Electromagnetic fluctuations during fast reconnection in a laboratory plasma

Anomalous Resistivity and Electron Heating by Lower Hybrid Drift Waves during Magnetic Reconnection with a Guide Field

The lower hybrid drift wave (LHDW) has been a candidate for anomalous resistivity and electron heating inside the electron diffusion region of magnetic reconnection. In a laboratory reconnection layer with a finite guide field, quasielectrostatic LHDW (ES-LHDW) propagating along the direction nearly perpendicular to the local magnetic field is excited in the electron diffusion region. ES-LHDW generates large density fluctuations (δn_{e}, about 25% of the mean density) that are correlated with fluctuations in the out-of-plane electric field (δE_{Y}, about twice larger than the mean reconnection electric field). With a small phase difference (∼30°) between two fluctuating quantities, the anomalous resistivity associated with the observed ES-LHDW is twice larger than the classical resistivity and accounts for 20% of the mean reconnection electric field. After we verify the linear relationship between δn_{e} and δE_{Y}, anomalous electron heating by LHDW is estimated by a quasilinear analysis. The estimated electron heating is about 2.6±0.3  MW/m^{3}, which exceeds the classical Ohmic heating of about 2.0±0.2  MW/m^{3}. This LHDW-driven heating is consistent with the observed trend of higher electron temperatures when the wave amplitude is larger. Presented results provide the first direct estimate of anomalous resistivity and electron heating power by LHDW, which demonstrates the importance of wave-particle interactions in magnetic reconnection.

Laboratory Study of Collisionless Magnetic Reconnection

A concise review is given on the past two decades' results from laboratory experiments on collisionless magnetic reconnection in direct relation with space measurements, especially by the Magnetospheric Multiscale (MMS) mission. Highlights include spatial structures of electromagnetic fields in ion and electron diffusion regions as a function of upstream symmetry and guide field strength, energy conversion and partitioning from magnetic field to ions and electrons including particle acceleration, electrostatic and electromagnetic kinetic plasma waves with various wavelengths, and plasmoid-mediated multiscale reconnection. Combined with the progress in theoretical, numerical, and observational studies, the physics foundation of fast reconnection in collisionless plasmas has been largely established, at least within the parameter ranges and spatial scales that were studied. Immediate and long-term future opportunities based on multiscale experiments and space missions supported by exascale computation are discussed, including dissipation by kinetic plasma waves, particle heating and acceleration, and multiscale physics across fluid and kinetic scales.

MHD instability dynamics and turbulence enhancement towards the plasma disruption at the HL-2A tokamak

The evolutions of MHD instability behaviors and enhancement of both electrostatic and electromagnetic turbulence towards the plasma disruption have been clearly observed in the HL-2A plasmas. Two types of plasma disruptive discharges have been investigated for similar equilibrium parameters: one with a distinct stage of a small central temperature collapse ([Formula: see text] 5-10%) around 1 millisecond before the thermal quench (TQ), while the other without. For both types, the TQ phase is preceded by a rotating 2/1 tearing mode, and it is the development of the cold bubble from the inner region of the 2/1 island O-point along with its inward convection that causes the massive energy loss. In addition, the micro-scale turbulence, including magnetic fluctuations and density fluctuations, increases before the small collapse, and more significantly towards the TQ. Also, temperature fluctuations measured by electron cyclotron emission imaging enhances dramatically at the reconnection site and expand into the island when approaching the small collapse and TQ, and the expansion is more significant close to the TQ. The observed turbulence enhancement near the X-point cannot be fully interpreted by the linear stability analysis by GENE. Evidences suggest that nonlinear effects, such as the reduction of local [Formula: see text] shear and turbulence spreading, may play an important role in governing turbulence enhancement and expansion. These results imply that the turbulence and its interaction with the island facilitate the stochasticity of the magnetic flux and formation of the cold bubble, and hence, the plasma disruption.

