Final Report for DoE Grant DE-FG02-06ER54878, Laboratory Studies of Reconnection in Magnetically Confined Plasmas

The study of the collisionless magnetic reconnection constituted the primary work carried out under this grant. The investigations utilized two magnetic configurations with distinct boundary conditions. Both configurations were based upon the Versatile Toroidal Facility (VTF). The first configuration is characterized by open boundary conditions where the magnetic field lines interface directly with the vacuum vessel walls. The reconnection dynamics for this configuration has been methodically characterized and it has been shown that kinetic effects related to trapped electron trajectories are responsible for the high rates of reconnection observed. This type of reconnection has not been investigated before. Nevertheless, the results are directly relevant to observations by the Wind spacecraft of fast reconnection deep in the Earth magnetotail. The second configuration was developed to be specifically relevant to numerical simulations of magnetic reconnection, allowing the magnetic field-lines to be contained inside the device. The configuration is compatible with the presence of large current sheets in the reconnection region and reconnection is observed in fast powerful bursts. These reconnection events facilitate the first experimental investigations of the physics governing the spontaneous onset of fast reconnection. In this Report we review the general motivation of this work, the experimental set-up, and the main physics results

Final Report: Laboratory Studies of Spontaneous Reconnection and Intermittent Plasma Objects

The study of the collisionless magnetic reconnection constituted the primary work carried out under this grant. The investigations utilized two magnetic configurations with distinct boundary conditions. Both configurations were based upon the Versatile Toroidal Facility (VTF) at the MIT Plasma Science and Fusion Center and the MIT Physics Department. The NSF/DOE award No. 0613734, supported two graduate students (now Drs. W. Fox and N. Katz) and material expenses. The grant enabled these students to operate the VTF basic plasma physics experiment on magnetic reconnection. The first configuration was characterized by open boundary conditions where the magnetic field lines interface directly with the vacuum vessel walls. The reconnection dynamics for this configuration has been methodically characterized and it has been shown that kinetic effects related to trapped electron trajectories are responsible for the high rates of reconnection observed. This type of reconnection has not been investigated before. Nevertheless, the results are directly relevant to observations by the Wind spacecraft of fast reconnection deep in the Earth magnetotail. The second configuration was developed to be relevant to specifically to numerical simulations of magnetic reconnection, allowing the magnetic field-lines to be contained inside the device. The configuration is compatible with the presence of large current sheets in the reconnection region and reconnection is observed in fast powerful bursts. These reconnection events facilitate the first experimental investigations of the physics governing the spontaneous onset of fast reconnection. In the Report we review the general motivation of this work and provide an overview of our experimental and theoretical results enabled by the support through the awards

Final Report for DoE Grant DE-FG02-06ER54878, Laboratory Studies of Reconnection in Magnetically Confined Plasmas

The study of the collisionless magnetic reconnection constituted the primary work carried out under this grant. The investigations utilized two magnetic configurations with distinct boundary conditions. Both configurations were based upon the Versatile Toroidal Facility (VTF). The first configuration is characterized by open boundary conditions where the magnetic field lines interface directly with the vacuum vessel walls. The reconnection dynamics for this configuration has been methodically characterized and it has been shown that kinetic effects related to trapped electron trajectories are responsible for the high rates of reconnection observed. This type of reconnection has not been investigated before. Nevertheless, the results are directly relevant to observations by the Wind spacecraft of fast reconnection deep in the Earth magnetotail. The second configuration was developed to be specifically relevant to numerical simulations of magnetic reconnection, allowing the magnetic field-lines to be contained inside the device. The configuration is compatible with the presence of large current sheets in the reconnection region and reconnection is observed in fast powerful bursts. These reconnection events facilitate the first experimental investigations of the physics governing the spontaneous onset of fast reconnection. In this Report we review the general motivation of this work, the experimental set-up, and the main physics results

Final Report: Laboratory Studies of Spontaneous Reconnection and Intermittent Plasma Objects

