Session DI1: Flux Tubes and Reconnection

Dynamic and Stagnating Plasma Flow Leading to Magnetic-Flux-Tube Collimation

This talk presents experimental observations, first reported by You, Yun, Bellan in PRL (art. 045002, 2005), strongly supporting the ``MHD pump-collimation'' model proposed by Bellan in Phys.~Plasmas (vol. 10, p.1999, 2003). Collimated, plasma-filled, magnetic flux tubes are observed over a tremendous range of scales. In laboratory plasmas, on the surface of the Sun, or jetting out of galactic cores, these flux tubes are extremely collimated, with cross-sections that do not vary much along the length of the tube even in the absence of external magnetic fields or any significant ambient medium pressure. Furthermore, these flux tubes are not in static equilibrium but exhibit strong plasma flows on a rapid time-scale compared to their overall motion within their surroundings. The Caltech experiment simulates magnetically-driven astrophysical jets at the laboratory scale by imposing boundary conditions analogous to astrophysical jet boundary conditions and with plasma dimensionless numbers comparable to numerical MHD simulations. Observations show a distinct sequence of events. The initial flux tubes flare out into the large vacuum, because the magnetic field weakens away from the source. As electrical current flows, the flux tubes become denser and more collimated while sucking plasma from gas sources at the system boundary, effectively acting like a magnetohydrodynamic pump. These flux tubes then merge together into a single column which jets out into the vacuum. The jet continues the same pumping process, to become even denser and more collimated, until either the electrical current or the supply of particles stop. The strong plasma flow convects frozen-in magnetic flux to regions of weaker magnetic field at the end of the tube, and as the flow stagnates there, magnetic flux piles up, pinching the tube into a collimated filament.   

Single versus multiple helicity reconnection in fusion relevant plasmas

Magnetic field line reconnection in collisionless regimes is of great relevance to both space and laboratory fusion plasmas. In present-day tokamak experiments, collisionless reconnection is believed to account for fast relaxation events (such as the sawtooth crash). In such devices the relaxation time can be shorter than the electron-ion collision time and electron inertia becomes the driving mechanism for the breaking of magnetic field lines. We treat this process with both 2D and 3D models. The main feature in the 2D approximation is that, while the magnetic field changes its topology, the topology of generalized fields is preserved. The process is characterized by the presence of very narrow scale lengths, and by current density and vorticity layers. The typical growth time is of the order of the linear growth time. When a magnetic island grows, interactions with neighboring islands of different helicities become more likely, signifying ultimately a 3D process. This allows magnetic field lines to follow chaotic trajectories. The questions we address here are whether the characteristic 2D small scale structures survive in a chaotic configuration, and the problem of the proper definition of reconnected flux in 3D. We find that the small scale structures obtained in 2D simulations persist and we propose a definition of the reconnected flux in 3D. Finally, we find that, even in a complex 3D setting, the characteristic growth time of the collisionless reconnection process is of the order of the exponential growth time found in the small-amplitude linear phase.   

Experimental Study of Non-MHD Effects During Fast Reconnection

Magnetic reconnection is the topological change of magnetic field lines during which magnetic energy is converted to plasma energy. One of the important goals in magnetic reconnection research is to explain the fast reconnection rate observed in laboratory, space and astrophysical plasmas. Recent breakthroughs show that non-MHD effects, including the Hall effect and electromagnetic fluctuations, can facilitate fast magnetic reconnection. The Hall effect has been found to drive fast magnetic reconnection in 2D numerical simulations [1]. A hallmark of the Hall effect is an out-of-plane quadrupole field in the reconnection region, which has been clearly observed in the Magnetic Reconnection Experiment (MRX) [2,3]. The spatial scale of the quadrupole field and the electron flow pattern agree well with numerical simulations. Measurements also show that the Hall effect is more significant in the collisionless regime than in the collisional regime, indicating that the Hall effect plays an important role in collisionless fast reconnection. Furthermore, Mach probe measurements demonstrate that the ion outflow is much slower than the electron outflow. Magnetic fluctuations in the lower-hybrid frequency range have been observed along with the quadrupole field; these fluctuations happen not only in the current sheet center region [4], but also in the outflow region. Comparisons with space observations will be discussed. In collaboration with M. Yamada, H. Ji, S. P. Gerhardt, A. Kuritsyn, R. Kulsrud and H. Torreblanca. \newline \newline [1] J. Birn et al., J. Geophys. Res., 106, 3715, 2001 \newline [2] Y. Ren et al., Phys. Rev. Lett., 95, 055003, 2005 \newline [3] M. Yamada et al., Phys. Plasmas, 13, 052119, 2006 \newline [4] H. Ji et al., Phys. Rev. Lett., 92, 115001, 2004   

Fast Collisionless Reconnection in Electron-Positron Plasmas

There has been growing interest in pair plasmas for their applications to astrophysical as well as laboratory plasma physics. Important astrophysical applications include extragalactic jets, and winds and jets from pulsars.~In addition to these applications, studies of magnetic reconnection in electron-positron plasmas present a new opportunity to examine critically the question of the ingredients that are essential in realizing regimes of fast magnetic reconnection. In a pair plasma, the electron and ion skin depth parameters are identical, the Hall current cancels out exactly, and whistler waves do not exist. We demonstrate, by means of two-dimensional particle-in-cell simulations, that fast reconnection occurs in a pair plasma without a separation of spatial scales between electron and positron flows, and without the intervention of the Hall current.~Despite the absence of the Hall current and whistler waves, our numerical results provide clear evidence of fast collisionless reconnection due to the localization caused by the off-diagonal components of the pressure tensors [N. Bessho and A. Bhattacharjee, Phys. Rev. Lett., 95, 245001 (2005)].~We have carried out simulations in non-relativistic as well as relativistic regimes, the latter with drifting J\"{u}ttner-Synge distribution functions. When the Alfv\'{e}n speed is close to the speed of light, the outflow speed due to reconnection also becomes close to the speed of light. Ultrarelativistic particles are generated by reconnection. We will discuss the energy spectrum of accelerated particles and the mechanisms of their acceleration.