# Laser opacity in underdense preplasma of solid targets due to quantum electrodynamics effects

## TL;DR

This paper demonstrates that quantum electrodynamics effects, such as pair production and gamma-ray emission, cause laser opacity in underdense preplasmas at 10-200 PW, impacting laser-target interactions and proton acceleration.

## Contribution

It reveals how QED processes induce opacity in underdense preplasmas at ultra-high laser powers, a phenomenon not accounted for in classical plasma physics.

## Key findings

- QED effects generate a pair plasma that blocks laser transport.
- Laser energy is efficiently transferred to photons via QED cascades.
- Preplasmas become opaque at 1 μm scale length, affecting experiments.

## Abstract

We investigate how next-generation laser pulses at 10 PW $-$ 200 PW interact with a solid target in the presence of a relativistically underdense preplasma produced by amplified spontaneous emission (ASE). Laser hole boring and relativistic transparency are strongly restrained due to the generation of electron-positron pairs and $\gamma$-ray photons via quantum electrodynamics (QED) processes. A pair plasma with a density above the initial preplasma density is formed, counteracting the electron-free channel produced by the hole boring. This pair-dominated plasma can block the laser transport and trigger an avalanche-like QED cascade, efficiently transfering the laser energy to photons. This renders a 1-$\rm\mu m$-scalelength, underdense preplasma completely opaque to laser pulses at this power level. The QED-induced opacity therefore sets much higher contrast requirements for such pulse in solid-target experiments than expected by classical plasma physics. Our simulations show for example, that proton acceleration from the rear of a solid with a preplasma would be strongly impaired.