Strong Azimuthal Magnetic Field Research Articles

Three-dimensional modeling of a plasma in a strong azimuthal magnetic field.

Three-dimensional magnetohydrodynamic simulations are able to model the generation of disk-shaped plasma, driven by laser ablation from a current-carrying rod in a pulsed-power machine producing azimuthal magnetic fields of 2-3 MG. The plasma at such extreme conditions is unique in that the parameter space for the plasma β and Hall parameter χ transition from below unity to greater than unity at different stages of the plasma generation. In simulations, the formation of the plasma disk in the azimuthal direction is driven by heat flux from the laser spot and depends on the set of transport coefficients used in simulations. The most recent set of transport coefficients leads to the formation of plasma ejecta at the back end of the rod, which qualitatively matches experiments. Specifically, the cross-gradient Nernst effect, which twists the magnetic field, is shown to have a large effect on the shape of the back-end ejecta. In the direction along the axis of the rod, there is propagation of perturbations from the disk as observed in experiments. In simulations, the period of temperature perturbations is in good agreement with experimental results. An instability due to coupling of heat flux and the magnetic field advection provides a possible explanation for perturbation growth along the axis of the rod, and the instability growth rate is consistent with experimental results.

Magnetic pinching of relativistic particle beams: a new approach to strong-field QED physics

Quantum electrodynamics (QED) is a foundation of modern physics, yet access to the strong-field QED regime in the laboratory remains a formidable challenge. Currently, high-power lasers at the multi-petawatt level and above are generally believed to be an important approach to test QED physics. Here, we present a different approach by use of an electron beam self-pinched to near-solid-density. The beam self-pinching is realized while it transports through a properly designed hollow cone target, where strong azimuthal magnetic fields are generated by the beam-induced plasma return currents at the inner surface of the cone target. In this way, the beam diameter can be reduced by more than an order of magnitude down to submicron and its density is increased by hundreds of times. The produced ultradense electron beams can unlock a new regime of QED-dominated beam–plasma interactions, for example, more than 60% of the beam energy can be converted into GeV gamma-rays with unprecedented brilliance when such a beam passes through a thin solid foil. Moreover, with proper parameter design, this beam-focusing scheme can also be applied to positron beams and thus may find applications in broad areas, such as particle colliders and strong-field physics.

Detecting deep axisymmetric toroidal magnetic fields in stars

Context. Asteroseismology has revealed small core-to-surface rotation contrasts in stars in the whole Hertzsprung–Russell diagram. This is the signature of strong transport of angular momentum (AM) in stellar interiors. One of the plausible candidates to efficiently carry AM is magnetic fields with various topologies that could be present in stellar radiative zones. Among them, strong axisymmetric azimuthal (toroidal) magnetic fields have received a lot of interest. Indeed, if they are subject to the so-called Tayler instability, the accompanying triggered Maxwell stresses can transport AM efficiently. In addition, the electromotive force induced by the fluctuations of magnetic and velocity fields could potentially sustain a dynamo action that leads to the regeneration of the initial strong axisymmetric azimuthal magnetic field. Aims. The key question we aim to answer is whether we can detect signatures of these deep strong azimuthal magnetic fields. The only way to answer this question is asteroseismology, and the best laboratories of study are intermediate-mass and massive stars with external radiative envelopes. Most of these are rapid rotators during their main sequence. Therefore, we have to study stellar pulsations propagating in stably stratified, rotating, and potentially strongly magnetised radiative zones, namely magneto-gravito-inertial (MGI) waves. Methods. We generalise the traditional approximation of rotation (TAR) by simultaneously taking general axisymmetric differential rotation and azimuthal magnetic fields into account. Both the Coriolis acceleration and the Lorentz force are therefore treated in a non-perturbative way. Using this new formalism, we derive the asymptotic properties of MGI waves and their period spacings. Results. We find that toroidal magnetic fields induce a shift in the period spacings of gravity (g) and Rossby (r) modes. An equatorial azimuthal magnetic field with an amplitude of the order of 105 G leads to signatures that are detectable in period spacings for high-radial-order g and r modes in γ Doradus (γ Dor) and slowly pulsating B (SPB) stars. More complex hemispheric configurations are more difficult to observe, particularly when they are localised out of the propagation region of MGI modes, which can be localised in an equatorial belt. Conclusions. The magnetic TAR, which takes into account toroidal magnetic fields in a non-perturbative way, is derived. This new formalism allows us to assess the effects of the magnetic field in γ Dor and SPB stars on g and r modes. We find that these effects should be detectable for equatorial fields thanks to modern space photometry using observations from Kepler, TESS CVZ, and PLATO.

