Magnetic Reconnection Experiment Research Articles

Global axisymmetric simulations of two-fluid reconnection in an experimentally relevant geometry

To address the interplay between local and global effects in magnetic reconnection, axisymmetric numerical simulations for the Magnetic Reconnection Experiment [M. Yamada et al., Phys. Plasmas 4, 1936 (1997)] are performed using the NIMROD code [C. R. Sovinec et al., J. Comput. Phys. 195, 355 (2004)]. The “pull” and “push” modes of the device are simulated both with and without two-fluid effects in the generalized Ohm’s law. As in experiment, the pull reconnection rate is slowed due to the presence of downstream pressure associated with the outflow. Effects induced by toroidicity include a radially inward drift of the current sheet during pull reconnection and a radially outward displacement of the X-point during push reconnection. These effects result from the inboard side of the current sheet having less volume than the outboard side, facilitating the formation of large scale pressure gradients since the inboard side is more susceptible to a buildup or depletion of density. Toroidicity also leads to asymmetry of the quadrupole field during two-fluid simulations. During pull reconnection, the outboard lobes of the quadrupole typically peak close to the X-point, whereas the inboard quadrupole lobes peak near the flux core surfaces. At experimentally relevant parameters, the reconnection rate is found to depend more on the mode of operation than on the inclusion of two-fluid effects. The current sheet in two-fluid co-helicity simulations tilts due to a Lorentz force associated with the guide field and the outflowing electrons, resulting in asymmetric flow patterns for both ions and electrons. In two-fluid counter-helicity simulations, the Hall effect leads to a radial shift in position of the X-point and an asymmetric outflow pattern, which is examined in terms of separate force-density contributions. In general, asymmetry due to toroidicity or the Hall effect often leads to uneven outflow, which then feeds back on the reconnection process through large scale pressure gradients.

Inductive Sustainment of a Field-Reversed Configuration Stabilized by Shaping, Magnetic Diffusion, and Finite-Larmor-Radius Effects

Oblate field-reversed configuration (FRC) plasmas are sustained for up to 350 micros, or approximately 15 poloidal flux-confinement times, in the magnetic reconnection experiment. The diamagnetic equilibrium is maintained in argon plasmas as a balance of an inward pinch and outward diffusion. Numerical and analytic models show that the observed stability is provided by a combination of plasma shaping, magnetic diffusion, and finite-Larmor radius effects. FRCs formed with lighter ions, which benefit less from these stabilizing effects, succumb to rapid instability and cannot be sustained.

Effects of global boundary and local collisionality on magnetic reconnection in a laboratory plasma

The magnetic reconnection process is studied in a wide range of operating conditions in the well-controlled Magnetic Reconnection Experiment. The reconnection rate is observed to be a function of both global (i.e., system size) and local (collisionality) plasma parameters. When only local collisionality is lowered, the current sheet is shortened while effective resistivity is enhanced, both accelerating reconnection rates. At a fixed collisionality, the current sheet length increases with system size, resulting in the reduction of the reconnection rate. These results quantitatively agree with a generalized Sweet-Parker analysis.

Progress in understanding magnetic reconnection in laboratory and space astrophysical plasmas

This paper reviews the progress in understanding the fundamental physics of magnetic reconnection, focusing on significant results in the past decade from dedicated laboratory experiments, numerical simulations, and space astrophysical observations. Particularly in the area of local reconnection physics, many important findings have been made with respect to two-fluid dynamics, the profile of the neutral sheet, the effects of guide field, and scaling laws with respect to collisionality. Notable findings have been made on global reconnection dynamics through detailed documentation of magnetic self-organization phenomena in fusion plasmas as well as in solar flares. After a brief review of the well-known early work, we will discuss representative recent experimental and theoretical work and attempt to interpret the essence of significant modern findings. Especially, the recent data on local reconnection physics from the Magnetic Reconnection Experiment device [M. Yamada , Phys. Plasmas13, 052119 (2006)] are used to compare experimental and numerical results.

A Self-Organized Plasma with Induction, Reconnection, and Injection Techniques: the SPIRIT Concept for Field Reversed Configuration Research

A comprehensive research concept, known as SPIRIT, is described for the investigation of the formation, stability, and sustainment of oblate field reversed configurations (FRCs). This concept, whose name stands for Self-organized Plasma with Induction, Reconnection, and Injection Techniques (SPIRIT), allows for the study of FRC stability properties on time scales much longer than the energy confinement time. Counter-helicity merging of inductively formed spheromaks is utilized to form large-flux FRCs. These FRCs are sustained by neutral beam injection with the initial aid of compact ohmic solenoids. Stability to n = 1 tilt/shift modes is provided by plasma shaping and conducting shells. Stability to n ≥ 2 co-interchange modes is achieved by a distribution of highenergy non-thermal ions provided by the neutral beam. The combination of plasma shaping, conducting shells, current sustainment, and the non-thermal beam component are expected to lead to a configuration with stability to all global MHD modes, a regime recently discovered through hybrid-MHD simulation using the HYM code. An experimental test of the concept, utilizing the existing Magnetic Reconnection Experiment (MRX) facility, is described. Initial experiments in MRX have confirmed the viability of the SPIRIT concept, and calculations indicate that the confinement of high-energy ions in MRX should be sufficient to test the SPIRIT concept. c

