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.
Highlights
Electrons move and can gain energy in response to the electromagnetic fields of a laser pulse; the coupling of the laser pulse energy to the electrons regulates the entire relativistic intensity laser-plasma interaction
Taking into account that R ≈ 2w0, we find that |Ex| ≈ 0.05|Ey| ≤ 0.05E0, where E0 is the amplitude of the transverse electric field in the focal plane of the incoming laser pulse
We have shown that laser beam propagation in near-critical plasmas, where nc < ne < nγc, can create conditions favorable for electron heating to energies well beyond what is achievable using transverse electric field direct laser acceleration (DLA) in such plasmas
Summary
Electrons move and can gain energy in response to the electromagnetic fields of a laser pulse; the coupling of the laser pulse energy to the electrons regulates the entire relativistic intensity laser-plasma interaction. Electrons are expelled from regions of highest intensity and, if the ponderomotive force persists to balance the electric field acting to return the electrons, a cavitated channel can form [32, 33, 34] In this regime, direct laser acceleration (DLA) assisted by quasi-static transverse and longitudinal electric fields of the channel may become the dominant mechanism generating an electron population with characteristic energies many times greater than Up [35, 36, 37, 38, 39, 40, 41, 42, 43]. The two-dimensional particle-in-cell simulations show one of the dominant energy transfer mechanisms into the high-energy tail is mediated by the evolving longitudinal electric fields within the main plasma volume causing the electrons to experience huge, rapid acceleration via this mechanism This is in stark contrast to previously identified DLA mechanisms that have either occurred in the very underdense region or essentially in vacuum with the overdense region serving as a source of electrons
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
