Opacity of relativistically underdense plasmas for extremely intense laser pulses

TL;DR

This paper challenges the common belief that relativistically underdense plasmas are transparent to intense laser pulses, showing through simulations that at extremely high intensities, the plasma becomes opaque due to cascade effects and dense pair plasma formation.

Contribution

The study demonstrates that at ultra-high laser intensities, relativistically underdense plasmas become opaque due to electromagnetic cascades and pair plasma formation, contrary to prior assumptions.

Findings

Laser absorption increases sharply above 3 x 10^{24} W/cm^2

Dense electron-positron plasma forms inside the laser field

Plasma opacity is maintained despite increasing laser intensity

Abstract

It is generally believed that relativistically underdense plasma is transparent for intense laser radiation. However, particle-in-cell simulations reveal abnormal laser field absorption above the intensity threshold about~$3 \times 10^{24}~\mathrm{W}\,\mathrm{cm}^{-2}$ for the wavelength of $1~\mu \mathrm{m}$. Above the threshold, the further increase of the laser intensity doesn't lead to the increase of the propagation distance. The simulations take into account emission of hard photons and subsequent pair photoproduction in the laser field. These effects lead to onset of a self-sustained electromagnetic cascade and to formation of dense electron-positron ($e^+e^-$) plasma right inside the laser field. The plasma absorbs the field efficiently, that ensures the plasma opacity. The role of a weak longitudinal electron-ion electric field in the cascade growth is discussed.