The unexpected role of evolving longitudinal electric fields in generating energetic electrons in relativistically transparent plasmas

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.

Energy absorption and coupling to electrons in the transition from surface- to volume-dominant intense laser–plasma interaction regimes

“…DLA has been investigated previously in the case of uniform near critical density plasma [34,35]. The main driver for DLA is the formation of strong longitudinal electric field structures due to self-focusing of the laser pulse and reflection from the overdense regions of the plasma [36]. To demonstrate this effect, the longitudinal electric fields are plotted in figures 6(a) and (b) for l = 376 nm and l = 40 nm, respectively, sampled at t = 0.2 ps.…”

Laser reflection as a catalyst for direct laser acceleration in multipicosecond laser-plasma interaction

“…Next, we estimate the phase velocity of the forwardpropagating laser in the PIC simulation, which we find differs substantially from the cold neutral plasma value. Established graphical methods for directly measuring the phase velocity in PIC simulation 48,49 are challenging to apply and interpret in the presence of interference patterns, such as results from the strong laser reflection in our simulation. Instead, we constrain the phase velocity by examining the laser work (W y ) done on high energy electrons as they slip in phase.…”

Interaction of relativistically intense laser pulses with long-scale near critical plasmas for optimization of laser based sources of MeV electrons and gamma-rays

“…One of the possibilities to increase the electron beam charge above a nC level keeping the electron energy at a level of tens up to hundreds of MeV, is to use the advantage of relativistic laser interaction with plasmas of subcritical and near critical density (NCD) [21][22][23][24]. The critical electron density is defined as n m e 4 L cr 2 2 w p = ( ) / where m and e are the mass of electron at rest and its charge and L w is the laser frequency.…”

“…The experiments showed a strong increase of the 'temperature' and number of supra-ponderomotive electrons caused by the increased length of a relativistic ion channel. New results on the electron acceleration from pre-ionized foam layers conducted at the Titan laser system were reported in [24]. There, 250 μm thick foam layers with mean densities from 3 up to 100 mg cm −3 (n e =0.9-30×10 21 cm −3 ) were pre-ionized by ns ASE-prepulse.…”

“…This method can be applied if the energy of the ns pulse is large enough to generate a hohlraum radiation temperature above 30-40 eV. If the energy of the ns-pulse is restricted or one uses ns-prepulse to drive a super-sonic ionization, like it was done in [24], direct laser irradiation of the foam layers has to be used.…”