The study of the collisionless magnetic reconnection constituted the primary work carried out under this grant. The investigations utilized two magnetic configurations with distinct boundary conditions. Both configurations were based upon the Versatile Toroidal Facility (VTF) at the MIT Plasma Science and Fusion Center and the MIT Physics Department. The NSF/DOE award No. 0613734, supported two graduate students (now Drs. W. Fox and N. Katz) and material expenses. The grant enabled these students to operate the VTF basic plasma physics experiment on magnetic reconnection. The first configuration was characterized by open boundary conditions where the magnetic field lines interface directly with the vacuum vessel walls. The reconnection dynamics for this configuration has been methodically characterized and it has been shown that kinetic effects related to trapped electron trajectories are responsible for the high rates of reconnection observed. This type of reconnection has not been investigated before. Nevertheless, the results are directly relevant to observations by the Wind spacecraft of fast reconnection deep in the Earth magnetotail. The second configuration was developed to be relevant to specifically to numerical simulations of magnetic reconnection, allowing the magnetic field-lines to be contained inside the device. The configuration is compatible with the presence of large current sheets in the reconnection region and reconnection is observed in fast powerful bursts. These reconnection events facilitate the first experimental investigations of the physics governing the spontaneous onset of fast reconnection. In the Report we review the general motivation of this work and provide an overview of our experimental and theoretical results enabled by the support through the awards

Experimental Study of Current-Driven Turbulence During Magnetic Reconnection

CMPD Final Report Experimental Study of Current-Driven Turbulence During Magnetic Reconnection Miklos Porkolab, PI, Jan Egedal, co-PI, William Fox, graduate student. This is the final report for Grant DE-FC02-04ER54786, âÃÂÃÂMIT Participation in the Center for Multiscale Plasma Dynamics,âÃÂàwhich was active from 8/1/2004 to 7/31/2010. This Grant supported the thesis work of one MIT graduate student, William Fox, The thesis research consisted of an experimental study of the fluctuations arising during magnetic reconnection in plasmas on the Versatile Toroidal Facility (VTF) at MIT Plasma Science and Fusion Center (PSFC). The thesis was submitted and accepted by the MIT physics Department, âÃÂÃÂW. Fox, Experimental Study of Current-Driven Turbulence During Magnetic Reconnection, Ph.D. Thesis, MIT (2009)âÃÂÃÂ. In the VTF experiment reconnection and current-sheet formation is driven by quickly changing currents in a specially arranged set of internal conductors. Previous work on this device [Egedal, et al, PRL 98, 015003, (2007)] identified a âÃÂÃÂspontaneousâÃÂàreconnection regime. In this work fluctuations were studied using impedance-matched, high-bandwidth Langmuir probes. Strong, broadband fluctuations, with frequencies extending from near the lower-hybrid frequency [fLH = (fcefci)1/2] to the electron cyclotron frequency fce were found to arise during the reconnection events. Based on frequency and wavelength measurements, lower-hybrid waves and Trivelpiece-Gould waves were identified. The lower-hybrid waves are easiest to drive with strong perpendicular drifts or gradients which arise due to the reconnection events; an appealing possibility is strong temperature gradients. The Trivelpiece-Gould modes can result from kinetic, bump-on-tail instability of a runaway electron population energized by the reconnection events. We also observed that the turbulence is often spiky, consisting of discrete positive-potential spikes, which were identified as âÃÂÃÂelectron phase-space holes,âÃÂàa class of nonlinear solitary wave known to evolve from a strong beam-on-tail instability. We established that fast electrons were produced by magnetic reconnection. Overall, these instabilities were found to be a consequence of reconnection, specifically the strong energization of electrons, leading to steep gradients in both coordinate- and velocity-space. Estimates (using quasi-linear theory) of the anomalous resistivity due to these modes did not appear large enough to substantially impact the reconnection process. Relevant publications: âÃÂâ W. Fox, M. Porkolab, et al, Phys. Rev. Lett. 101, 255003 (2008). âÃÂâ W. Fox, M. Porkolab, et al, Phys. Plasmas 17, 072303, (2010)

