Magnetic field line reconnection experiments: 5. Current disruptions and double layers

Abstract

In a large laboratory plasma a current sheet is generated in the process of magnetic field line reconnection. The stability of the sheet with respect to local current increases is investigated. When the current density in the center of the sheet exceeds a critical value, spontaneous local current disruptions are observed. The current from the center of the sheet moves out to the sides. Magnetic flux variations in regions remote from the current sheet generate an inductive voltage in the current loop that drops off inside the plasma in the form of a potential double layer. This leads to particle acceleration with velocities much larger than those expected from the steady state electric fields in the plasma. The particle beams acquire their energy at the expense of the stored magnetic field energy of the current system. Beam‐plasma instabilities are generated that dissipate some of the directed kinetic energy and heat the background plasma. A model for the mechanism of the current disruptions is formulated. The potential structure leads to ion expulsion creating a localized density drop. The associated current drop in an inductive circuit drives the potential structure, thereby providing feedback for the disruptive instability. It saturates at a total current loss upon which the current system recovers, and the process repeats randomly. Similarities and differences to magnetospheric substorm phenomena are pointed out.

Identification of Y-Shaped and O-Shaped Diffusion Regions During Magnetic Reconnection in a Laboratory Plasma

“…Even in the co-helicity case, if BT is small (Br/Bz 5 I), the island is unstable resulting in a thin sheet current which is driven by incoming and oppositely directed magnetic fields. Interestingly, this result is consistent with previous results obtained in an electron MHD plasma where ions were not magnetized [18].…”

Review of controlled laboratory experiments on physics of magnetic reconnection

“…Two distinctively different shapes of diffusion regions were identified both in the electron MHD plasma regime of LPD [Stenzel et al, 1983] Another important finding to date is that the enhancement factor of the measured resistivity over its classical values (or "the anomaly factor") is strongly dependent on the collisionality. In the collisional regime where the mean-free-path is comparable to the current sheet thickness, the anomaly factor is near unity, that is, almost no resistivity enhancement is found.…”

“…Two distinctively different shapes of diffusion regions were identified both in the electron MHD plasma regime of LPD [Stenzel et al, 1983] and in the MHD plasmas in MRX [Yamada et al, 1997a]. The familiar 2-D feature, a double-Yshaped diffusion region, was identified when there is no or very little axial magnetic field (third vector component).…”

Laboratory Observations of Spontaneous Magnetic Reconnection

“…Until now, experimental observation of spontaneous reconnection has been limited to linear devices [11][12][13], which all include strong three dimensional (3D) reconnection dynamics. Meanwhile, space observations show that the spatial scale along reconnecting current sheets can be several orders of magnitude (10 4 ) larger than the spatial scale perpendicular to the current sheets [14], implying that reconnection in nature can also occur in systems for which the global structure is mostly two dimensional.…”