Generation of Cold Magnetized Relativistic Plasmas at the Rear of Thin Foils Irradiated by Ultra-High-Intensity Laser Pulses

A scheme to generate magnetized relativistic plasmas in a laboratory setting is proposed. It is based on the interaction of ultra-high-intensity sub-picosecond laser pulses with few-micron-thick foils or films. By means of Particle-In-Cell simulations, it is shown that energetic electrons produced by the laser and evacuated at the rear of the target trigger an expansion of the target, building up a strong azimuthal magnetic field. It is shown that in the expanding plasma sheath, a ratio of the magnetic pressure and the electron rest-mass energy density exceeds unity, whereas the plasma pressure is lower than the magnetic pressure and the electron gyroradius is lower than the plasma dimension. This scheme can be utilized to study astrophysical extreme phenomena such as relativistic magnetic reconnection in laboratory.

Direct laser acceleration of electrons assisted by strong laser-driven azimuthal plasma magnetic fields.

A high-intensity laser beam propagating through a dense plasma drives a strong current that robustly sustains a strong quasistatic azimuthal magnetic field. The laser field efficiently accelerates electrons in such a field that confines the transverse motion and deflects the electrons in the forward direction. Its advantage is a threshold rather than resonant behavior, accelerating electrons to high energies for sufficiently strong laser-driven currents. We study the electron dynamics via a test-electron model, specifically deriving the corresponding critical current density. We confirm the model's predictions by numerical simulations, indicating energy gains two orders of magnitude higher than achievable without the magnetic field.

MHD Instabilities in Accretion Disks and Their Implications in Driving Fast Magnetic Reconnection

Abstract Magnetohydrodynamic instabilities play an important role in accretion disk systems. Besides the well-known effects of magnetorotational instability (MRI), the Parker–Rayleigh–Taylor instability (PRTI) also arises as an important mechanism to help in the formation of the coronal region around an accretion disk and in the production of magnetic reconnection events similar to those occurring in the solar corona. In this work, we have performed three-dimensional magnetohydrodynamical (3D-MHD) shearing-box numerical simulations of accretion disks with an initial stratified density distribution and a strong azimuthal magnetic field with a ratio between the thermal and magnetic pressures of the order of unity. This study aimed at verifying the role of these instabilities in driving fast magnetic reconnection in turbulent accretion disk/corona systems. As we expected, the simulations showed an initial formation of large-scale magnetic loops due to the PRTI followed by the development of a nearly steady-state turbulence driven by both instabilities. In this turbulent environment, we have employed an algorithm to identify the presence of current sheets produced by the encounter of magnetic flux ropes of opposite polarity in the turbulent regions of both the corona and the disk. We computed the magnetic reconnection rates in these locations, obtaining average reconnection velocities in Alfvén speed units of the order of 0.13 ± 0.09 in the accretion disk and 0.17 ± 0.10 in the coronal region (with mean peak values of order 0.2), which are consistent with the predictions of the theory of turbulence-induced fast reconnection.

Effects of plasma density on laser-generated energetic electron generation and transport in a plasma channel

We use two-dimensional particle-in-cell simulations to investigate how the plasma density n0 of the channel target affects energetic-electron generation and transportation. The simulations show that the optimum plasma-density regime is 10 ≤ n0 ≤ 25 for the present simulation parameters, which results in a peak energy flux and coupling efficiency from laser to electrons. In this case, the laser beam propagates stably in the channel, which has the advantage of increasing the acceleration length and more effectively generating high-energy electrons. Furthermore, the high-current electron beam and the density modulation induce strong azimuthal magnetic fields and double-layer radial electric fields around the inner surface of the channel, which consistently guide and collimate the hot-electron bunch so that it propagates over rather long times and distances. Upon further increasing the plasma density n0, the hot electrons are scattered out of the channel by the damped laser pulse and the reduced quasistatic interface electromagnetic fields, reducing the long-time transport. The use of a proper plasma-density channel stably guides the relativistically intense laser pulse and greatly improves the properties of the electron beam.

Magnetorotational dynamo action in the shearing box

Magnetic dynamo action caused by the magnetorotational instability is studied in the shearing-box approximation with no imposed net magnetic flux. Consistent with recent studies, the dynamo action is found to be sensitive to the aspect ratio of the box: it is much easier to obtain in tall boxes (stretched in the direction normal to the disc plane) than in long boxes (stretched in the radial direction). Our direct numerical simulations indicate that the dynamo is possible in both cases, given a large enough magnetic Reynolds number. To explain the relatively larger effort required to obtain the dynamo action in a long box, we propose that the turbulent eddies caused by the instability most efficiently fold and mix the magnetic field lines in the radial direction. As a result, in the long box the scale of the generated strong azimuthal (stream-wise directed) magnetic field is always comparable to the scale of the turbulent eddies. In contrast, in the tall box the azimuthal magnetic flux spreads in the vertical direction over a distance exceeding the scale of the turbulent eddies. As a result, different vertical sections of the tall box are permeated by large-scale non-zero azimuthal magnetic fluxes, facilitating the instability. In agreement with this picture, the cases when the dynamo is efficient are characterized by a strong intermittency of the local azimuthal magnetic fluxes.

