Magnetic Reconnection between Colliding Magnetized Laser-Produced Plasma Plumes

Observations of magnetic reconnection between colliding plumes of magnetized laser-produced plasma are presented. Two counterpropagating plasma flows are created by irradiating oppositely placed plastic (CH) targets with 1.8-kJ, 2-ns laser beams on the Omega EP Laser System. The interaction region between the plumes is prefilled with a low-density background plasma and magnetized by an externally applied magnetic field, imposed perpendicular to the plasma flow, and initialized with an $X$-type null point geometry with $B=0$ at the midplane and $B=8\mathrm{T}$ at the targets. The counterflowing plumes sweep up and compress the background plasma and the magnetic field into a pair of magnetized ribbons, which collide, stagnate, and reconnect at the midplane, allowing the first detailed observations of a stretched current sheet in laser-driven reconnection experiments. The dynamics of current sheet formation are in good agreement with first-principles particle-in-cell simulations that model the experiments.

Figure 1

Experimental setup. Two counterpropagating drive plasma plumes are obtained by irradiating two opposing plastic (CH) ablator targets. An external magnetic field was created by pulsing an electric current through conductors located directly behind each target. The region between the ablator targets was prefilled by a tenuous background plasma created by a dedicated laser-ablator pair. A multi-MeV proton beam (not shown) generated with a high-intensity short-pulse laser beam was used to probe the dynamics and topology of the magnetic field in the interaction region.

Figure 2

Proton radiographic images of the magnetic field evolution. The ablator targets are situated at the left and right borders of each frame. Dark areas correspond to high proton fluence. The series illustrates four stages in the magnetic field evolution: (a) formation of magnetic “ribbons” and the sweeping up of magnetic field, (b) magnetic ribbon collision, (c) reconnection, and (d) magnetic field annihilation. Frame (e) shows a proton radiography image without the background (BG) plasma. The time stamps on each frame show the time when the proton beam fired relative to the drive beams. The horizontal and vertical scales are the same. Results of simulated proton radiography at the corresponding times are shown in the right column [(f)–(j)], with overlaid magnetic field lines (red).

Figure 3

Measured time dependance of the separation $d$ between the outer caustic boundaries of the magnetic ribbons (data points) compared with particle-in-cell (PIC) simulations (curve) showing inflow and stagnation of the flows.