Magnetic Reconnection Experiment Research Articles

Design of Three-Dimensional Magnetic Probe System for Space Plasma Environment Research Facility (SPERF).

A three-dimensional magnetic probe system has been designed and implemented at the Space Plasma Environment Research Facility (SPERF). This system has been developed to measure the magnetic field with high spatial and temporal resolution, enabling studies of fundamental processes in space physics, such as magnetic reconnection at the Earth's magnetopause, on the basis of SPERF. The system utilizes inductive components as sensors, arranged in an array and soldered onto a printed circuit board (PCB), achieving a spatial resolution of 2.5 mm. The system's electrical parameters have been measured, and its amplitude-frequency response characteristics have been simulated. The system has demonstrated good performance with response capabilities below 50 kHz. The experimental setup and results are discussed, highlighting the system's effectiveness in accurately measuring weak magnetic signals and its suitability for magnetic reconnection experiments.

Radiatively cooled magnetic reconnection experiments driven by pulsed power

We present evidence for strong radiative cooling in a pulsed-power-driven magnetic reconnection experiment. Two aluminum exploding wire arrays, driven by a 20 MA peak current, 300 ns rise time pulse from the Z machine (Sandia National Laboratories), generate strongly driven plasma flows (MA≈7) with anti-parallel magnetic fields, which form a reconnection layer (SL≈120) at the mid-plane. The net cooling rate far exceeds the Alfvénic transit rate (τcool−1/τA−1≫1), leading to strong cooling of the reconnection layer. We determine the advected magnetic field and flow velocity using inductive probes positioned in the inflow to the layer, and inflow ion density and temperature from analysis of visible emission spectroscopy. A sharp decrease in x-ray emission from the reconnection layer, measured using filtered diodes and time-gated x-ray imaging, provides evidence for strong cooling of the reconnection layer after its initial formation. X-ray images also show localized hotspots, regions of strong x-ray emission, with velocities comparable to the expected outflow velocity from the reconnection layer. These hotspots are consistent with plasmoids observed in 3D radiative resistive magnetohydrodynamic simulations of the experiment. X-ray spectroscopy further indicates that the hotspots have a temperature (170 eV) much higher than the bulk layer (≤75 eV) and inflow temperatures (about 2 eV) and that these hotspots generate the majority of the high-energy (>1 keV) emission.

Plasmoid Formation and Strong Radiative Cooling in a Driven Magnetic Reconnection Experiment.

We present the first experimental study of plasmoid formation in a magnetic reconnection layer undergoing rapid radiative cooling, a regime relevant to extreme astrophysical plasmas. Two exploding aluminum wire arrays, driven by the Z machine, generate a reconnection layer (S_{L}≈120) in which the cooling rate far exceeds the hydrodynamic transit rate (τ_{hydro}/τ_{cool}>100). The reconnection layer generates a transient burst of >1 keV x-ray emission, consistent with the formation and subsequent rapid cooling of the layer. Time-gated x-ray images show fast-moving (up to 50 km s^{-1}) hotspots in the layer, consistent with the presence of plasmoids in 3D resistive magnetohydrodynamic simulations. X-ray spectroscopy shows that these hotspots generate the majority of Al K-shell emission (around 1.6keV) prior to the onset of cooling, and exhibit temperatures (170eV) much greater than that of the plasma inflows and the rest of the reconnection layer, thus providing insight into the generation of high-energy radiation in radiatively cooled reconnection events.

Local dynamic responses of magnetic reconnection to three-dimensional perturbations in a laboratory plasma

The dynamic responses of magnetic reconnection to localized three-dimensional (3D) magnetic field perturbations imposed by a pair of figure-8-shaped coils are investigated in the Magnetic Reconnection Experiment (MRX) device. Both the magnetic field geometry and current sheet profiles are altered by external perturbations. For the case when the inductive electric field associated with these perturbations aligns with the preexisting reconnection electric field, O-type magnetic structures appear within an elongated current sheet. When these magnetic structures are ejected downstream at the speed close to the ion outflow velocity, the inductive electric field is enhanced considerably. Despite that the imposed perturbation amplitude is larger than 30% of the original reconnecting magnetic field, the overall reconnection process remains robust without current sheet disruptions. This technique to form O-type magnetic structures can serve as an additional experimental knob for future systematic laboratory investigations of 3D magnetic reconnection and related instabilities without disrupting two-dimensional current sheet.

