Weibel-Induced Filamentation during an Ultrafast Laser-Driven Plasma Expansion

The development of current instabilities behind the front of a cylindrically expanding plasma has been investigated experimentally via proton probing techniques. A multitude of tubelike filamentary structures is observed to form behind the front of a plasma created by irradiating solid-density wire targets with a high-intensity ($I\sim {10}^{19}\mathrm{W}/{\mathrm{cm}}^2$), picosecond-duration laser pulse. These filaments exhibit a remarkable degree of stability, persisting for several tens of picoseconds, and appear to be magnetized over a filament length corresponding to several filament radii. Particle-in-cell simulations indicate that their formation can be attributed to a Weibel instability driven by a thermal anisotropy of the electron population. We suggest that these results may have implications in astrophysical scenarios, particularly concerning the problem of the generation of strong, spatially extended and sustained magnetic fields in astrophysical jets.

Figure 1

Selection of RCF images describing the ${\mathrm{CPA}}_1$-wire interaction with $t_r$ values of (a) $\sim 10$, (b) $\sim 30$, and (c) $\sim 50\mathrm{ps}$. The laser is incident from the right. The faint red lines of images (b) and (c) indicate the approximate positions of the $n_p$ pileups caused by the deflection of probe protons by the radially expanding proton front [10].

Figure 2

(a) Magnified region from the bottom-right quadrant of Fig. 1b. The axis of the wire image runs along the left-hand margin. (b) Matched particle tracing output obtained with $B_p=15\mathrm{T}$, $E_p=6\times {10}^8\mathrm{V}{\mathrm{m}}^{-1}$, and $d=40\mu \mathrm{m}$.

Figure 3

Schematic describing the random placement of tubular filaments originating from the wire surface and the coordinate system used in the particle tracing simulations.

Figure 4

Sample of PIC simulation results. (a) Electron plasma frequency ${\omega }_{pe}$ (in units of the initial electron plasma frequency ${\omega }_{p0}$), thermal anisotropy $A(r)$, and magnetic energy density $E_B(r)$. (b) Magnetic field $B_y(x,z)$ and (c) total current density $J$ (in units of the maximum current). The results are plotted at the time $t=10\mathrm{ps}$.