skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Multiple-Scale Physics During Magnetic Reconnection

Abstract

Magnetic reconnection is a key fundamental process in magnetized plasmas wherein the global magnetic topology is modified and stored energy is transferred from fields to particles. Reconnection is an inherently local process, and mechanisms to couple global-scale dynamics are not well understood. This dissertation explores two different mechanisms for cross-scale coupling during magnetic reconnection. As one example, we theoretically examine reconnection in a collisionless plasma using particle-in-cell simulations and demonstrate that large scale reconnection physics can couple to and drive microscopic instabilities, even in two-dimensional systems if significant scale separation exists between the Debye length and the electron skin depth. The physics underlying these instabilities is explained using simple theoretical models, and their potential connection to existing discrepancies between laboratory experiments and numerical simulations is explored. In three-dimensional systems, these instabilities are shown to generate anomalous resistivity that balances a substantial fraction of the electric field. In contrast, we also use experiments to investigate cross-scale couplings during reconnection in a collisional plasma. A leading candidate for coupling global and local scales is the hierarchical breakdown of elongated, reconnecting current sheets into numerous smaller current sheets -– the plasmoid instability. In the Magnetic Reconnection Experiment (MRX), recent hardware improvements have extendedmore » the accessible parameter space allowing for the study of long-lived, elongated current sheets. Moreover, by using Argon, reproducible and collisional plasmas are produced, which allow for a detailed statistical study of collisional reconnection. As a result, we have conclusively measured the onset of sub-ion-scale plasmoids during resistive, anti-parallel reconnection for the first time. The current sheet thickness is intermediate between ion and electron kinetic scales such that the plasma is in the Hall-MHD regime. Surprisingly, plasmoids are observed at Lundquist numbers < 100 well below theoretical predictions (> 10,000). The number of plasmoids scales with both Lundquist number and current sheet aspect ratio. The Hall quadrupolar fields are shown to suppress plasmoids. Finally, plasmoids are shown to couple local and global physics by enhancing the reconnection rate. These results are compared with prior studies of tearing and plasmoid instability, and implications for astrophysical plasmas, laboratory experiments, and theoretical studies of reconnection are discussed.« less

Authors:
ORCiD logo [1]
  1. Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)
Publication Date:
Research Org.:
Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Fusion Energy Sciences (FES) (SC-24)
Contributing Org.:
Princeton University
OSTI Identifier:
1409082
DOE Contract Number:
AC02-09CH11466
Resource Type:
Thesis/Dissertation
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; Magnetic Reconnection; Plasmoid Instability

Citation Formats

Jara-Almonte, Jonathan. Multiple-Scale Physics During Magnetic Reconnection. United States: N. p., 2017. Web.
Jara-Almonte, Jonathan. Multiple-Scale Physics During Magnetic Reconnection. United States.
Jara-Almonte, Jonathan. Thu . "Multiple-Scale Physics During Magnetic Reconnection". United States. doi:.
@article{osti_1409082,
title = {Multiple-Scale Physics During Magnetic Reconnection},
author = {Jara-Almonte, Jonathan},
abstractNote = {Magnetic reconnection is a key fundamental process in magnetized plasmas wherein the global magnetic topology is modified and stored energy is transferred from fields to particles. Reconnection is an inherently local process, and mechanisms to couple global-scale dynamics are not well understood. This dissertation explores two different mechanisms for cross-scale coupling during magnetic reconnection. As one example, we theoretically examine reconnection in a collisionless plasma using particle-in-cell simulations and demonstrate that large scale reconnection physics can couple to and drive microscopic instabilities, even in two-dimensional systems if significant scale separation exists between the Debye length and the electron skin depth. The physics underlying these instabilities is explained using simple theoretical models, and their potential connection to existing discrepancies between laboratory experiments and numerical simulations is explored. In three-dimensional systems, these instabilities are shown to generate anomalous resistivity that balances a substantial fraction of the electric field. In contrast, we also use experiments to investigate cross-scale couplings during reconnection in a collisional plasma. A leading candidate for coupling global and local scales is the hierarchical breakdown of elongated, reconnecting current sheets into numerous smaller current sheets -– the plasmoid instability. In the Magnetic Reconnection Experiment (MRX), recent hardware improvements have extended the accessible parameter space allowing for the study of long-lived, elongated current sheets. Moreover, by using Argon, reproducible and collisional plasmas are produced, which allow for a detailed statistical study of collisional reconnection. As a result, we have conclusively measured the onset of sub-ion-scale plasmoids during resistive, anti-parallel reconnection for the first time. The current sheet thickness is intermediate between ion and electron kinetic scales such that the plasma is in the Hall-MHD regime. Surprisingly, plasmoids are observed at Lundquist numbers < 100 well below theoretical predictions (> 10,000). The number of plasmoids scales with both Lundquist number and current sheet aspect ratio. The Hall quadrupolar fields are shown to suppress plasmoids. Finally, plasmoids are shown to couple local and global physics by enhancing the reconnection rate. These results are compared with prior studies of tearing and plasmoid instability, and implications for astrophysical plasmas, laboratory experiments, and theoretical studies of reconnection are discussed.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Thu Jun 01 00:00:00 EDT 2017},
month = {Thu Jun 01 00:00:00 EDT 2017}
}

