The effects of partial ionization (n(i) / n(n) ≤ 1%) on magnetic reconnection in the Hall regime have been studied systematically in the Magnetic Reconnection Experiment. It is shown that, when neutrals are added, the Hall quadrupole field pattern and thus electron flow are unchanged while the ion outflow speed is reduced due to ion-neutral drag. However, in contrast to theoretical predictions, the ion diffusion layer width does not change appreciably. Therefore, the total ion outflow flux and the normalized reconnection rate are reduced.

We review the theory of magnetic reconnection in weakly ionized gases. The theory is relevant to reconnection in the interstellar medium, protostellar and protoplanetary disks, the outer envelopes of cool stars, and a new laboratory experiment. In general, partial ionization introduces three effects beyond the obvious one: increased resistivity due to electron-neutral collisions. First, magnetic neutral sheets are steepened by plasma-neutral drift, setting up the conditions for reconnection. Second, when ion-neutral friction is strong, the effective ion mass is increased by ρ/ρi, the ratio of total to plasma mass density. This reduces the Alfven speed vA by a factor of ρ/ρi and increases the ion skin depth δi by ρ/ρi. As a result, entrainment of neutrals slows MHD reconnection but permits the onset of fast collisionless reconnection at a larger Lundquist number S, or for a longer current sheet, than in the fully ionized plasma case. These effects, taken together, promote fast collisionless reconnection when the ionization fraction is of order 10% to 1%, but reconnection is slowed down for much smaller ionization fractions. Finally, ion-neutral friction can be a strong heating mechanism throughout the inflow and outflow regions. These effects are under study at the Magnetic Reconnection Experiment (MRX).

Magnetic field line reconnection is still considered, by some, to be one of the most important topics in plasma physics. It has been in this category for close to 30 years and the 'problem of reconnection' has still not been solved. Magnetic field topologies are part and parcel of the current systems within a plasma whatever their source. Plasma currents may initially be induced or injected but they soon become entangled or part of the currents of plasma waves, flows and structures. We first present experimental results of undriven reconnection, which occurs when two magnetic flux ropes are generated from initially adjacent pulsed current channels in a background magnetoplasma (length 18 m, diameter 60 cm). The second example presented is the three-dimensional (3D) magnetic fields and currents associated with colliding laser-produced plasmas. The currents in this situation are those of shear Alfven waves. The wave magnetic field is a small fraction of the background field; nevertheless, reconnection regions, multiple magnetic 'X' points (which are 3D) and induced electric fields are observed. The first involves the interaction of magnetic flux ropes and the second localized reconnection sites in the current system of Alfven waves.

Two and three magnetic flux ropes are created and studied in a well-diagnosed laboratory experiment. The twisted helical bundles of field lines rotate and collide with each other over time. In the two rope case, reverse current layers indicative of reconnection are observed. Using a high spatial and temporal resolution three-dimensional volume data set in both cases, quasi-separatrix layers (QSLs) are identified in the magnetic field. Originally developed in the context of solar magnetic reconnection, QSLs are thought to be preferred sites for reconnection. This is verified in these studies. In the case of three flux ropes there are multiple QSLs, which come and go in time. The divergence of the field lines within the QSLs and the field line motion is presented. In all cases, it is observed that the reconnection is patchy in space and bursty in time. Although it occurs at localized positions it is the result of the nonlocal behavior of the flux ropes.

The concept of quasiseparatrix layers (QSLs) has emerged as a powerful tool to study the connectivity of magnetic field lines undergoing magnetic reconnection in solar flares. Although they have been used principally by the solar physics community until now, QSLs can be employed to shed light on all processes in which reconnection occurs. We present the first application of this theory to an experimental flux rope configuration. The three-dimensional data set acquired in this experiment makes the determination of the QSL possible.