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Plasmoid

A plasmoid is a coherent, compact structure consisting of intertwined with , often characterized by its self-contained magnetic topology that confines the against expansion. These structures typically form through processes like , where oppositely directed in a break and reform, leading to the ejection of plasmoids as magnetic islands or flux ropes laden with and energetic particles. In astrophysical contexts, plasmoids play a crucial role in explosive energy release events, such as solar flares and coronal mass ejections, by facilitating rapid transport of and across vast distances in space environments like planetary magnetospheres and relativistic jets from active galactic nuclei. experiments have successfully generated plasmoids using techniques like theta-pinches or rotating , demonstrating their potential applications in advanced systems, such as plasmoid thrusters that achieve high specific impulses through inductive . Additionally, plasmoids have been proposed as a model for rare atmospheric phenomena like ball lightning, where they manifest as luminous, long-lived balls during thunderstorms, though this remains a subject of ongoing research.

Definition and History

Definition

A plasmoid is a coherent, self-contained structure consisting of plasma and embedded , typically exhibiting toroidal or elongated geometries. The term was coined in 1956 by Winston H. Bostick to describe a "plasma-magnetic entity" observed in experiments with high-velocity jets interacting with . Unlike diffuse plasmas, which lack defined boundaries and dissipate readily, plasmoids maintain structural integrity due to the frozen-in that confine and stabilize the plasma against external disruptions. These structures are often formed through processes like , where oppositely directed break and reform, releasing plasmoids. Key laboratory examples of plasmoids include compact toroids, field-reversed configurations, spheromaks, and variants produced in devices. In natural settings, plasmoids manifest as magnetospheric bubbles in planetary magnetotails, discrete objects within cometary tails, structures in the , and features along the .

Historical Development

The concept of the plasmoid originated in laboratory plasma physics during the mid-1950s, when Winston H. Bostick introduced the term to describe coherent, self-contained structures of ionized matter observed in experiments involving magnetic fields. In his seminal 1956 paper published in Physical Review, Bostick reported observations from a plasma gun that projected ionized matter across a transverse magnetic field, revealing elongated plasma cylinders with embedded magnetic moments that behaved as discrete entities rather than diffuse clouds. Building on these initial findings, Bostick conducted further experiments between 1957 and 1958 at the Radiation Laboratory, utilizing plasma guns to generate and photograph plasmoids in a controlled . These studies demonstrated that plasmoids could maintain structural integrity while traversing , exhibiting measurable magnetic moments and curved trajectories influenced by Lorentz forces, as captured through and magnetic probe measurements. Bostick also briefly referenced potential applications of these plasmoid behaviors to astrophysical phenomena, such as galaxy formation, by analogy to interacting structures. During the 1950s and 1960s, plasmoid research intersected with early efforts in controlled thermonuclear fusion, where plasma confinement challenges drove investigations into coherent plasma structures amid the broader declassification of fusion-related plasma physics in 1958. This period saw influences from Hannes Alfvén's foundational work on magnetohydrodynamics and plasma cosmology, which emphasized the role of magnetic fields in shaping cosmic plasma dynamics and inspired laboratory simulations of such processes. By the 1970s and 1980s, satellite observations began confirming plasmoid-like structures in space plasmas, with Edward W. Hones, Jr., identifying them in Earth's magnetotail using data from the IMP-8 spacecraft, revealing bipolar magnetic signatures associated with substorms. Voyager missions in the late 1970s and 1980s further extended these identifications to outer planetary magnetospheres, detecting plasma blobs with embedded fields during flybys of Jupiter and Saturn.

Physical Characteristics

Structure and Properties

Plasmoids consist of ionized embedded with that are frozen into the fluid due to the high electrical of the , allowing the lines to move with the plasma particles. The structural coherence of plasmoids is sustained by Lorentz forces, where the of and (\mathbf{j} \times \mathbf{B}) provides the necessary confinement against expansion or dispersion. In equilibrium, plasmoids typically exhibit cylindrical or toroidal shapes, often elongated along the direction of the embedded lines, forming coherent, self-contained configurations such as flux ropes or compact toroids. These structures possess measurable attributes, including sizes ranging from micrometers in -scale reconnection events to thousands of kilometers in magnetotail plasmoids, translational velocities approaching Alfvén speeds (up to several hundred km/s in observations), transverse (on the order of mV/m), and magnetic moments that indicate internal field strengths sufficient for self-confinement. In astrophysical contexts, such as coronal plasmoids, typical densities are on the order of $10^9–$10^{10} cm^{-3} and temperatures around 1–8 , while laboratory plasmoids may have densities of $10^{14}–$10^{16} cm^{-3} and temperatures of 2000–5000 K. The equilibrium conditions of plasmoids are governed by pressure balance, where the internal total pressure—comprising the gas pressure of the plasma (p) and the magnetic pressure (B^2 / 2\mu_0)—equals the external confining pressure (p_{\rm ext}): p + \frac{B^2}{2\mu_0} = p_{\rm ext} This balance maintains the plasmoid's radius; in a field-free vacuum, the absence of external pressure leads to expansion or dissipation of the structure.