Pile-up Type Magnetic Reconnection Experiment by Compression of Current Sheet

Transient effect of fast magnetic reconnection was investivated in TS-4 torus plasma merging device. Reconnection speed of two merging tokamaks under low inflow and strong guiding(toroidal) field B⊥ agrees with the theoretical value for the Sweet-Parker model. However, when current-sheet was compressed by two pairs of acceleration coils, the reconnection speed became three times as much as the value for the Sweet-Parker model under constant resistivity of the sheet. The inflow flux was found to exceed the outflow flux, indicating density piled-up in current sheet. The measured inflow speed is consistent with a new model where the flux pile-up effect is inclued in convensional Sweet-Parker model.

Recent Advances in the SPIRIT (Self-organized Plasma with Induction, Reconnection, and Injection Techniques) Concept

Since its inception at the 1997 Innovative Confinement Concept meeting, the Self-organized Plasma with Induction, Reconnection, and Injection Techniques (SPIRIT) concept has been continuously advanced both theoretically and experimentally. The main features of this concept are: (1) formation of large-flux Field Reversed Configuration (FRC) plasmas by merging two spheromaks with opposite helicities; (2) flexibility to assess FRC stability by varying the plasma shape and kinetic parameter, by using passive stabilizers, and by injecting energetic ions; (3) sustainment of the FRC for a time significantly longer than the energy confinement time using an ohmic transformer and/or neutral beam injection. Experiments carried out in TS-3/4 and SSX and more recently in Magnetic Reconnection Experiment (MRX) have further verified the effectiveness of this formation scheme for large-flux FRCs. An improved understanding of FRC stability over plasma shape and kinetic parameter has been obtained in MRX. New numerical simulations showed that FRC plasmas can be globally stabilized by injecting energetic ions. Many of these aspects of the SPIRIT concept can be further studied in the current MRX device.

Equilibrium and stability studies of oblate field-reversed configurations in the Magnetic Reconnection Experiment

The equilibrium and stability of oblate field-reversed configurations (FRCs) have been studied in the Magnetic Reconnection Experiment [M. Yamada et al., Phys. Plasmas 4, 1936 (1997)]. In the absence of a passive stabilization, tilt and shift instabilities often become unstable, with the tilt in particular limiting the plasma lifetime. The tilt instability can be mitigated by either including a passive stabilizing conductor, or by forming very oblate plasmas. Large perturbations (n=2 and 3) may still remain after passive stabilization is applied. These perturbations have the characteristics of co-interchange modes, which have never been observed, and can lead to the early termination of the plasma. The co-interchange modes can be minimized through the formation of plasmas with a very oblate shape, leading to the maximum FRC lifetime. A code has been developed to calculate equilibria for these plasmas. A rigid-body model explains the improved stability of oblate plasmas to n=1 tilt modes. Numerical calculations indicate improved stability to n⩾2 co-interchange modes for the very oblate plasma shapes.

Local measurements of plasma ion dynamics with optical probes

Two insertable optical probes have been constructed to measure local ion temperature and flow velocity using the idea proposed by Fiksel et al. [Rev. Sci. Instrum. 69, 2024 (1998)]. The light from plasma is collected by an optical fiber bundle and transported to a high resolution spectrometer. Spatial resolution of a few centimeters is achieved by using a collimator and a view dump. One ion dynamics spectroscopy (IDS) probe is employed in the edge plasma of Madison Symmetric Torus reversed field pinch in combination with the high throughput (f∕4.5) and high resolution (0.15nm∕mm) IDS-II spectrometer. It has provided local ion temperature measurements of carbon and helium impurities with temporal resolution of 10μs and accuracy of about 5eV. The second instrument is used on the Magnetic Reconnection eXperiment, where the local temperature of helium ions (∼10eV) has been measured with 1eV accuracy. Details of the designs, calibrations, and data analysis are described.

Coupling between Global Geometry and the Local Hall Effect Leading to Reconnection-Layer Symmetry Breaking

The coupling between the global reconnection geometry and the local microphysics, caused by the Hall effect, is studied during counterhelicity plasma merging in the magnetic reconnection experiment. The structure of the reconnection layer is significantly modified by reversing the sign of the toroidal fields, which affects the manifestation of the Hall effect in the collisionless regime. The local two-fluids physics changes the global boundary conditions, and this combination effect consequently provides different reconnection rates, magnetic field structure, and plasma flow patterns for two different counterhelicity merging cases in the collisionless regime.

