Attosecond-scale absorption at extreme intensities

Abstract

A novel non-ponderomotive absorption mechanism, originally presented by Baeva et al. [Phys. Plasmas 18, 056702 (2011)] in one dimension, is extended into higher dimensions for the first time. This absorption mechanism, the Zero Vector Potential (ZVP), is expected to dominate the interactions of ultra-intense laser pulses with critically over-dense plasmas such as those that are expected with the Extreme Light Infrastructure laser systems. It is shown that the mathematical form of the ZVP mechanism and its key scaling relations found by Baeva et al. in 1D are identically reproduced in higher dimensions. The two dimensional particle-in-cell simulations are then used to validate both the qualitative and quantitative predictions of the theory.

Energy absorption in the laser-QED regime

“…These high-momentum bunches of electrons, such as those shown in Fig. 1 are seen to co-propagate with the zeroes in the vector potential, hence the nomenclature ZVP absorption 27,33 .

Figure 1A momentum-position phase space plot in the laser-forward direction demonstrating the high-momentum peaks characteristic of the ZVP mechanism. The laser amplitude, was set to and the electron density was 50 times critical.

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“…These regimes are often identified by how the “hot” electron energy scales with the electron density, n e , and/or the normalised amplitude of the laser, a 0 :where A is the vector potential, I the intensity, and λ the wavelength of the incident laser pulse, c is the speed of light in vacuum, and e and m e are the electronic charge and rest mass. Here, as in previous work 27 , and throughout the rest of this manuscript, the vector potential is defined in the Lorenz gauge.…”

“…Notable, and dominant, examples include the explicitly ponderomotive mechanism 30 and Brunel (or vacuum) heating 31,32 . Theoretical and numerical work in recent years has predicted that when moving into the regime of “ultra-relativistic” lasers incident on relativistically over-dense targets, absorption processes are dominated by non-ponderomotive mechanisms such as the zero-vector-potential (ZVP) absorption mechanism 27,33 . This regime is identified by , and where n c is the critical density of the plasma, above which the plasma is opaque to light of wavelengths less than or equal to λ 0 :…”

“…In the case of the ZVP mechanism, the radiation pressure from the laser field displaces the electron fluid from the ion background, setting up a pseudo-capacitor system. When the vector potential of the laser field passes through zero, the radiation pressure instantaneously vanishes and the electric field of the pseudo-capacitor causes the electrons to shuttle across the pseudo-capacitor, and be accelerated to energies on the order of several keV up to MeV 27 . These high-momentum bunches of electrons, such as those shown in Fig.…”

“…The non-relativistic, ponderomotive, and non-ponderomotive regimes are distinguished experimentally by the scaling of the “hot electron” energy with a 0 . Specifically, , where: for the non-relativistic case 29 , in the ponderomotive regime 26 , and in the non-ponderomotive regime 27,33 . In this paper, we propose an alteration to the ZVP model devised by Baeva et al .…”