Laboratory Observation of Resistive Electron Tearing in a Two-Fluid Reconnecting Current Sheet

The spontaneous formation of plasmoids via the resistive electron tearing of a reconnecting current sheet is observed in the laboratory. These experiments are performed during driven, antiparallel reconnection in the two-fluid regime within the Magnetic Reconnection Experiment. It is found that plasmoids are present even at a very low Lundquist number, and the number of plasmoids scales with both the current sheet aspect ratio and the Lundquist number. The reconnection electric field increases when plasmoids are formed, leading to an enhanced reconnection rate.

Figure 1

Schematic of the Magnetic Reconnection Experiment showing the internal coils used to induce and drive reconnection, as well as the array of pickup coils used to measure the magnetic field. MRX is cylindrically symmetric, and the coordinate system used here is $(r,y,z)$ corresponding to the inflow, out-of-plane, and outflow directions, respectively.

Figure 2

Example discharges showing plasmoid formation either via a chain of small plasmoids [(a)–(c)] or via a single large plasmoid [(d)–(f)]. Contours of the poloidal flux and the locations of identified $X$ points and $O$ points are shown along with the out-of-plane current density (a),(d) and the out-of-plane Hall quadrupolar fields (b),(e). For clarity, flux contours outside of the central current sheet have been suppressed. (c),(f) The profile of the normal magnetic field $B_r$ at $r=37.5\mathrm{cm}$ [red line in (a), (b), (d), and (e)], along with typical experimental uncertainties in the magnetic field measurement.

Figure 3

(a) The observed time-averaged number of plasmoids $⟨n_O⟩$ and (b) the current sheet aspect ratio versus the Lundquist number. The mean is given by the black line, whereas the red shaded region denotes the standard deviation, and the blue region is the standard error of the mean. On average, more plasmoids are seen in elongated, high $S$, current sheets.

Figure 4

Example of the correlation between plasmoids and enhancement of the reconnection electric field in a single discharge. The shaded region in (a) indicates a conservative estimate of 10% for the experimental uncertainty in the electric field measurement.

Figure 5

Enhancement of the reconnection electric field during plasmoid formation. (a) Ensemble average of the time history of $\sim 500$ plasmoid formation events. The black line is the mean of all events, and the shaded region shows the standard error of the mean. (b) Statistically, more than $3/4$ of events result in an increased electric field immediately after plasmoid formation, $E_{t1}$, relative to that before plasmoid formation, $E_{t0}$. (c) Relationship between the global reconnection rate and the number of plasmoids for 200 discharges. Error bars correspond to the standard deviation.