Stability and Dynamics

Plasmoids exhibit a range of interaction behaviors when encountering or other plasmoids in settings. Under the influence of a transverse , plasmoids follow curved orbital paths due to the on their charged components, with the force given by \mathbf{F} = q (\mathbf{v} \times \mathbf{B}), where q is charge, \mathbf{v} is , and \mathbf{B} is the . In low-pressure gases, such as at approximately $10^{-3} mm , plasmoids can spiral inward and come to a stop as collisions with gas molecules dissipate their kinetic energy. Upon collision with another plasmoid, they often fragment into smaller structures, as observed in early projection experiments across . These interactions highlight the dynamic responsiveness of plasmoids to external forces and densities. Dissipation of plasmoids occurs through several mechanisms, particularly in unconfined environments. In vacuum, plasmoids expand rapidly due to internal electromagnetic stresses, leading to quick dissipation unless stabilized by resonant conditions or surface currents. Merging with other plasmoids can also lead to dissipation, where smaller entities coalesce into a larger one, often accompanied by fragmentation in the intervening current sheets driven by tearing-mode instabilities. Additionally, plasmoids may be absorbed into larger magnetic structures, where their energy and fields integrate into broader configurations during processes like outflows. In observations, plasmoid lifetimes typically range from seconds to minutes, influenced by confinement and environmental conditions. For instance, in controlled discharges, plasmoids persist for tens of seconds before significant motion or decay becomes evident, with stability enhanced by magnetic trapping or low collision rates. These durations allow for detailed study of their evolution, though they shorten in high-vacuum or high-energy setups.

Formation Mechanisms

In Magnetic Reconnection

Plasmoids form during as a result of the plasmoid instability, a secondary tearing mode that arises in thin current sheets. This instability occurs when the current sheet elongates sufficiently, leading to rapid growth of magnetic islands that evolve into chains of plasmoids along the reconnection layer. In the classical Sweet-Parker model of reconnection, the current sheet has an aspect ratio determined by the balance of inflow and outflow, resulting in a slow reconnection rate that scales inversely with the of the Lundquist number S = \frac{L V_A}{\eta}, where L is the system length scale, V_A is the Alfvén speed, and \eta is the magnetic diffusivity. The normalized reconnection rate in resistive (MHD) is thus approximately \frac{V_{\rm in}}{V_A} \sim S^{-1/2}. This scaling predicts impractically slow reconnection for high-S plasmas typical in astrophysical environments, where S > 10^4. In contrast, the Petschek model proposes a faster, localized reconnection mechanism involving standing slow-mode shocks, achieving rates around 0.1 independent of S, but it requires specific nonuniform resistivity profiles that are not always realized. The plasmoid instability resolves this discrepancy by destabilizing the Sweet-Parker sheet above a critical Lundquist number S_c \approx 10^4, triggering multiple tearing modes that fragment the sheet into a chain of plasmoids. This process accelerates reconnection, yielding rates weakly dependent on S (around 0.01–0.1) in high-S regimes through hierarchical plasmoid formation and coalescence. Numerical evidence from 2D resistive MHD simulations of Harris current sheets confirms the rapid onset of plasmoid chains, with the number of plasmoids scaling as S^{3/8} and the instability growth rate as S^{1/4}. Extending to 3D MHD models, simulations of Harris-like configurations reveal similar chain formation, though with additional obliquity effects that can enhance or suppress plasmoid alignment depending on guide-field strength.