Design and development of a single-laser 2D Thomson scattering measurement system for TS-6 merging tokamak/magnetic reconnection experiments

Nd:YAG Thomson scattering diagnostics is reliable measurement of electron temperature and density, but its 2D extension conventionally needs as many polychromators as measuring points and as many lasers as measuring paths. To reduce the numbers and costs, we designed and developed a single-laser 2D measurement system by use of multiple reflection and time-of-flight of laser light. A single laser pulse reflects between mirrors on both sides of the vacuum vessel and makes seven round trips, which enables measurement at m × 7 points with only m polychromators and a single laser. Here, we present the design to satisfy the resolution required for TS-6 merging tokamak and magnetic reconnection experiments and the system development, especially the time-separation of scattered light signals from different points with time-of-flight delays. The measured Thomson scattering signals from 7 × 2 points and the measured Raman scattered signals from points in four laser paths indicate the feasibility of Thomson scattering measurement at m × 4 points.

Double-sided electron energy analyzer for measurement of non-Maxwellian electron energy distributions.

A double-sided electron energy analyzer is developed for studies of magnetic reconnection. It can measure electron energy distribution functions along two directions opposite to each other at the same time. Each side is composed of a floating reference grid, an energy selector grid, and a collector plate. The voltage of the selector grid is swept from -40 to 0V with respect to the reference grid with a frequency of 1MHz. This fast sweeping is required to resolve sub-Alfvénic changes in plasma quantities of the Magnetic Reconnection Experiment, where the typical Alfvénic time is a few microseconds. The reliability of the energy analyzer is checked in Maxwellian plasmas away from the reconnection region. In this case, the electron temperature computed from the electron energy distribution function agrees with measurements of a reference triple Langmuir probe. When it is located near the reconnection region, the temperatures of the tail electron population from both sides, facing into the electron flow and facing away from it, exceed the bulk electron temperature measured by the Langmuir probe by a factor of about 2.

Measurement of ion temperature and toroidal flow during magnetic reconnection with a large guide field

Ion temperature and toroidal flow along the guide field direction are measured using a new ion tomographic diagnostic on the Magnetic Reconnection eXperiment (MRX) during magnetic reconnection with a guide field strength of about 1.4 and 2.1 times the strength of the reconnecting component. Strong toroidal flows, beyond what has been measured in anti-parallel and lower guide field conditions on MRX, are observed. Sustained ion heating with no discernible structure within the measurement region is also observed. Probe measurements including Langmuir and Mach probe measurements are made to support the tomographic inversion of line-integrated measurements, as well as to provide local measurements of plasma parameters. Measurements of toroidal velocity and ion temperature are supported with time series data. Energy flow into and out of the X-line region is estimated using a guiding center framework and presented in the Appendix of this manuscript, suggesting an outsized role played by parallel electric field in energizing ions. The guiding center approximation is not well satisfied in the region of interest; however, the estimates provide a springboard for future, further experimentation.

A nuclear-reaction-based method for probing the nonthermal ion energy spectrum in high energy density laboratory plasmas

The nuclear reactions in a plasma system with particle distributions deviated from the Maxwellian are proved to have some unique characteristics, in particular, in their reaction product energy spectra. Based on this, a new nuclear-reaction-based method for probing the nonthermal ion energy spectrum in high-energy-density (HED) laboratory plasmas is proposed, where the energy spectrum of the nonthermal ion high-energy tails can be accurately evaluated through analysis from the spread and peak of the product energy spectrum. The principle of this diagnostic method is theoretically derived and verified by two-dimensional particle-in-cell simulations that self-consistently includes the nuclear reaction calculations. As an example, our simulations demonstrate clearly how this method is applied for probing the nonthermal high-energy protons produced in the HED magnetic reconnection experiment, where a small ratio of boron element is dopped in the laser-ablated hydrocarbon target and the proton-boron (pB) reaction is chosen as the referenced nuclear reaction. The simulations also show that the pB reaction rate is increased by four orders of magnitude and the peak of the energy spectrum of the generated alpha particles shift significantly towards the high-energy range due to the nonthermal protons accelerated from the reconnections.

Control of Transient Static Electric Field in the Magnetic Reconnection Experiment with Large Guide Field

Control of Transient Static Electric Field in the Magnetic Reconnection Experiment with Large Guide Field

3D simulation of lower-hybrid drift waves in strong guide field asymmetric reconnection in laboratory experiments

The magnetic reconnection experiment has recently seen short wavelength ( k ρ e ∼ 1) lower-hybrid waves near the electron diffusion region in strong guide field reconnection. Based on plasma parameters from the experiment, we perform a three-dimensional fully kinetic simulation in order to investigate the generation of the lower-hybrid waves and their effects on the reconnection process. We find that the low-beta regions around the reconnection site are unstable to the lower-hybrid drift instability propagating in the outflow direction, driven by the difference between the electron and ion outflows. The waves modify the electron distributions, leading to periodic opening and closing of gaps in electron velocity space, and provide a small contribution to the anomalous resistivity. Finally, the simulation results are discussed in the context of space observations and laboratory experiments.

