We present a detailed study of magnetic reconnection in a quasi-two-dimensional pulsed-power driven laboratory experiment. Oppositely directed magnetic fields (B ¼ 3 T), advected by supersonic, subAlfvénic carbon plasma flows (V in ¼ 50 km=s), are brought together and mutually annihilate inside a thin current layer (δ ¼ 0.6 mm). Temporally and spatially resolved optical diagnostics, including interferometry, Faraday rotation imaging, and Thomson scattering, allow us to determine the structure and dynamics of this layer, the nature of the inflows and outflows, and the detailed energy partition during the reconnection process. We measure high electron and ion temperatures (T e ¼ 100 eV, T i ¼ 600 eV), far in excess of what can be attributed to classical (Spitzer) resistive and viscous dissipation. We observe the repeated formation and ejection of plasmoids, consistent with the predictions from semicollisional plasmoid theory. DOI: 10.1103/PhysRevLett.118.085001 Magnetic reconnection is the rapid change of magnetic field topology in a plasma, accompanied by bulk heating and particle acceleration [1,2]. Reconnection is a ubiquitous process that occurs across a vast region of parameter space, including the collisionless plasmas at the heliopause [3] and the dense, hot plasmas deep in the solar convection zone [4,5]. Our understanding of magnetic reconnection has improved over the years thanks to dedicated laboratory experiments. In facilities like MRX [6][7][8] and TREX [9] the magnetic energy is much larger than the other plasma energy components. In contrast, laser-driven high energy density experiments are strongly driven-the kinetic and thermal energies are much larger than the magnetic energy [10,11], and reconnection heating is small [12].In this Letter we present experimental studies of high energy density magnetic reconnection driven by a new pulsed-power platform. The reconnection layer was created by the interaction of magnetized plasma flows in a quasi-2D geometry, which we studied using high resolution, nonperturbative measurements of the temperature, flow velocity, electron density, and magnetic field in the reconnection layer. The colliding plasma flows were supersonic (M s ∼ 1.6) but sub-Alfvénic (M A ∼ 0.7), and therefore the thermal and dynamic plasma betas (ratio of the thermal or ram pressure to the magnetic pressure) are close to unity (β th ∼ 0.7, β dyn ∼ 0.9). These parameters are significantly different from those found both in magnetically driven experiments, such as MRX, and in laser driven experiments, and we believe our experiments are the first to make a detailed study of this regime. We observed the formation of a reconnection layer with an aspect ratio of L=δ > 10, which existed for at least ten hydrodynamic flow times δ=V in , where L is the layer half length and δ is the layer half width [ Fig. 1(a)]. The annihilation of the magnetic flux caused strong plasma heating in the reconnection layer (T i ≈ 600 eV,ZT e ≈ 600 with T e ≈ 100 eV in a carbon plasma with average ionizationZ ≈ 6)...

We present an experimental study of the development and structure of bow shocks produced by the interaction of a magnetised, collisional, super-Alfv enic plasma flow with conducting cylindrical obstacles. The plasma flow with an embedded, frozen-in magnetic field (Re M $ 20) is produced by the current-driven ablation of fine aluminium wires in an inverse, exploding wire array z-pinch. We show that the orientation of the embedded field with respect to the obstacles has a dramatic effect on the bow shock structure. When the field is aligned with the obstacle, a sharp bow shock is formed with a global structure that is determined simply by the fast magneto-sonic Mach number. When the field is orthogonal to the obstacle, magnetic draping occurs. This leads to the growth of a magnetic precursor and the subsequent development of a magnetised bow shock that is mediated by two-fluid effects, with an opening angle and a stand-off distance, that are both many times larger than in the parallel geometry. By changing the field orientation, we change the fluid regime and physical mechanisms that are responsible for the development of the bow shocks. MHD simulations show good agreement with the structure of well-developed bow shocks. However, collisionless, two-fluid effects will need to be included within models to accurately reproduce the development of the shock with an orthogonal B-field. Published by AIP Publishing.

We describe magnetic reconnection experiments using a new, pulsed-power driven experimental platform in which the inflows are super-sonic but sub-Alfvénic. The intrinsically magnetised plasma flows are long lasting, producing a well-defined reconnection layer that persists over many hydrodynamic time scales. The layer is diagnosed using a suite of high resolution laser based diagnostics which provide measurements of the electron density, reconnecting magnetic field, inflow and outflow velocities and the electron and ion temperatures. Using these measurements we observe a balance between the power flow into and out of the layer, and we find that the heating rates for the electrons and ions are significantly in excess of the classical predictions. The formation of plasmoids is observed in laser interferometry and optical self-emission, and the magnetic O-point structure of these plasmoids is confirmed using magnetic probes.

This work presents a magnetic reconnection experiment in which the kinetic, magnetic and thermal properties of the plasma each play an important role in the overall energy balance and structure of the generated reconnection layer. Magnetic reconnection occurs during the interaction of continuous and steady flows of super-Alfvénic, magnetized, aluminum plasma, which collide in a geometry with two-dimensional symmetry, producing a stable and long-lasting reconnection layer. Optical Thomson scattering measurements show that when the layer forms, ions inside the layer are more strongly heated than electrons, reaching temperatures of T i~Z ̅ T e ≳ 300 eVmuch greater than can be expected from strong shock and viscous heating alone. Later in time, as the plasma density in the layer increases, the electron and ion temperatures are found to equilibrate, and a constant plasma temperature is achieved through a balance of the heating mechanisms and radiative losses of the plasma. Measurements from Faraday rotation polarimetry also indicate the presence of significant magnetic field pile-up occurring at the boundary of the reconnection region, which is consistent with the super-Alfvénic velocity of the inflows.

A supersonic flow of magnetized plasma is produced by the application of a 1 MA-peak, 500 ns current pulse to a cylindrical arrangement of parallel wires, known as an inverse wire array. The plasma flow is produced by the × acceleration of the ablated wire material, and a magnetic field of several Tesla is embedded at source by the driving current. This setup has been used for a variety of experiments investigating the interactions of magnetized plasma flows. In experiments designed to investigate magnetic reconnection, the collision of counter-streaming flows, carrying oppositely directed magnetic fields, leads to the formation of a reconnection layer in which we observe ions reaching temperatures much greater than predicted by classical heating mechanisms. The breakup of this layer under the plasmoid instability is dependent on the properties of the inflowing plasma, which can be controlled by the choice of the wire array material. In other experiments, magnetized shocks were formed by placing obstacles in the path of the magnetized plasma flow. The pile-up of magnetic flux in front of a conducting obstacle produces a magnetic precursor acting on upstream electrons at the distance of the ion inertial length. This precursor subsequently develops into a steep density transition via ion-electron fluid decoupling. Obstacles which possess a strong private magnetic field affect the upstream flow over a much greater distance, providing an extended bow shock structure.In the region surrounding the obstacle the magnetic pressure holds off the flow, forming a void of plasma material, analogous to the magnetopause around planetary bodies with self-generated magnetic fields.