Collisionless Shocks Mediated by Shear-Flow Magnetic Fields in Ultraintense-Laser-Produced Counter-Streaming Plasmas

Abstract

The formation of ion-Weibel-mediated collisionless shocks (IW-CSs) in ultraintense-laser-produced counter-streaming plasmas is investigated using particle-in-cell simulations. Analysis of the underlying microphysics reveals that a shear-flow ion-Weibel instability generates magnetic fields, which isotropize the incoming flow and mediate shock formation. An analytical expression for the shear-flow magnetic field is derived, and a scaling law relating the magnetic field amplitude to the laser intensity is established. This mechanism reduces the shock formation time and the required laser energy by three and two orders of magnitude, respectively, compared to high-power laser experiments, making it feasible to produce IW-CSs using existing multi-kilojoule, picosecond ultraintense laser facilities.

Figures (5)

• Figure 1: Ion phase-space density distributions (logarithmic scale) in counter-streaming plasmas produced by ultraintense laser ($a_0=10$). (a,c) $x-P_x$ distribution. (b,d) $P_x-P_y$ distribution for ions between $x=-20\mu m$ and 20μ m. (a,b) and (c,d) show simulation results at 6ps and 16ps, respectively.
• Figure 2: Self-generated magnetic field and ion density distributions in counter-streaming plasmas produced by ultraintense laser ($a_0=10$). (a,d) Self-generated magnetic field. (b,e) Ion density. (c,f) Magnetic field overlaid with ion density of left plasma flow. (a-c) and (d-f) show simulation results at 6ps and 16ps, respectively.
• Figure 3: Profiles of dimensionless physical quantities (DPQs) along the $y$-direction in a local region ($x=-20\mu m$ to 15μ m, $y=-55\mu m$ to -28μ m) at simulation time $t=6ps$.
• Figure 4: Scaling law of the shear-flow magnetic field with the normalized laser amplitude. The red solid line represents the least-squares fit. Error bars for the magnetic field data are determined from the statistical variation of local maximum value within the shock region.
• Figure 5: Ultraintense-laser-produced collisionless shock in a reduced transverse scale simulation (at 16ps). (a) Self-generated magnetic field. (b) Ion density. (c) Magnetic field overlaid with ion density of left plasma flow.