Direct observations of pure electron outflow in magnetic reconnection

Magnetic reconnection is a universal process in space, astrophysical, and laboratory plasmas. It alters magnetic field topology and results in energy release to the plasma. Here we report the experimental results of a pure electron outflow in magnetic reconnection, which is not accompanied with ion flows. By controlling an applied magnetic field in a laser produced plasma, we have constructed an experiment that magnetizes the electrons but not the ions. This allows us to isolate the electron dynamics from the ions. Collective Thomson scattering measurements reveal the electron Alfvénic outflow without ion outflow. The resultant plasmoid and whistler waves are observed with the magnetic induction probe measurements. We observe the unique features of electron-scale magnetic reconnection simultaneously in laser produced plasmas, including global structures, local plasma parameters, magnetic field, and waves.

Structure of a Perturbed Magnetic Reconnection Electron Diffusion Region in the Earth's Magnetotail

We report in situ observations of an electron diffusion region (EDR) and adjacent separatrix region in the Earth's magnetotail. We observe significant magnetic field oscillations near the lower hybrid frequency which propagate perpendicularly to the reconnection plane. We also find that the strong electron-scale gradients close to the EDR exhibit significant oscillations at a similar frequency. Such oscillations are not expected for a crossing of a steady 2D EDR, and can be explained by a complex motion of the reconnection plane induced by current sheet kinking propagating in the out-of-reconnection-plane direction. Thus, all three spatial dimensions have to be taken into account to explain the observed perturbed EDR crossing. These results shed light on the interplay between magnetic reconnection and current sheet drift instabilities in electron-scale current sheets and highlight the need for adopting a 3D description of the EDR, going beyond the two-dimensional and steady-state conception of reconnection.

Probes to measure kinetic and magnetic phenomena in plasmas

Diagnostic tools are of fundamental importance in experimental research. In plasma physics, probes are usually used to obtain the plasma parameters, such as density, temperature, electromagnetic fields, and waves. This Review focuses on low-temperature plasma diagnostics where in situ probes can be used. Examples of in situ and remote diagnostics will be shown, proven by many experimental verifications. This Review starts with Langmuir probes and then continues with other diagnostics such as waves, beams, and particle collectors, which can provide high accuracy. A basic energy analyzer has been advanced to measure distribution functions with three-dimensional velocity resolution, three directions in real space and time resolution. The measurement of the seven-dimensional distribution function is the basis for understanding kinetic phenomena in plasma physics. Non-Maxwellian distributions have been measured in magnetic reconnection experiments, scattering of beams, wakes of ion beams, etc. The next advance deals with the diagnostics of electromagnetic effects. It requires magnetic probes that simultaneously resolve three field components, measured in three spatial directions and with time resolution. Such multi-variable data unambiguously yield field topologies and related derivatives. Examples will be shown for low frequency whistler modes, which are force-free vortices, flux ropes, and helical phase rotations. Thus, with advanced probes, large data acquisition and fast processing further advance in the fields of kinetic plasma physics and electromagnetic phenomena can be expected. The transition from probes to antennas will also be stimulated. Basic research with new tools will also lead to new applications.

Probe measurements of electric field and electron density fluctuations at megahertz frequencies using in-shaft miniature circuits

A four-tip electrostatic probe is constructed to measure high-frequency (0.1-10 MHz) fluctuations in both the electric field (one component) and electron density in a laboratory plasma. This probe also provides data for the local electron temperature and density. Circuits for high-frequency measurements are fabricated on two miniature boards, which are embedded in the probe shaft, near the tips to minimize the pickup of common-mode signals. The amplitude and phase response of two circuits to sinusoidal test signals are measured and compared with results from modeling. For both circuits, the phase shift between input and output signals is relatively small (<30°). The performance of the probe is verified in a high-density (∼10¹³ cm^-3) and low-temperature (≲10 eV) plasma. The probe successfully measures high-frequency (∼2 MHz) fluctuations in the electric field and density, which are associated with lower hybrid drift waves. This probe can provide information on the wave-associated anomalous drag, which can be compared with the classical resistivity.

Effects of plasma turbulence on the nonlinear evolution of magnetic island in tokamak

Magnetic islands (MIs), resulting from a magnetic field reconnection, are ubiquitous structures in magnetized plasmas. In tokamak plasmas, recent researches suggested that the interaction between an MI and ambient turbulence can be important for the nonlinear MI evolution, but a lack of detailed experimental observations and analyses has prevented further understanding. Here, we provide comprehensive observations such as turbulence spreading into an MI and turbulence enhancement at the reconnection site, elucidating intricate effects of plasma turbulence on the nonlinear MI evolution.