Laboratory study of the failed torus mechanism in arched, line-tied, magnetic flux ropes

Coronal mass ejections (CMEs) are some of the most energetic and violent events in our solar system. The prediction and understanding of CMEs are of particular importance due to the impact that they can have on Earth-based satellite systems and, in extreme cases, ground-based electronics. CMEs often occur when long-lived magnetic flux ropes (MFRs) anchored to the solar surface destabilize and erupt away from the Sun. One potential cause for these eruptions is an ideal magnetohydrodynamic (MHD) instability, such as the kink or torus instability. Previous experiments on the magnetic reconnection experiment revealed a class of MFRs that were torus-unstable but kink-stable, which failed to erupt. These “failed-tori” went through a process similar to Taylor relaxation, where the toroidal current was redistributed before the eruption ultimately failed. We have investigated this behavior through additional diagnostics that measure the current distribution at the foot points and the energy distribution before and after an event. These measurements indicate that ideal MHD effects are sufficient to explain the energy distribution changes during failed torus events. This excludes Taylor relaxation as a possible mechanism of current redistribution during an event. A new model that only requires non-ideal effects in a thin layer above the electrodes is presented to explain the observed phenomena. This work broadens our understanding of the stability of MFRs and the mechanism behind the failed torus through the improved prediction of the torus instability and through new diagnostics to measure the energy inventory and current profile at the foot points.

Push and pull magnetic reconnection driven by intense laser interaction with double-coil capacitor target

Push and pull magnetic reconnection (MR) experiments using high-power laser irradiating a capacitor target with the plates connected by a pair of U-shaped coils are carried out. During the beginning (end) of the laser-target interaction that creates a hot plasma in the region, the rise (fall) stages of the coil currents generates expanding (contracting) magnetic fields that reconnect in the midplane between the coils, resulting in push (pull) MR. Proton radiography and proton ray-tracing simulation are used to track the evolution of the magnetic fields. The proton accumulation and void formation between the coils are related to the oppositely directed current-sheet currents during the push and pull MR stages. The directions of the plasma electron outflows during these two MR phases are obtained by monitoring the soft x-ray emission. Our results suggest that the double-coil capacitor target may be useful for laboratory modeling of fast MR and related phenomena in astrophysical plasmas.

Effects of Coulomb collisions on lower hybrid drift waves inside a laboratory reconnection current sheet

We have developed a local, linear theoretical model for lower hybrid drift waves that can be used for plasmas in the weakly collisional regime. Two cases with typical plasma and field parameters for the current sheet of the magnetic reconnection experiment have been studied. For a case with a low electron beta (βe=0.25, high guide field case), the quasi-electrostatic lower hybrid drift wave is unstable, while the electromagnetic lower hybrid drift wave has a positive growth rate for a high-βe case (βe=8.9, low guide field case). For both cases, including the effects of Coulomb collisions reduces the growth rate but collisional impacts on the dispersion and growth rate are limited (≲20%).

A time-resolved imaging system for the diagnosis of x-ray self-emission in high energy density physics experiments.

A diagnostic capable of recording spatially and temporally resolved x-ray self-emission data was developed to characterize experiments on the MAGPIE pulsed-power generator. The diagnostic used two separate imaging systems: a pinhole imaging system with two-dimensional spatial resolution and a slit imaging system with one-dimensional spatial resolution. The two-dimensional imaging system imaged light onto the image plate. The one-dimensional imaging system imaged light onto the same piece of image plate and a linear array of silicon photodiodes. This designallowed the cross-comparison of different images, allowing a picture of the spatial and temporal distribution of x-ray self-emission to be established. The designwas tested in a series of pulsed-power-driven magnetic-reconnection experiments.

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.

Ion temperature measurements from tomographic reconstruction of Doppler spectra in the presence of multi-component flow in two dimensions.

A new ion Doppler diagnostic has been constructed to measure ion temperature profiles in the presence of multi-component flow during magnetic reconnection experiments. The inversion technique and diagnostic setup are applicable to axisymmetric plasmas with two-component flow across the measurement cross section, which occurs during magnetic reconnection. The particular design discussed here is optimized for operation on the Magnetic Reconnection eXperiment (MRX) at Princeton Plasma Physics Laboratory. To prove the viability of this diagnostic for MRX and the future Facility for Laboratory Reconnection Experiments, measurements have been taken and ion temperature and perpendicular flow profiles have been obtained. The radial velocity on MRX does not contribute to the Doppler shift of the measured spectra but does contribute to the broadening of the spectra, while toroidal flow contributes to both. It is shown that neglecting the radial velocity for vR = 20km/s leads to an error in the ion temperature inversion of 20%. Results from MRX discharges are shown, and the impact of radial velocity on ion temperature inversions is discussed.

