Relativistic Plasma Polarizer: Impact of Temperature Anisotropy on Relativistic Transparency

3D particle-in-cell simulations demonstrate that the enhanced transparency of a relativistically hot plasma is sensitive to how the energy is partitioned between different degrees of freedom. For an anisotropic electron distribution, propagation characteristics, like the critical density, will depend on the polarization of the electromagnetic wave. Despite the onset of the Weibel instability in such plasmas, the anisotropy can persist long enough to affect laser propagation. This plasma can then function as a polarizer or a wave plate to dramatically alter the pulse polarization.

Figure 1

Surfaces of constant intensity of the reflected and transmitted pulses 140 fs after the pulse hits the target (top panel). $E_y$ and $E_z$ cross sections at $z=0$ are given on the bottom and side of the box, respectively, along with $n_i$. The magnetic field energy and $\sqrt{⟨p_y^2⟩/⟨p_z^2⟩}$ are plotted throughout the simulation time (bottom panel).

Figure 2

Ratio of critical densities, $n_{\perp }/n_{\parallel }$, as a function of effective electron temperature $1/\alpha$ and the degree of anisotropy $\epsilon$. The solid lines indicate the contours of constant average electron energy.

Figure 3

Plots of the incident pulse (at $t=0$), the transmitted pulse through the relativistic plasma (at $t=190\mathrm{fs}$, $\alpha =8.0$, and $\epsilon =0.45$), and the transmitted pulse through the nonrelativistic plasma (at $t=280\mathrm{fs}$, $\alpha =500$, and $\epsilon =0.45$) are denoted in black, red, and green, respectively. The inset represents the relativistic distribution function in ($p_y,p_z$) space.