Magnetic reconnection and plasma turbulence occur in atmospheric and magnetized laboratory plasmas. Here the authors report evolution of magnetic islands and plasma turbulence in tokamak plasmas using high resolution 2D electron cyclotron emission diagnostics.

Lower-Hybrid Drift Waves Driving Electron Nongyrotropic Heating and Vortical Flows in a Magnetic Reconnection Layer

We report measurements of lower-hybrid drift waves driving electron heating and vortical flows in an electron-scale reconnection layer under a guide field. Electrons accelerated by the electrostatic potential of the waves exhibit perpendicular and nongyrotropic heating. The vortical flows generate magnetic field perturbations comparable to the guide field magnitude. The measurements reveal a new regime of electron-wave interaction and how this interaction modifies the electron dynamics in the reconnection layer.

Anomalous Heating and Plasmoid Formation in a Driven Magnetic Reconnection Experiment

We present a detailed study of magnetic reconnection in a quasi-two-dimensional pulsed-power driven laboratory experiment. Oppositely directed magnetic fields (B=3  T), advected by supersonic, sub-Alfvénic carbon plasma flows (V_{in}=50  km/s), are brought together and mutually annihilate inside a thin current layer (δ=0.6  mm). Temporally and spatially resolved optical diagnostics, including interferometry, Faraday rotation imaging, and Thomson scattering, allow us to determine the structure and dynamics of this layer, the nature of the inflows and outflows, and the detailed energy partition during the reconnection process. We measure high electron and ion temperatures (T_{e}=100  eV, T_{i}=600  eV), far in excess of what can be attributed to classical (Spitzer) resistive and viscous dissipation. We observe the repeated formation and ejection of plasmoids, consistent with the predictions from semicollisional plasmoid theory.

Perspectives on magnetic reconnection

Magnetic reconnection is a topological rearrangement of magnetic field that occurs on time scales much faster than the global magnetic diffusion time. Since the field lines break on microscopic scales but energy is stored and the field is driven on macroscopic scales, reconnection is an inherently multi-scale process that often involves both magnetohydrodynamic (MHD) and kinetic phenomena. In this article, we begin with the MHD point of view and then describe the dynamics and energetics of reconnection using a two-fluid formulation. We also focus on the respective roles of global and local processes and how they are coupled. We conclude that the triggers for reconnection are mostly global, that the key energy conversion and dissipation processes are either local or global, and that the presence of a continuum of scales coupled from microscopic to macroscopic may be the most likely path to fast reconnection.

Design and construction of Keda Space Plasma Experiment (KSPEX) for the investigation of the boundary layer processes of ionospheric depletions

In this work, the design and construction of the Keda Space Plasma EXperiment (KSPEX), which aims to study the boundary layer processes of ionospheric depletions, are described in detail. The device is composed of three stainless-steel sections: two source chambers at both ends and an experimental chamber in the center. KSPEX is a steady state experimental device, in which hot filament arrays are used to produce plasmas in the two sources. A Macor-mesh design is adopted to adjust the plasma density and potential difference between the two plasmas, which creates a boundary layer with a controllable electron density gradient and inhomogeneous radial electric field. In addition, attachment chemicals can be released into the plasmas through a tailor-made needle valve which leads to the generation of negative ions plasmas. Ionospheric depletions can be modeled and simulated using KSPEX, and many micro-physical processes of the formation and evolution of an ionospheric depletion can be experimentally studied.

Laboratory study of magnetic reconnection with a density asymmetry across the current sheet

The effects of a density asymmetry across the current sheet on anti-parallel magnetic reconnection are studied systematically in a laboratory plasma. Despite a significant density ratio of up to 10, the in-plane magnetic field profile is not significantly changed. On the other hand, the out-of-plane Hall magnetic field profile is considerably modified; it is almost bipolar in structure with the density asymmetry, as compared to quadrupolar in structure with the symmetric configuration. Moreover, the ion stagnation point is shifted to the low-density side, and the electrostatic potential profile also becomes asymmetric with a deeper potential well on the low-density side. Nonclassical bulk electron heating together with electromagnetic fluctuations in the lower hybrid frequency range is observed near the low-density-side separatrix. The dependence of the ion outflow and reconnection electric field on the density asymmetry is measured and compared with theoretical expectations. The measured ion outflow speeds are about 40% of the theoretical values.

