Slowing of Magnetic Reconnection Concurrent with Weakening Plasma Inflows and Increasing Collisionality in Strongly Driven Laser-Plasma Experiments

An evolution of magnetic reconnection behavior, from fast jets to the slowing of reconnection and the establishment of a stable current sheet, has been observed in strongly driven, $\beta \lesssim 20$ laser-produced plasma experiments. This process has been inferred to occur alongside a slowing of plasma inflows carrying the oppositely directed magnetic fields as well as the evolution of plasma conditions from collisionless to collisional. High-resolution proton radiography has revealed unprecedented detail of the forced interaction of magnetic fields and super-Alfvénic electron jets ($V_{\text{jet}}\sim 20V_A$) ejected from the reconnection region, indicating that two-fluid or collisionless magnetic reconnection occurs early in time. The absence of jets and the persistence of strong, stable magnetic fields at late times indicates that the reconnection process slows down, while plasma flows stagnate and plasma conditions evolve to a cooler, denser, more collisional state. These results demonstrate that powerful initial plasma flows are not sufficient to force a complete reconnection of magnetic fields, even in the strongly driven regime.

Figure 1

Proton radiography setup for reconnection experiments on the OMEGA EP laser system.

Figure 2

Proton radiography images at different times relative to the onset of the interaction beams, with dark areas representing greater proton fluence. The image at (a) 0.3 ns is produced by $\sim 18\text{−}\mathrm{MeV}$ protons, while the images at (b) 0.9 ns, (c) 1.3 ns, and (d) 1.8 ns are produced by $\sim 24\text{−}\mathrm{MeV}$ protons. Contrast has been optimized differently in each image to reveal details. The times indicated are between the onset of the interaction beams and the arrival of backlighter protons.

Figure 3

draco-simulated electron density ($n_e$, green line) and electron temperature ($T_e$, red line) around the collision region as a function of time. As the density increases and the temperature decreases, the Lundquist number lower bound ($S$, blue line) decreases, the collisionality upper bound (${\delta }_{\mathrm{SP}}/d_i$, thick black line) increases, and the plasma enters the regime of single-fluid reconnection.