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

We investigate how next-generation laser pulses at $10\text{–}200\mathrm{PW}$ 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 hole boring. This pair-dominated plasma can block laser transport and trigger an avalanchelike QED cascade, efficiently transferring the laser energy to the photons. This renders a $1\text{−}\mu \mathrm{m}$ scale-length, underdense preplasma completely opaque to laser pulses at this power level. The QED-induced opacity therefore sets much higher contrast requirements for such a 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.

Figure 1

(a) Temporal profile of the laser power passing through the preplasma rear. In (a)–(f), the red dashed line and the blue solid line correspond to the simulations without and with the QED effects, respectively. (b), (e) Peak energy of the protons at $80{\tau }_0$ vs the laser power. (c), (d), (f), (g) Proton energy spectra at $80{\tau }_0$ with different laser powers. (b)–(d) are the results with a foil with a preplasma. (e)–(g) are the results with a polished foil without a preplasma.

Figure 2

(a), (b) Temporal evolution of residual energies of the laser pulse, target particles, photons, pairs, and the photons generated via the cascade (or only via the pairs), normalized by the total laser energy ${\varepsilon }_0$, where the laser power is taken as 200 PW. (a) and (b) correspond to the simulations with and without the QED effects, respectively. All particles exiting the simulation box are recorded and counted while the laser energy transported away is not counted. (c) Absorbed laser energy via the cascade in the preplasma region vs laser powers, where the energy is normalized by the total laser energy absorbed in this region.

Figure 3

Snapshots of (a), (d) laser electric fields $e{E}_y/m_e{\omega }_{0}c$, (b), (e) pair-plasma electron densities $n_e^{\text{pair}}/n_c$, (c), (f) positron densities $n_p^{\text{pair}}/n_c$ at different times, (g) Au-target electron density $n_e^{\text{Au}}/n_c$, and (h) electrostatic field $e{E}_y^{\text{ES}}/m_e{\omega }_{0}c$, where the maximum value is suppressed in (g) for clarity. (i) Number of generated photons and pairs vs their transverse position $y$, where the pair number is scaled by a factor of 40.

Figure 4

The energy absorption efficiency of lasers with different powers in the preplasma region vs preplasma density scale lengths $L$.

Figure 5

Temporal profile of laser power passing through the preplasma rear, where a controlled prepulse is taken 120 fs ahead of the main pulse in the red line and no prepulse is taken in the blue dashed line. (a) A 40-PW main pulse and a 4-PW prepulse are adopted. (b) A 200-PW main pulse and a 10-PW prepulse are adopted.