Observation of ion acceleration and heating during collisionless magnetic reconnection in a laboratory plasma

The ion dynamics in a collisionless magnetic reconnection layer are studied in a laboratory plasma. The measured in-plane plasma potential profile, which is established by electrons accelerated around the electron diffusion region, shows a saddle-shaped structure that is wider and deeper towards the outflow direction. This potential structure ballistically accelerates ions near the separatrices toward the outflow direction. Ions are heated as they travel into the high-pressure downstream region.

Evolution of an electron current layer prior to reconnection onset

Electron current layers (ECLs) are the sites where magnetic reconnection initiates in a current sheet. Using three-dimensional particle-in-cell simulations, we study the plasma processes that occur in an ECL as it evolves rapidly over a short time scale much shorter than the ion cyclotron period. The processes include its thinning, generation of electrostatic instabilities, trapping and heating of electrons in growing waves, its rebroadening, generation of anomalous resistivity, and eventually the generation of large-amplitude magnetic fluctuations. These fluctuations could be interpreted in terms of electron tearing and/or Weibel instabilities, which are commonly invoked as mechanisms for the magnetic reconnection onset. The widths of the broadened ECL are compared with those measured in the magnetic reconnection experiment, showing excellent agreement.

Plasmoid ejection and secondary current sheet generation from magnetic reconnection in laser-plasma interaction

Reconnection of the self-generated magnetic fields in laser-plasma interaction was first investigated experimentally by Nilson et al. [Phys. Rev. Lett. 97, 255001 (2006)] by shining two laser pulses a distance apart on a solid target layer. An elongated current sheet (CS) was observed in the plasma between the two laser spots. In order to more closely model magnetotail reconnection, here two side-by-side thin target layers, instead of a single one, are used. It is found that at one end of the elongated CS a fanlike electron outflow region including three well-collimated electron jets appears. The (>1 MeV) tail of the jet energy distribution exhibits a power-law scaling. The enhanced electron acceleration is attributed to the intense inductive electric field in the narrow electron dominated reconnection region, as well as additional acceleration as they are trapped inside the rapidly moving plasmoid formed in and ejected from the CS. The ejection also induces a secondary CS.

Influence of the lower-hybrid drift instability on magnetic reconnection in asymmetric configurations

Using fully kinetic 3D simulations of magnetic reconnection in asymmetric antiparallel configurations, we demonstrate that an electromagnetic lower-hybrid drift instability (LHDI) localized near the X line can substantially modify the reconnection mechanism in the regimes with large asymmetry, a moderate ratio of electron to ion temperature, and low plasma β. However, the mode saturates at a small amplitude in the regimes typical of Earth's magnetopause. In these cases, LHDI-driven turbulence is predominantly localized along the separatrices on the low-β side of the current sheet, in agreement with spacecraft observations.

Whistler mode based explanation for the fast reconnection rate measured in the mit versatile toroidal facility

Despite the widely discussed role of whistler waves in mediating magnetic reconnection (MR), the direct connection between such waves and the MR has not been demonstrated by comparing the characteristic temporal and spatial features of the waves and the MR process. Using the whistler wave dispersion relation, we theoretically predict the experimentally measured rise time (τ(rise)) of a few microseconds for the fast rising MR rate in the Versatile Toroidal Facility at MIT. The rise time is closely given by the inverse of the frequency bandwidth of the whistler waves generated in the evolving current sheet. The wave frequencies lie much above the ion cyclotron frequency, but they are limited to less than 0.1% of the electron cyclotron frequency in the argon plasma. The maximum normalized MR rate R=0.35 measured experimentally is precisely predicted by the angular dispersion of the whistler waves.