Physical origin of the quadrupole out-of-plane magnetic field in Hall-magnetohydrodynamic reconnection

A quadrupole pattern of the out-of-plane component of the magnetic field inside a reconnection region is seen as an important signature of the Hall-magnetohydrodynamic regime of reconnection. It has been first observed in numerical simulations and just recently confirmed in the Magnetic Reconnection Experiment [Y. Ren, M. Yamada, S. Gerhardt, H. Ji, R. Kulsrud, and A. Kuritsin, Phys. Rev. Lett. 95, 055003 (2005)] and also seen in spacecraft observations of Earth’s magnetosphere. In this study, the physical origin of the quadrupole field is analyzed and traced to a current of electrons that flows along the lines in and out of the inner reconnection region to maintain charge neutrality. The role of the quadrupole magnetic field in the overall dynamics of the reconnection process is discussed. In addition, the bipolar poloidal electric field is estimated and its effect on ion motions is emphasized.

Experimental study of two-fluid effects on magnetic reconnection in a laboratory plasma with variable collisionality

This article describes the recent findings on two-fluid effects on magnetic reconnection in plasmas with variable collisionality in the magnetic reconnection experiment (MRX) [M. Yamada et al., Phys. Plasmas 4, 1936 (1997)]. The MRX device has been upgraded to accommodate a variety of reconnection operation modes and high energy density experiments by increasing its capacitor bank energy and extending the discharge duration. As our experimental operation regime has moved from the collisional to the collision-free, two-fluid effects have become more evident. It is observed that the two-dimensional profile of the neutral sheet is changed significantly from the rectangular shape of the familiar Sweet-Parker type to a double wedge shape as the collisionality is reduced and the reconnection rate increases. The recent evolution of our experimental research from the magnetohydrodynamics (MHD) to the two-fluid analysis is presented to illuminate the physics of Hall MHD in a collision-free reconnection layer. In particular, a clear experimental verification of an out-of-plane quadrupole field, a characteristic signature of the Hall MHD, has been made in the MRX neutral sheet, where the sheet width is comparable to the ion skin depth. It is important to note that the Hall effect, which occurs due to two-dimensional laminar flows of electrons in the reconnection plane, is observed together with the presence of low and high frequency magnetic turbulence, which often has three-dimensional structures. These observations in MRX have striking similarities to the recent magnetospheric measurements of reconnection region, in which the quadrupole component has been detected together with magnetic fluctuations.

Magnetic merging in the MRX

Electromagnetic fluctuations within the heart of a controlled magnetic reconnection experiment could provide an explanation for the unusual rates observed, and provide another piece in the puzzle of how magnetic fields couple to plasmas.

Experimental Verification of the Hall Effect during Magnetic Reconnection in a Laboratory Plasma

In this Letter we report a clear and unambiguous observation of the out-of-plane quadrupole magnetic field suggested by numerical simulations in the reconnecting current sheet in the magnetic reconnection experiment. Measurements show that the Hall effect is large in the collision-less regime and becomes small as the collisionality increases, indicating that the Hall effect plays an important role in collision-less reconnection.

An electromagnetic drift instability in the magnetic reconnection experiment and its importance for magnetic reconnection

The role which resistivity plays in breaking magnetic field lines, heating the plasma, and plasma-field slippage during magnetic reconnection is discussed. Magnetic fluctuations are observed in the MRX (magnetic reconnection experiment) [M. Yamada, H. Ji, S. Hsu, T. Carter, R. Kulsrud, N. Bertz, F. Jobes, Y. Ono, and F. Perkins, Phys. Plasmas 4, 1936 (1997)] that are believed to provide resistive friction or wave resistivity. A localized linear theory has been proposed for their origin as an obliquely propagating lower hybrid drift instability. In this paper, the linear theory of the instability is summarized, and the resulting heating and slippage are calculated from quasilinear theory. Making use of measured amplitudes of the magnetic fluctuations in the MRX, the amount of these effects is estimated. Within the experimental uncertainties they are shown to be quite important for the magnetic reconnection process.

