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

Abstract

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.

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].…”

“…As the interaction of the magnetized flows occurs over a relatively long timescale, with a continuous inflow of the magnetized plasma for many times the hydrodynamic timescale of the system (T ≫ L V flow ⁄ ), the power balance can be monitored as the reconnection process develops and the system evolves. The results have shown that there is a transfer of the kinetic and magnetic energy of the inflowing plasma to the kinetic and thermal energy inside the layer [25,[30][31][32].…”

“…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.…”

An experimental platform for pulsed-power driven magnetic reconnection

“…Experiments with carbon wires produced super-sonic (M S = 1.7) but sub-Alfvénic (M A = 0.7) flows, which were accelerated to V = 50 km/s and advected a field of B = 3 T. 24,25 The flows propagated radially outwards from each wire, without shock formation between adjacent flows (Fig. 5a), which suggests the flows have a fast magneto-sonic Mach number…”

“…For the parameters measured, plasmoids are expected to form in carbon, but not in aluminium, consistent with our observations. 15,25 There are clear differences between experiments with aluminium and carbon wires. The inflows in aluminium are more strongly driven and denser than those in carbon, and with more pronounced density modulations.…”