Thesis/Dissertation:
Other availability
Please see Document Availability for additional information on obtaining the full-text document. Library patrons may search WorldCat to identify libraries that hold this thesis or dissertation.

Save / Share:
  • Energy conversion from magnetic energy to particle energy during magnetic reconnection is studied in the collisionless plasma of the Magnetic Reconnection Experiment (MRX). The plasma is in the two-fluid regime, where the ion motion is decoupled from that of the electron within the so-called ion diffusion region. For ion heating and acceleration, the role of the in-plane (Hall) electric field is emphasized. The in-plane potential responsible for the Hall electric field is established by electrons that are accelerated near the small electron diffusion region. The in-plane electrostatic potential profile shows a well structure along the direction normal to the reconnectionmore » current sheet that becomes deeper and wider downstream as its boundary expands along the separatrices where the in-plane electric field is strongest. Since the Hall electric field is 3--4 times larger than the reconnection electric field, unmagnetized ions obtain energy mostly from the in-plane electric field, especially near the separatrices. The Hall electric field ballistically accelerates ions near the separatrices toward the outflow direction. After ions are accelerated, they are heated as they travel into the high pressure downstream region. This downstream ion heating cannot be explained by classical, unmagnetized transport theory, which suggests that the magnetic field should be important due to an effect called re-magnetization. Electrons are also significantly heated during reconnection. The electron temperature sharply increases across the separatrices and peaks just outside of the electron diffusion region. Unlike ions, electrons acquire energy mostly from the reconnection electric field and the energy gain is localized near the X-point. However, the electron bulk flow energy increase remains negligible. These observations support the assertion that efficient electron heating mechanisms exist around the electron diffusion region and that the generated heat is quickly transported along the magnetic field due to the high parallel thermal conductivity of electrons. Classical Ohmic dissipation based on the perpendicular Spitzer resistivity is too small to compensate the heat flux, indicating the presence of anomalous electron heating. Finally, a total energy inventory is calculated based on analysis of the Poynting, enthalpy, flow energy, and heat flux in the measured diffusion layer. More than half of the incoming magnetic energy is converted to particle energy during collisionless reconnection. Unlike in the Sweet-Parker model, the outgoing Poynting flux is not negligible, which is due to considerable Hall fields, i.e., the quadrupole out-of-plane magnetic field and the in-plane electric field. The total ion energy gain during reconnection is larger than that of electrons, since the energy gain occurs over a broader region. The total ion thermal energy gain is larger than the increase of the ion flow energy. Finally, the electron thermal energy gain is comparable to the ion thermal energy gain, while the electron flow energy remains insignificant.« less
  • The processes associated with reconnecting magnetic field lines have been studied in a large experimental laboratory plasma. Detailed time- and space-resolved probe measurements of the plasma density, temperature, potential and electric and magnetic fields are discussed. Plasma currents are seen to modify the vacuum magnetic field topology. A flat neutral sheet develops along the separatrix where magnetic flux is transferred from regions of private to common flux. Forced tearing and magnetic island formation are also observed. Rapid electron heating, density and temperature nonuniformities and plasma potential gradients are all observed. The pressure is found to peak at the two edgesmore » of the neutral sheet. The dissipation E.J is determined and analyzed in terms of particle heating and fluid acceleration. A consistent, detailed picture of the energy flow via Poynting's theorem is also described. Significant temporal fluctuations in the magnetic fields and electron velocity distribution are measured and seen to give rise to anomalously high values for the plasma resistivity, the ion viscosity and the cross-field thermal conductivity. Electron temperature fluctuations, double layers associated with partial current disruptions, and whistler wave magnetic turbulence have all been identified and studied during the course of the reconnection event.« less
