Formation of collisionless shocks driven by strongly magnetized relativistic electrons in the laboratory

In experiments performed with the OMEGA EP laser system, proton deflectometry captured magnetic field dynamics consistent with collisionless shock formation driven by strongly magnetized relativistic electrons. During laser-foil interactions, shocks can form as relativistic electrons and strong surface magnetic fields generated by a short-pulse laser impinge on a cooler plasma produced by a longer-pulse laser. Three-dimensional particle-in-cell simulations reproduce the magnetic draping and fast formation speeds measured in the experiment and reveal that this relativistic-electron-driven shock forms at an interface that is unstable to shear and streaming instabilities. The simulation results provide insight into the microphysics that may influence high-energy shocks observed in extreme astrophysical environments.

Figure 1

(a) Schematic of the experimental setup. The high-intensity SP laser arrives at the main target at $t=t_0$ (after the LP-produced magnetic fields had already evolved for 750 ps). The overlaid image representing the radiochromic film (RCF) stack shows the magnetic field conditions at $t_0$. (b) A time series of proton images using a Cu target show the fast field dynamics on picosecond timescales. Snapshots from face-on and side-on imaging of the interaction with a CH target are annotated in (c) and (d), respectively. Axes are scaled by the probing magnification.

Figure 2

(a) A time series of experimental proton images (left half of each time slice) from a single shot with a CH foil are compared to corresponding forward modeling results (right half). (b) An example field map from the forward modeling analysis for $t=t_0+9$ ps. Streamlines of a path-integrated magnetic field are overlaid on a map of the horizontal field component. (c) Lineouts taken at $z=0$ from the field profiles for each time slice in (a) demonstrate the ultrafast temporal evolution of the magnetic fields and the modulation of the LP-produced Biermann fields at the interface. Rear-surface fields are excluded from (c) to emphasize the front-surface dynamics. Dashed vertical lines indicate the zero-crossing point between the SP- and LP-produced fields. Shaded areas show the modulated field region caused by the draped magnetic fields at the discontinuity. The inset plot in (c) shows an isolated 2D view of the draped magnetic field feature from the dashed box region in (b).

Figure 3

A 3D surface of electron density at $t=2667{\omega }_0^{-1}$ is plotted with a cutout to show SP-driven magnetic field lines draped around the LP plume. The fields are highly perturbed due to the unstable interface which results in strong regions of $B_z$ (slice plotted on the $-z$ wall) and complex structures in $B_x$ (slice on the $+x$ wall).

Figure 4

Lineouts are taken from the 3D simulation at $y\approx 95(c/{\omega }_0), z=0$ at several time steps to form space-time plots of ion charge density $Z{n}_i$, magnetic field $B_z$, and electric field $E_x$. Lines are plotted over the electric field that follow the initial compression and late-time shock propagation with velocities $v_c$ and $v_{sh}$ and sonic Mach number $M$ shown beside the respective lines. See Supplemental Fig. 2 for 2D slices of ion charge density showing the shock structure [32].