Laboratory Generation

Plasmoids in laboratory settings are generated using specialized devices that accelerate and confine ionized gas within to form coherent, self-contained structures. Key devices include coaxial plasma guns, (DPF) machines, theta-pinch systems, and magnetized plasma guns. These apparatuses operate primarily in chambers to minimize interactions with ambient air, enabling the formation of plasmoids with embedded . Coaxial plasma guns, pioneered by Bostick in the , employ a central surrounded by an outer , where a high-voltage ionizes gas and propels along the via Lorentz forces. This process creates elongated plasmoids that resemble twisted tubes, with magnetic moments measurable up to several gauss-centimeters. Dense focus devices initiate plasmoids through electromagnetic compression: an initial current sheath forms a parabolic structure that accelerates along electrodes, ballooning at the end into a dense plasmoid via pinch dynamics and instabilities. Theta-pinch machines use azimuthal magnetic fields from a single-turn coil to inductively heat and compress , forming (FRC) plasmoids that can be accelerated to velocities exceeding 100 km/s. Magnetized guns, often variants, inject pre-magnetized to produce compact toroids with embedded poloidal and fields, suitable for applications like refueling. The generation process typically involves injecting into a controlled environment, where electromagnetic forces shape it into coherent structures with closed field lines. For instance, in magnetized guns, is accelerated by J × B forces, forming plasmoids with velocities on the order of tens to hundreds of km/s. Recent experiments from 2022 to 2024 have advanced this by using algorithms to high-velocity plasmoids ejected from coaxial guns, revealing propagation speeds up to 50 km/s and confirming their morphology through frame-by-frame analysis. These studies often trigger formation via magnetic reconnection-like processes but focus on empirical rather than . Diagnostics for laboratory plasmoids rely on non-invasive techniques to probe structure and dynamics without disrupting the plasma. Visible imaging with high-speed cameras captures evolution and trajectory, while magnetic probes (e.g., B-dot coils) measure field strengths on the order of 0.1–1 T and infer current distributions. Velocity measurements, achieved via time-of-flight methods or Doppler , routinely report speeds from 5 km/s in early setups to over 300 km/s in optimized theta-pinch accelerators. These tools enable tracking of plasmoid integrity during propagation. A primary challenge in plasmoid generation is maintaining outside chambers, where atmospheric interactions cause rapid and fragmentation of the coherent within milliseconds. Even in controlled environments, instabilities like tilting or limit propagation distances to meters, necessitating ongoing refinements in field configuration and injection parameters.

Natural Occurrences

In Astrophysics

In astrophysics, plasmoids are coherent plasma structures often formed through processes in cosmic environments, playing a key role in release and particle across , heliospheric, and galactic scales. These structures manifest as magnetic islands or flux ropes that detach from current sheets, contributing to dynamic phenomena like ejections and in magnetized s. Observations from and ground-based telescopes have revealed plasmoids in various astrophysical contexts, linking them to observable emissions and large-scale plasma dynamics. Solar occurrences of plasmoids are prominently observed in coronal mass ejections (CMEs) and flares, where they emerge from elongated sheets during reconnection events. In CMEs, plasmoids form as merging magnetic islands that accelerate outward, driving the ejection of and into the . For instance, high-resolution imaging from missions like Hinode has captured plasmoid ejections in flares, where fast reconnection in the sheet produces bidirectional outflows accompanied by plasmoid formation. These plasmoids typically exhibit speeds of hundreds of km/s and sizes on the order of 10^4–10^5 km, facilitating rapid energy transport from the to interplanetary . In the (HCS), plasmoids appear as flux-rope-like or magnetic-island structures embedded in the sheet, resulting from reconnection in the warped HCS as it propagates outward from . Spacecraft such as and have detected these plasmoids as low-beta (<0.5) structures with lengths up to 0.1 , often qualifying as slow-mode interplanetary CMEs. Recent simulations and observations indicate that tearing instabilities in the HCS lead to plasmoid chains, enhancing and particle in the . Additionally, 2025 studies of flares from the Sgr A* have shown plasmoid-dominated current sheets in the , where reconnection ejects plasmoids producing variable near-infrared and emissions, supporting models of plasmoid-mediated variability in galactic centers. Magnetospheric examples include flux ropes and plasma bubbles in Earth's magnetotail, where plasmoids form during substorms via reconnection at the near-Earth neutral line. The mission has provided direct evidence of three-dimensional flux ropes flanked by X-lines, with plasmoids observed as tailward-moving structures at speeds of 200–600 km/s, often associated with auroral brightenings. These magnetotail plasmoids, with diameters of ~1–10 radii, detach and propagate downtail, releasing stored and accelerating particles. In cometary and interstellar environments, plasmoid-like structures arise from solar wind interactions with cometary plasma tails, forming discrete blobs or knots through reconnection in the draped magnetic field. Rosetta observations of comet 67P/Churyumov-Gerasimenko revealed plasma structures resembling magnetic islands in the tail, influenced by solar wind compression and turbulence. In interstellar contexts, similar dynamics occur in solar wind-cosmic ray interactions, producing plasmoid chains that modulate particle propagation. Observational for plasmoid ejections includes and radio emissions, which trace nonthermal electrons accelerated during reconnection. Soft plasmoids in solar flares, imaged by Yohkoh and Hinode, correlate with metric/decimetric radio bursts from gyrosynchrotron emission in the ejecting structures. In galactic settings like Sgr A*, flares from plasmoid ejections exhibit variability on timescales of minutes, linked to synchrotron and inverse-Compton processes in the plasmoid chains.

