electron density is below the critical density (“underdense” plasma), as well as at the surface of the critical density. The surface of the critical density could exhibit a rich physics. Namely, the incident laser radiation can get converted in transverse electromagnetic waves of significantly higher amplitudes than the incident radiation, due to various nonlinear processes. We proposed a diagnostic method based on the laser-produced satellites of hydrogenic spectral lines in plasmas. The method allows measuring both the laser field (or more generally, the field of the resulting transverse electromagnetic wave) and the opacity from experimental spectrum of a hydrogenic line exhibiting satellites. This spectroscopic diagnostic should be useful for a better understanding of laser-plasma interactions, including relativistic laser-plasma interactions.

Highlights

In experimental studies of laser-plasma interactions, the laser radiation cannot penetrate plasma regions of the electron density Ne > Nc, where Nc is the critical density defined by the equation
We presented the method allowing to measure both the laser field and the opacity from experimental spectrum of a hydrogenic line exhibiting satellites
The method is appropriate for a linearly-polarized laser field at the surface of the critical density or in underdense plasma regions

Summary

Introduction

In experimental studies of laser-plasma interactions, the laser radiation cannot penetrate plasma regions (called “overdense “plasma) of the electron density Ne > Nc , where Nc is the critical density defined by the equation. At the so-called “relativistic” laser intensities—typically the intensities exceeding 1018 W/cm , the critical density increases (despite the laser frequency is fixed) [1,2,3]. As mentioned in some of the above sources, the laser field (or more generally, the field of the resulting transverse electromagnetic wave) in laser-produced plasmas were measured in experiments by using satellites of dipole-forbidden spectral lines of helium-like ions—see, e.g., papers [13,14] and references therein. In the present paper we proposed the method appropriate for the above situation—the method that allows measuring both the laser field and the opacity from experimental spectrum of a hydrogenic line exhibiting satellites. We obtain the necessary theoretical results analytically and show how to use them for this purpose

The Method

Conclusions