Formation of a localized acceleration potential during magnetic reconnection with a guide field

Magnetic reconnection near the surface of the sun and in the Earth’s magnetotail is associated with the production of highly energetic electrons. Direct acceleration in the reconnection electric field has been proposed as a possible mechanism for energizing these electrons. Here, however, we use kinetic simulations of guide-field reconnection to show that in two-dimensional (2D) reconnection the parallel electric field, E[subscript II] in the reconnection region is localized and its structure does not permit significant energization of the electrons. Rather, a large fraction of the electrons become trapped due to a sign reversal in E[subscript II], imposing strict constraints on their motions and energizations. Given these new results, simple 2D models, which invoke direct acceleration for energizing electrons during a single encounter with a reconnection region, need to be revised.United States. Dept. of Energy (Junior Faculty Grant DE-FG02-06ER54878

Laboratory observations of electron energization and associated lower-hybrid and Trivelpiece–Gould wave turbulence during magnetic reconnection

This work presents an experimental study of current-driven turbulence in a plasma undergoing magnetic reconnection in a low-β, strong-guide-field regime. Electrostatic fluctuations are observed by small, high-bandwidth, and impedance-matched Langmuir probes. The observed modes, identified by their characteristic frequency and wavelength, include lower-hybrid fluctuations and high-frequency Trivelpiece–Gould modes. The observed waves are believed to arise from electrons energized by the reconnection process via direct bump-on-tail instability (Trivelpiece–Gould) or gradients in the fast electron population (lower-hybrid)

Regimes of the Electron Diffusion Region in Magnetic Reconnection

The electron diffusion region during magnetic reconnection lies in different regimes depending on the pressure anisotropy, which is regulated by the properties of thermal electron orbits. In kinetic simulations at the weakest guide fields, pitch angle mixing in velocity space causes the outflow electron pressure to become nearly isotropic. Above a threshold guide field that depends on a range of parameters, including the normalized electron pressure and the ion-to-electron mass ratio, electron pressure anisotropy develops in the exhaust and supports extended current layers. This new regime with electron current sheets extending to the system size is also reproduced by fluid simulations with an anisotropic closure for the electron pressure. It offers an explanation for recent spacecraft observations.United States. Dept. of Energy (DOE Junior Faculty Grant No. DE-FG02-06ER54878)United States. National Aeronautics and Space Administration (NASA Grant No. NNH11CC65C

Cluster observations of bidirectional beams caused by electron trapping during antiparallel reconnection

[1] So-called bidirectional electron beams have been observed by a number of spacecraft missions mainly in the inflow of reconnection regions. Here we show that these beam-like features in the electron distribution function are explained by electron trapping. The trapping is mainly controlled by a positive acceleration potential, Φ||, which is related to the structure of the parallel electric fields in the vicinity of the reconnection region. Guided by the results of a kinetic simulation, we extend a recent analytical model for the electron distribution function applicable to the inflow region in antiparallel reconnection. The model is successfully compared to data observed by the four Cluster spacecraft inside an active reconnection region. The anisotropy recorded in the electron distributions is consistent with mainly electric trapping of electrons by Φ||. In the analysis we determine the profiles of Φ|| along the paths of the Cluster spacecraft during their encounter with a reconnection region. Typical values of eΦ|| are in excess of 1 keV (much higher than the electron temperature in the ambient lobe plasma) and Φ||traps all thermal electrons. This is important for the internal structure of the Hall current system associated with the ion diffusion region because extended trapping significantly alters the electrical and kinematic properties of the electron fluid. Finally, at the boundary between the inflow and exhaust regions the values of eΦ|| in the inflow region smoothly approach the shoulder energies (up to 15 keV) of the so-called flat-top distribution observed in the reconnection exhaust. This suggests that Φ|| may be important also to the formation of the flat-top distributions.National Science Foundation (U.S.) (United States. Dept. of Energy Award PHY 0613734)United States. Dept. of Energy (Junior Faculty Award ER54878