Terrestrial Phenomena

Plasmoids have been hypothesized to explain , a rare atmospheric phenomenon characterized by luminous, spherical orbs that persist for seconds to minutes and exhibit erratic motion during thunderstorms. These orbs are often reported to float, pass through solid objects, and dissipate with explosions or silently, based on numerous eyewitness accounts spanning centuries. Laboratory experiments have generated plasmoid-like structures mimicking these properties, supporting the plasma-magnetic entity model for . In , researchers produced stable atmospheric-pressure plasmoids using solutions, observing lifetimes of hundreds of milliseconds and diameters up to 0.2 meters, with structures featuring a hot core surrounded by a cooler shell that enhances stability and mobility—traits aligning with observations. These plasmoids formed via electrochemical discharges at the air-water interface, dissipating energies on the order of 1-10 joules while remaining luminous and coherent. Another study demonstrated plasmoid generation from underwater electrical discharges propagating into air, creating buoyant, long-lived formations at that replicate the glowing, spherical appearance and brief persistence of . Such lab simulations, lasting from milliseconds to seconds, provide evidence for plasmoids as localized plasma-magnetic entities arising from lightning-induced discharges. Plasmoid-like structures have also been proposed in other terrestrial events, such as upper-atmospheric transient luminous events including sprites and blue jets, which occur above thunderstorms and involve high-speed ejections from cloud tops. These phenomena manifest as red, jellyfish-shaped sprites reaching altitudes of 50-90 km or conical blue jets extending to 40 km, potentially driven by similar electromagnetic processes that form coherent plasma blobs. lights, luminous phenomena preceding or accompanying seismic activity, may similarly involve plasmoids generated by geological stresses, such as piezoelectric effects in quartz-rich rocks or / degassing along faults, producing fireballs or glowing auras lasting 20-100 seconds. Observations in , including photographed lights during the 1965-1967 Matsushiro (magnitudes up to 5.4), and in the , where plasmoids up to 500 m in size move at speeds of 1500 km/h, link these to tectonic and fault ruptures creating ionized gas entities.

Applications and Research

In Space Propulsion

Plasmoid thrusters represent an innovative approach to electric for space missions, leveraging compact, self-confined structures to generate . In a 2001 NASA study, researchers proposed a plasmoid thruster concept that produces (FRC)-like plasmoids using a theta-pinch and accelerates them electromagnetically for ejection, aiming to achieve high values in the range of 5,000 to 15,000 seconds. This design draws on laboratory generation techniques, such as inductive formation, to create stable plasmoids without electrodes, thereby minimizing and enabling the use of diverse propellants like or . The primary mechanism involves magnetic acceleration of plasmoids, where induced azimuthal currents in the plasma interact with external magnetic fields to produce Lorentz forces that propel the structures at high velocities. The self-confined magnetic fields within plasmoids reduce beam divergence, enhancing propulsion efficiency to potentially 50-80% by maintaining plasma coherence during exhaust. In prototypes, such as the Plasmoid Thruster Experiment (PTX), initial tests demonstrated ejection velocities of 5.3 km/s, with theoretical models and similar experiments indicating capabilities exceeding 50 km/s. Coaxial plasma gun-based systems have been explored as scalable prototypes for , utilizing magnetized configurations to form and accelerate FRC plasmoids through helicity injection and . For instance, the Magnetically Accelerated (MAP) thruster employs rotating magnetic fields for electrodeless formation and sequenced coils for , achieving ejection velocities of at least 180 km/s and specific impulses of 6,000 to 8,000 seconds in optimized setups. An Alfvenic reconnecting plasmoid thruster variant, based on electrodes with static fields, generates plasmoids via current-sheet instabilities and reports simulated exhaust velocities from 20 to 500 km/s, controllable by coil currents. Compared to traditional ion thrusters, plasmoid systems offer higher thrust density, on the order of 10^5 N/m², enabling greater power scaling to megawatt levels while maintaining high specific impulse for deep-space applications. This advantage stems from the plasmoids' ability to deliver pulsed, high-density exhaust without the low-thrust limitations of gridded ion accelerators, potentially supporting missions requiring both efficiency and substantial acceleration.