  • Classical models of magnetic reconnection consist of a small diffusion region bounded by two slow shocks, across which the plasma is accelerated. Most space plasma current sheets separate two different plasmas, violating symmetry conditions across the current sheet. One form of asymmetry is a sheared plasma flow. In this thesis, the author investigates the magnetic reconnection process in the presence of a shear flow across the current sheet using two-dimensional magnetohydrodynamic simulations. The results show that only for sheared flow below the average Alfven velocity of the inflow regions can steady state magnetic reconnection occur. A detailed examination of themore » Rankine-Hugoniot jump conditions reveals that the two slow shocks of earlier models are replaced by a strong intermediate shock and a weaker slow shock in the presence of shear flow. Both symmetric and asymmetric density profiles are examined. Depending upon the direction of the flow in the adjacent inflow region, the effects from the sheared flow and the effects from the density asymmetry will compete with each other. The results are applied to the dayside and flank regions of the magnetosphere. For tailward flow in the flanks, the two asymmetries compete making the magnetic field transition layer broad with the high speed flow contained within the transition region. For the dayside region, the magnetic field transition region is thin and the accelerated flow is earthward of the sharp current layer (magnetopause). These results are consistent with the data. A velocity shear in the invariant direction was examined under otherwise symmetric conditions. With the magnetic field initially only in the x - y plane, B(z), and consequently field-aligned current, is generated by the initial v(z). The field-aligned current depends on the velocity profiles in all directions. For a velocity sheared in both the z and the y direction, the results show a very localized region of large field-aligned currents.« less
  • Internal magnetic field probes are used to study the magnetic field-line reconnection during the formation of field-reversed configurations (FRC's) in a low-compression theta-pinch. Measurements of the reversed trapped flux indicated that most of the loss of the initial bias flux occurs during the radial implosion. Therefore, the flux loss is not a consequence of plasma-wall contact during field-reversal. An operating boundary in the parameter space of filling pressure, bias field, and external field is found for formation of FRC's with equilibrium lengths shorter than the coil. Measurements of the internal magnetic fields near the ends of the theta-pinch indicate thatmore » FRC formation can be delayed by plasma flow out the ends. The addition of independently driven magnetic mirrors extends the operating boundary. An axial array of magnetic islands forms during the early stages of the discharge. These islands then coalesce into large units. Several possible explanations of their formation are given. In addition, the growth rate for coalescence obtained from MHD simulations is compared with the experimental results. With the addition of a third magnetic mirror near the midplane of the device the formation of a two-cell FRC is observed.« less
  • Magnetic reconnection and tearing can play an important role in fusion experiments and in space plasma. This thesis is devoted to the magnetohydrodynamic (MHD) study of the linear and evolution of the resistive tearing mode instability in the presence of equilibrium shear flow, and the reconnection of an x-point magnetic field configuration. Numerical solutions of the linearized time-dependent MHD equations and growth rate scaling are obtained. The results of computations are compared to previous work, and the computed growth rate scalings agree with analytical predictions. The introduction of viscosity and small equilibrium shear flow alters the growth rate scaling considerably.more » When the shear flow is large, the growth rate behaves in a more complex way, and Kelvin-Helmholtz instability effects are present. The linear evolution of the double tearing mode with equilibrium shear flow and viscosity is investigated numerically. The dispersion relation for the growth rate of the double tearing instability is generalized to include flow. Relatively small shear flow at the resonant surfaces has a stabilizing effect on the double tearing mode. For Reynolds number comparable or larger than the magnetic Reynolds number a stabilizing effect is found. The nonlinear evolution of the tearing mode instability with equilibrium shear flow is investigated via numerical solutions of the resistive incompressible 2-D MHD equations. The simulations are initiated with solutions of the linearized MHD equations. Magnetic energy release decreases, and the saturation time increases with shear flow. The validity of the numerical solutions is tested by verifying that the total energy and helicity are conserved.« less