The Unexpected Role of Evolving Longitudinal Electric Fields in Generating Energetic Electrons in Relativistically Transparent Plasmas

TL;DR

This paper reveals that in relativistically transparent plasmas, evolving longitudinal electric fields generated by laser reflections play a crucial role in accelerating electrons to super-ponderomotive energies, alongside magnetic fields.

Contribution

It uncovers the significant impact of propagating longitudinal electric fields, produced by reflections, on energetic electron generation in relativistically transparent plasmas, a previously underappreciated mechanism.

Findings

Longitudinal electric fields are 30 times stronger than the incident laser field.

Reflections within the plasma funnel generate fields responsible for electron acceleration.

Direct laser acceleration accounts for about one-third of the super-ponderomotive electron energy.

Abstract

Superponderomotive-energy electrons are observed experimentally from the interaction of an intense laser pulse with a relativistically transparent target. For a relativistically transparent target, kinetic modeling shows that the generation of energetic electrons is dominated by energy transfer within the main, classically overdense, plasma volume. The laser pulse produces a narrowing, funnel-like channel inside the plasma volume that generates a field structure responsible for the electron heating. The field structure combines a slowly evolving azimuthal magnetic field, generated by a strong laser-driven longitudinal electron current, and, unexpectedly, a strong propagating longitudinal electric field, generated by reflections off the walls of the funnel-like channel. The magnetic field assists electron heating by the transverse electric field of the laser pulse through deflections, whereas the longitudinal electric field directly accelerates the electrons in the forward direction. The longitudinal electric field produced by reflections is 30 times stronger than that in the incoming laser beam and the resulting direct laser acceleration contributes roughly one third of the energy transferred by the transverse electric field of the laser pulse to electrons of the super-ponderomotive tail.