Radiation-pressure-driven ion Weibel instability and collisionless shocks

The Weibel instability from counterstreaming plasma flows is a basic process highly relevant for collisionless shock formation in astrophysics. In this paper we investigate, via two- and three-dimensional simulations, suitable configurations for laboratory investigations of the ion Weibel instability (IWI) driven by a fast quasineutral plasma flow launched into the target via the radiation pressure of an ultra-high-intensity laser pulse (“hole-boring” process). The use of S-polarized light at oblique incidence is found to be an optimal configuration for driving IWI, as it prevents the development of surface rippling observed at normal incidence that would lead to strong electron heating and would favor competing instabilities. Conditions for the evolution of IWI into a collisionless shock are also investigated.

Figure 1

(a) Schematic presentation of the investigated set-up. The incident angle $\theta$ of the S-polarized laser pulse is defined in the $x-y$ plane. (b) Ion Weibel instability from radiation-pressure-driven flows in a 3D PIC simulation with realistic ion mass. Magnetic field $B_z$ and slice in the $y-z$ plane at $x=14{\lambda }_L$ of the ion current $J_{x,i}$ at $t=65t_L$. (c) Ion density ${\langle n_i\left(x\right)\rangle }_{y,z}$ averaged over the $y-z$ plane. The laser pulse (not shown) propagates along the $x>0$ direction, and the laser-plasma interaction surface at this time is located at $x\simeq 10{\lambda }_L$.

Figure 2

Radiation-pressure-driven collisionless shock formation in a 2D3V simulation with reduced ion mass $m_i=100m_e$. [(a) and (b)] Ion distribution in $p_x-p_y$ at $t=30t_L$ (before shock formation) and $t=515t_L$ (after shock formation), respectively. (c) Ion density $n_i(x,y)$ and (d) averaged ion density ${\langle n_i\left(x\right)\rangle }_y$ at $t=515t_L$.

Figure 3

Effect of the laser polarization and angle of incidence on the surface instability and electron heating. 2D3V simulations with P- and C-polarized laser pulses at normal incidence $\theta =0^{\circ }$ (top panels and middle panels, respectively) and S-polarized pulse at oblique incidence $\theta ={45}^{\circ }$ (bottom panels), at $t=25t_L$. [(a), (d), and (g)] Electron density. (b), (e), and (h)] Magnetic field $B_z$ averaged on a laser period. [(c), (f), and (i)] $x-p_x$ electron phase space. The red and black lines denote the position of the laser-plasma interaction surface.

Figure 4

Mitigation of the surface instability by a surface current in a 2D3V simulation considering a finite laser spot size $10\mu \mathrm{m}$. Electron (a) and ion (b) current $J_y$ at $t=85t_L$.