Generation and Evolution of High-Mach-Number Laser-Driven Magnetized Collisionless Shocks in the Laboratory

We present the first laboratory generation of high-Mach-number magnetized collisionless shocks created through the interaction of an expanding laser-driven plasma with a magnetized ambient plasma. Time-resolved, two-dimensional imaging of plasma density and magnetic fields shows the formation and evolution of a supercritical shock propagating at magnetosonic Mach number $M_{\mathrm{ms}}\approx 12$. Particle-in-cell simulations constrained by experimental data further detail the shock formation and separate dynamics of the multi-ion-species ambient plasma. The results show that the shocks form on time scales as fast as one gyroperiod, aided by the efficient coupling of energy, and the generation of a magnetic barrier between the piston and ambient ions. The development of this experimental platform complements present remote sensing and spacecraft observations, and opens the way for controlled laboratory investigations of high-Mach number collisionless shocks, including the mechanisms and efficiency of particle acceleration.

Figure 1

Experimental setup. An external magnetic field was applied by pulsing current through conductors located behind opposing plastic (CH) piston targets. The volume between the piston targets was prefilled with an ambient plasma irradiated by a low-energy beam from a third CH target. Two counterpropagating piston plasma plumes, generated by ablating the opposing piston targets with two high-energy beams, were then driven through the magnetized ambient plasma. The resulting interaction was diagnosed with 2D refractive imaging (line-integrated along $\widehat{z}$) and proton radiography (line-integrated along $\widehat{y}$).

Figure 2

Refractive and proton radiographic images of collisionless shock evolution. In each image, the piston targets are located just outside the left and right borders, while the ambient target is located below the bottom border. The piston plasmas expand toward the center ($x=0$). The time stamps correspond to the time relative to the firing of the drive beams. The dashed rectangle in (a) represents the region of interest in Fig. 3. Panels (a)–(d) are images of angular filter refractometry. In (a), no shock is observed without an external magnetic field or ambient plasma. In (b)–(d), the shock is observed evolving from early to late times with an external field and ambient plasma. In (b), only one target was used, confirming that these features are independent of counterstreaming interactions between two pistons. In (e), proton radiography reveals the formation of strong magnetic field compressions (light ribbons) coincident with the shock at comparable times.

Figure 3

Evolution of line-integrated electron density profiles at (a) 2.35, (b) 2.85, and (c) 3.85 ns after laser ablation. For each, the density profiles (black) were reconstructed by linearly interpolating between the gradient density values associated with each AFR band edge and, in the regions of the density jumps, utilizing the shadowgraphy profiles. The constant density offset was estimated from simulations, and the shaded band corresponds to the uncertainty in this offset. Also shown are the corresponding profiles from psc PIC simulations (red). Additionally, in (c) the ambient (green) and piston (blue) contributions to the total electron density in the PIC simulations are shown. [(a), inset] Raw shadowgraphy signal (black) and reconstructed relative density (green) profile at 2.35 ns. [(b), inset] Direct comparison of the raw AFR signal (black) and corresponding synthetic simulation signal (red) at 2.85 ns. For both, the signals have been reduced to binary for simplicity. In all plots, the plasma moves toward $x=0$.

Figure 4

Results from 2D particle-in-cell simulations that show the formation of a high-Mach-number magnetized collisionless shock. The simulations consist of a CH piston plasma expanding into a CH ambient plasma in an externally applied magnetic field. Each panel is representative of a different time in units of the upstream C ion gyroperiod ${\omega }_{ci,\mathrm{C}}^{-1}$, from earliest (a) to latest (d). For each panel, the top three rows are phase space density plots of piston, ambient H, and ambient C ions in terms of the C magnetosonic Mach number $M_{\mathrm{ms}}$ and ambient C ion inertial length $d_i$. The bottom row are plots of magnetic field (black), total ion density (purple), ambient H ion density (red), ambient C ion density (green), and piston ion density (blue) relative to their upstream ambient values.