An 18.3 MJ charging and discharging pulsed power supply system for the Space Plasma Environment Research Facility (SPERF): The subsystem for the dipole coil

The Space Plasma Environment Research Facility (SPERF) currently under construction at the Harbin Institute of Technology in China is a user facility dedicated to studying space plasma physics on the ground. A coil system of the SPERF consists of seven types of coils, which are used to generate the magnetic fields and plasma required by the physical experiments. A dipole coil of the coil system works with four magnetosheath coils (flux cores) to build the magnetic fields resembling that of the Earth and solar wind. A capacitor-based pulsed power supply (PPS) system with a modular design is developed to excite the dipole coil to generate a magnetosphere-like magnetic field required by the magnetopause magnetic reconnection experiment. The PPS of the dipole coil has a longer pulse duration and more energy than that of other coils in the coil system, it delivers a pulsed current with a peak of more than 18 kA, and the duration of the current is not less than 95% of the peak over 10ms to the dipole coil when the charging voltage is not less than 20 kV. The detailed design of the PPS is discussed in this paper, and the discharge test of the PPS is carried out to verify the design of the PPS.

Potential distribution behind target in intense and short pulsed laser-driven magnetic reconnection

Recently, the short-pulse intense laser has become a common tool for studying the relativistic plasma with tremendous physical parameters. And the laser-driven magnetic reconnection is one of the hot topics and has received much attention. The laser-driven magnetic reconnection experiments are usually conducted by closely focusing two laser beams on a planar coil target. However, it is always hard to distinguish the physical property of magnetic reconnection from the complex background of laser-produced plasma. In this paper, we present the particle-in-cell simulation results of magnetic reconnection driven by two short-pulse lasers as well as a single laser pulse irradiating the solid planar target, and discuss the correlation between the potential distribution behind the target and the magnetic reconnection. When a single laser is used, the potential behind the target shows a double-peak distribution, which is in agreement with recent experimental results. When two lasers irradiate the target, the potential behind the target shows a three-peak distribution. The accumulated spatial distribution of plasma ions with fixed energy (4.5 and 6 MeV) at 3 μm behind the target shows several peaks, which is in agreement with the potential distribution when either a single laser or two lasers are used. In addition, after the laser pulse terminates, in the two-laser case there is extremely strong effect on the topological structure of the electric field compared with in the singlelaser case. When the magnetic reconnection happens (which can be identified through the reconnection electric field and the electron energy spectrum), the amplitude of the <i>x</i> component of the electric field has different evolution characteristics from the single laser case. The line outs of the <i>y</i> component of the electric field in two cases also have completely different shapes. In summary, the simulation results reveal that the potential distribution behind the target can directly affect the spatial distribution of the accelerated ions. This could be possibly used to identify the short pulse laser-driven magnetic reconnection in experiment.

Electron Magnetohydrodynamics Magnetic Reconnection Experiment on Keda Linear Magnetized Plasma Device**Supported by the National Natural Science Foundation of China under Grant Nos 41331067 and 41527804, the Key Research Program of Frontier Sciences of Chinese Academy of Sciences under Grant No QYZDJ-SSW-DQC010, and the Fundamental Research Funds for the Central Universities.

We conduct an electron magnetohydrodynamics magnetic reconnection experiment with guide-field in our Keda linear magnetized plasma device, in which two pulsed currents with the same direction are conducted in parallel with the axial direction of the main chamber of the device using two long aluminum sticks. After approximately , an X-type magnetic field line topology is formed at the center of the chamber. With the formation of the X-type topology of magnetic field lines, we can also find the rapid increase of the current and ratio of the common flux to the private flux in this area. Additionally, a reduction in the plasma density and the plasma density concentration along one pair of separatrices can also be found.

Energy Conversion by Parallel Electric Fields During Guide Field Reconnection in Scaled Laboratory and Space Experiments

AbstractWe present direct and scaled comparisons between laboratory and in situ space observations of magnetic reconnection with a guide field, comparing results from the Magnetospheric Multiscale Mission and the Magnetic Reconnection eXperiment (MRX). While Magnetospheric Multiscale Mission observations obtain high‐resolution and fully kinetic data, MRX observations fully cover the 2‐D reconnection plane near the current sheet, removing uncertainties in situating the measurements compared to the reconnection region. Through scaling transformations, we show a quantitative agreement in magnetic field and current density profiles, which agree within a factor of 2 from each other. The introduction of the guide field causes the energy conversion J·E in the current sheet to be dominated by J||E|| in both cases. However, parallel electric fields reported by recent spacecraft crossings are significantly (5–10 times) larger than values obtained on MRX, highlighting an important issue for understanding energy conversion by reconnection.