Magnetic Reconnection, Turbulence, and Collisionless Shock

A short summary of recent progress in measuring and understanding turbulence during magnetic reconnection in laboratory plasmas is given. Magnetic reconnection is considered as a primary process to dissipate magnetic energy in laboratory and astrophysical plasmas. A central question concerns why the observed reconnection rates are much faster than predictions made by classical theories, such as the Sweet-Parker model based on MHD with classical Spitzer resistivity. Often, the local resistivity is conjectured to be enhanced by turbulence to accelerate reconnection rates either in the context of the Sweet-Parker model or by facilitating setup of the Pestchek model. Measurements at a dedicated laboratory experiment, called MRX or Magnetic Reconnection Experiment, have indicated existence of strong electromagnetic turbulence in current sheets undergoing fast reconnection. The origin of the turbulence has been identified as right-hand polarized whistler waves, propagating obliquely to the reconnecting field, with a phase velocity comparable to the relative drift velocity. These waves are consistent with an obliquely propagating electromagnetic lower-hybrid drift instability driven by drift speeds large compared to the Alfven speed in high-beta plasmas. Interestingly, this instability may explain electromagnetic turbulence also observed in collisionless shocks, which are common in energetic astrophysical phenomena.

Kinetic simulation of the VTF magnetic reconnection experiment

A new numerical code is developed to assist the interpretation of experimental results from the Versatile Toroidal Facility (VTF) magnetic reconnection experiment. As input the code applies profiles of magnetic and electric fields consistent with experimental measurements. Applying Liouville's theorem the code calculates the electron distributions functions. The moments of the distribution function provide the profiles of the electron current density, which are in agreement with the experimental observations. Thus the code provides new insight into the origin of the fast reconnection rates observed in VTF.

Comparative experiments of co‐ and counter‐helicity merging in the compact toroid merging device TS‐4

AbstractFor the past 15 years, the TS‐3 merging project has explored several new types of CT/ST (Compact Tori and Spherical Tokamak): merging formation of FRCs (Field Reversed Configuration), magnetic reconnection experiment, and ultrahigh beta ST formation. The successful results of these experiments have enabled us to construct an upscaled merging device, namely, the TS‐4. The new TS‐4 has two flux cores (major radius = 0.5 m) for inductive plasma formation, a center coil (external toroidal field coil and OH coil), for q value control and plasma sustaining, and DC external field coils. The initial operation of the TS‐4 (since 2001) demonstrated co‐helicity and counter‐helicity merging of two spheromaks. The counter‐helicity merging revealed annihilation of toroidal magnetic field with a significant overshoot motion of reconnected field lines. It resulted in the formation of a high‐beta (≈ 0.8) FRC. The produced FRC has a significant field deformation, suggesting a contribution of the toroidal plasma flow to its equilibrium. The co‐helicity merging doubled the toroidal fluxes of spheromak, causing its flux conversion from toroidal to poloidal. The merging spheromaks relaxed into another spheromak with beta ≈ 0, suggesting the large thermal energy loss due to its non‐aximetric dynamo activity. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 145(4): 1–11, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10207

Magnetohydrodynamic modeling of two-dimensional reconnection in the Magnetic Reconnection Experiment

A two-dimensional magnetohydrodynamics (MHD) code is used to investigate the dynamical evolution of driven reconnection in the Magnetic Reconnection Experiment (MRX) [M. Yamada et al., Phys. Plasmas 7, 1781 (2000)]. The initial conditions and dimensionless parameters of the simulation are set to be similar to the experimental values. Many features of the time-evolution of magnetic configurations for both co- and counter-helicity reconnection in MRX are successfully reproduced in the framework of resistive MHD. The resistive MHD model is then augmented by the addition of a “model Hall” term to begin to assess the importance of two-fluid physics in the experiment. The effective decoupling of the ion fluid from the reconnecting magnetic field due to the model Hall term is shown to be important during the early dynamic X-phase of MRX reconnection, while effectively negligible during the late “steady-state” Y-phase, when plasma heating takes place. These results are consistent with the available experimental evidence. Based on simple symmetry considerations, an experiment to directly measure the Hall effect in MRX configuration is proposed and numerical evidence for the expected outcome is given.

Three-dimensional electron magnetohydrodynamic reconnection. III. Energy conversion and electron heating

Further observations are presented of a magnetic reconnection experiment with three-dimensional fields in the parameter regime of electron magnetohydrodynamics. The initial magnetic configuration is imposed via a Helmholtz coil, whose field is added to or subtracted from a uniform background magnetic field. Energy is transferred from the coil’s external power supply into thermal energy of electrons and kinetic energy of ions via the decay of the imposed magnetic field configuration. For the case when the Helmholtz coil field opposes the background field, thus creating a field-reversed configuration, the magnetic energy convects in the whistler mode and dissipates over large distances resulting in negligible heating. For the case when the Helmholtz coil field is added to the background field, magnetic field annihilation leads to strong localized electron heating and acceleration of unmagnetized ions via space-charge electric fields. The energy conversion to electron heat is observed in regions away from magnetic nulls and a new mechanism for breaking the frozen-in condition is proposed.