A current filamentation mechanism for breaking magnetic field lines during reconnection

During magnetic reconnection, the field lines must break and reconnect to release the energy that drives solar and stellar flares and other explosive events in space and in the laboratory. Exactly how this happens has been unclear, because dissipation is needed to break magnetic field lines and classical collisions are typically weak. Ion-electron drag arising from turbulence, dubbed 'anomalous resistivity', and thermal momentum transport are two mechanisms that have been widely invoked. Measurements of enhanced turbulence near reconnection sites in space and in the laboratory support the anomalous resistivity idea but there has been no demonstration from measurements that this turbulence produces the necessary enhanced drag. Here we report computer simulations that show that neither of the two previously favoured mechanisms controls how magnetic field lines reconnect in the plasmas of greatest interest, those in which the magnetic field dominates the energy budget. Rather, we find that when the current layers that form during magnetic reconnection become too intense, they disintegrate and spread into a complex web of filaments that causes the rate of reconnection to increase abruptly. This filamentary web can be explored in the laboratory or in space with satellites that can measure the resulting electromagnetic turbulence.

Observations of turbulence generated by magnetic reconnection

Spacecraft observations of turbulence within a magnetic reconnection (guide field approximately 0) ion diffusion region are presented. In the inertial subrange, electric and magnetic fluctuations both followed a -5/3 power law; at higher frequencies, the spectral indices were -1 and -8/3, respectively. The dispersion relation was found to be consistent with fast-mode-whistler waves rather than kinetic Alfvén-ion cyclotron waves. Lower hybrid waves, which could be enhanced by whistler mode conversion, were observed, but the associated anomalous resistivity was not found to significantly modify the reconnection rate.

Laboratory observation of electron phase-space holes during magnetic reconnection

We report the observation of large-amplitude, nonlinear electrostatic structures, identified as electron phase-space holes, during magnetic reconnection experiments on the Versatile Toroidal Facility at MIT. The holes are positive electric potential spikes, observed on high-bandwidth ( approximately 2 GHz) Langmuir probes. Investigations with multiple probes establish that the holes travel at or above the electron thermal speed and have a three-dimensional, approximately spherical shape, with a scale size approximately 2 mm. This corresponds to a few electron gyroradii, or many tens of Debye lengths, which is large compared to holes considered in simulations and observed by satellites, whose length scale is typically only a few Debye lengths. Finally, a statistical study over many discharges confirms that the holes appear in conjunction with the large inductive electric fields and the creation of energetic electrons associated with the magnetic energy release.

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.

Catastrophe model for fast magnetic reconnection onset

A catastrophe model for the onset of fast magnetic reconnection is presented that suggests why plasma systems with magnetic free energy remain apparently stable for long times and then suddenly release their energy. For a given set of plasma parameters there are generally two stable reconnection solutions: a slow (Sweet-Parker) solution and a fast (Alfvénic) Hall reconnection solution. Below a critical resistivity the slow solution disappears and fast reconnection dominates. Scaling arguments predicting the two solutions and the critical resistivity are confirmed with two-fluid simulations.

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.

Exact energy principle in magnetic reconnection for current-sheet models

On the basis of an exact nonlinear energy principle, it is shown that the change in magnetic topology (i.e., reconnection) in a finite-domain system leads to the conversion of magnetic field energy to particle energy. However, it is also shown that the conversion efficiency gradually disappears as the system size increases. This principle is demonstrated with model current-sheet equilibria including Harris and Fadeev solutions, as well as a current-sheet equilibrium which contains a singular current layer. The finding that energy conversion in reconnection is highly dependent on the system size may have an important implication for numerical simulations performed under finite geometry.

Nonlinear evolution of the lower-hybrid drift instability in a current sheet

The lower-hybrid drift instability is simulated in an ion-scale current sheet using a fully kinetic approach with values of the ion to electron mass ratio up to m(i)/m(e)=1836. Although the instability is localized on the edge of the layer, the nonlinear development increases the electron flow velocity in the central region resulting in a strong bifurcation of the current density and significant anisotropic heating of the electrons. This dramatically enhances the collisionless tearing mode and may lead to the rapid onset of magnetic reconnection for current sheets near the critical scale.