In Fusion and Plasma Physics

In , plasmoids play a central role through configurations such as spheromaks and field-reversed configurations (FRCs), which are compact plasmas characterized by closed lines and high values (the ratio of to , often exceeding 90%). Spheromaks feature linked and poloidal fields generated by internal currents, enabling as stable plasmoids without central solenoids, while FRCs lack field components entirely, relying on poloidal fields for confinement. These plasmoid-based systems offer advantages in simplicity and efficiency for reactors, as their high reduces the need for strong external magnets and supports compact designs. Experiments utilizing compact toroid (CT) injectors have demonstrated the potential of plasmoids for refueling and current drive in , where hypervelocity plasmoids are injected transversely to penetrate the plasma core without disruption. Merging of injected plasmoids with the target plasma enhances toroidal current, with studies showing up to 30% increases in tokamak current upon CT injection, supporting non-inductive sustainment. Repetitive plasmoid injection and merging processes are particularly effective for current drive, as they efficiently transfer and while minimizing losses. Plasmoids also provide critical insights into magnetic reconnection dynamics relevant to fusion instabilities, where thin current sheets in high-beta plasmas undergo tearing instabilities, forming chains of magnetic islands that accelerate reconnection rates. Laboratory experiments have observed plasmoid formation in laser-driven reconnection layers with electron temperatures around 2 keV and densities of approximately 3 × 10^{19} cm^{-3}, revealing super-Alfvénic inflows and flux pileup that mirror processes in edge-localized modes and disruptions. These studies advance understanding of instability mitigation in fusion devices by quantifying how plasmoids contribute to turbulent energy release. In dedicated spheromak experiments like the Sustained Spheromak Physics Experiment (SSPX), plasmoid confinement has achieved temperatures exceeding 200 (peaking at ~250 ) and densities on the order of 10^{19} m^{-3}, with low (χ_e ~ 10-20 m²/s) indicating viable confinement when magnetic fluctuations are minimized. These metrics highlight the plasmoid's potential for sustained high-temperature operation, essential for scaling to fusion-relevant conditions.

Recent Advances

In 2022, laboratory experiments at the Shenguang-II laser facility demonstrated plasmoid-dominated in a hybrid collisional-collisionless regime, where interactions between laser-produced plasmas and pre-embedded magnetic fields generated multiple plasmoids along the current sheet, achieving reconnection rates up to 0.1-0.2 times the Alfvén speed. This observation bridged theoretical predictions with empirical evidence, showing how plasmoids enhance reconnection efficiency in partially ionized plasmas relevant to astrophysical environments. Advancements in computational tools emerged in with the development of a watershed-based for automated plasmoid identification in two-dimensional simulations, incorporating contouring techniques to detect coherent structures in current sheets. Applied to Harris sheet models and general relativistic magnetohydrodynamic (GRMHD) simulations of accretion, this method provided updated statistics on plasmoid chains, revealing hierarchies with dozens of secondary plasmoids per primary sheet and enabling reconnection rates exceeding 0.3 in relativistic regimes. These statistics addressed previous gaps in quantifying plasmoid-mediated acceleration, facilitating faster reconnection models for astrophysical jets and flares. In 2025, simulations explored spin-polarized condensed plasmoids during radiation reaction-dominated reconnection, predicting their formation under extreme magnetic fields of approximately $10^{10} G in high-power laser facilities like the . These plasmoids exhibited high spin polarization (up to 50%) due to quantum radiation reaction, potentially observable through polarized gamma-ray emissions. Concurrent 2025 modeling of Sagittarius A* flares incorporated plasmoid current sheets in GRMHD simulations, linking episodic reconnection events to observed and variability with luminosities around $10^{35} erg/s. The analysis identified plasmoid ejections at current sheet boundaries as drivers of nonthermal particle acceleration, supporting flare durations of 10-30 minutes and spectral indices consistent with .