Anomalous Heating and Plasmoid Formation in a Driven Magnetic Reconnection Experiment

Abstract

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)...

Interactions of magnetized plasma flows in pulsed-power driven experiments

“…In carbon experiments however, the larger electron temperature of 60 eV results in a Lundquist number of S~100, which is sufficiently large that it is expected to place the layer in the plasmoid unstable, semi-collisional reconnection regime[41,42]. This is consistent with observations of the layer structure, which show the formation of plasmoids in both self-emission images [figure 3(b)] and laser interferometry[25,31,33]. Measurements with B-dot probes placed in the exhaust of the layer have indicated these indeed possess a magnetic O-point structure[25,31].…”

“…Magnetic reconnection is a complex and multifaceted process which is an active topic of research across space, astrophysics and plasma physics [28,29]. One of the most notable applications of the inverse wire array setup therefore has been to create a platform for studying this process in the laboratory [25,[30][31][32][33]. In these experiments two identical inverse wire arrays are fielded side-by-side on the MAGPIE generator, connected in parallel and in the same polarity to each receive a 50% share of the current supply.…”

“…(b) Carbon wire arrays produce a layer which is unstable to the production of plasmoids. Multi-frame videos showing the dynamics of the respective layers over each experiment can be found in the references[32] and[25].…”

“…Parameters of the plasma flow from inverse arrays of carbon (C), aluminium (Al) and tungsten (W) wires[13,19,20,[24][25][26][27]. Ranges indicate variability of the flow parameters over radial distance from the wires (~5-15 mm) and over the experimental timescale (~100-400 ns).…”

Formation and structure of a current sheet in pulsed-power driven magnetic reconnection experiments

“…The pulsed-power-driven reconnection platform 12,13 uses two exploding wire arrays (Fig. 2a), In the experiments described in this paper, each array consisted of 16 carbon wires (Staedler Mars Micro Carbon B) with a diameter of 400 µm, evenly spaced around a circle with a 16 mm diameter.…”