The effects of a magnetic field on planetary migration in laminar and turbulent discs

We investigate the migration of low-mass planets ($5 M_{\oplus}$ and $20 M_{\oplus}$) in accretion discs threaded with a magnetic field using 2D MHD code in polar coordinates. We observed that, in the case of a strong azimuthal magnetic field where the plasma parameter is $\beta\sim 1-2$, density waves at the magnetic resonances exert a positive torque on the planet and may slow down or reverse its migration. However, when the magnetic field is weaker (i.e., the plasma parameter $\beta$ is relatively large), then non-axisymmetric density waves excited by the planet lead to growth of the radial component of the field and, subsequently, to development of the magneto-rotational instability, such that the disc becomes turbulent. Migration in a turbulent disc is stochastic, and the migration direction may change as such. To understand migration in a turbulent disc, both the interaction between a planet and individual turbulent cells, as well as the interaction between a planet and ordered density waves, have been investigated.

Multiscale convection in a geodynamo simulation with uniform heat flux along the outer boundary

AbstractIt is generally expected that Earth's magnetic field, which is generated by convecting liquid metal within its core, will substantially alter that convection through the action of Lorentz forces. In most dynamo simulations, however, Lorentz forces do very little to change convective flow, which is predominantly fine‐scaled. An important exception to this observation is in dynamo models that employ uniform heat flux boundary conditions, rather than the usual uniform temperature conditions, in which multiscale convection is observed. We investigate the combined influence of thermal boundary conditions and magnetic fields using four simulations: two dynamos and two nonmagnetic models, with either uniform temperature or heat flux fixed at the outer boundary. Of the four, only the fixed‐heat‐flux dynamo simulation produces multiscale convective flow patterns. Comparison between the models suggests that the fixed‐flux dynamo generates large patches of strong azimuthal magnetic field that suppress small‐scale convective motions. By allowing temperature to vary along the outer boundary, the fixed‐flux dynamo generates stronger azimuthal flow and, in turn, stronger magnetic field, and the resulting Lorentz forces alter the nature of convective flow. Extrapolation of the analyses presented here suggests that magnetic fields may also suppress small‐scale convection in the Earth's core.

Long-Term Evolution of Broken Wakefields in Finite-Radius Plasmas

A novel effect of fast heating and charging a finite-radius plasma is discovered in the context of plasma wakefield acceleration. As the plasma wave breaks, most of its energy is transferred to plasma electrons. The electrons gain substantial transverse momentum and escape the plasma radially, which gives rise to a strong charge-separation electric field and azimuthal magnetic field around the plasma. The slowly varying field structure is preserved for hundreds of wakefield periods and contains (together with hot electrons) up to 80% of the initial wakefield energy.

Hemispherical Parker waves driven by thermal shear in planetary dynamos

Planetary and stellar magnetic fields are thought to be sustained by helical motions (α-effect) and, if present, differential rotation (Ω-effect). In the Sun, the strong differential rotation in the tachocline is responsible for an efficient Ω-effect creating a strong axisymmetric azimuthal magnetic field. This is a prerequisite for Parker dynamo waves that may be responsible for the solar cycle. In the liquid iron cores of terrestrial planets, the Coriolis force organizes convection into columns with a strong helical flow component. These likely dominate magnetic field generation while the Ω-effect is of secondary importance. Here we use numerical simulations to show that the planetary dynamo scenario may change when the heat flux through the outer boundary is higher in one hemisphere than in the other. A hemispherical dynamo is promoted that is dominated by fierce thermal wind responsible for a strong Ω-effect. As a consequence Parker dynamo waves are excited equivalent to those predicted for the Sun. They obey the same dispersion relation and propagation characteristics. We suggest that Parker waves may therefore also play a role in planetary dynamos for all scenarios where zonal flows become an important part of convective motions.

Experimental Evidence for a Transient Tayler Instability in a Cylindrical Liquid-Metal Column

In the current-driven, kink-type Tayler instability (TI) a sufficiently strong azimuthal magnetic field becomes unstable against nonaxisymmetric perturbations. The TI has been discussed as a possible ingredient of the solar dynamo mechanism and a source of the helical structures in cosmic jets. It is also considered as a size-limiting factor for liquid metal batteries. We report on a liquid metal TI experiment using a cylindrical column of the eutectic alloy GaInSn to which electrical currents of up to 8kA are applied. We present results of external magnetic field measurements that indicate the transient occurrence of the TI in good agreement with numerical predictions. The interference of TI with the competing large-scale convection, resulting from Joule heating, is also discussed.

A detailed analysis of a dynamo mechanism in a rapidly rotating spherical shell

AbstractMechanisms of magnetic field intensification by flows of an electrically conducting fluid in a rapidly rotating spherical shell are investigated using a numerical dynamo model with an Ekman number of $1{0}^{\ensuremath{-} 5} $. A strong dipolar solution with a magnetic energy 55 times larger than the kinetic energy of thermal convection is obtained. In a regime of small viscosity and inertia with the strong magnetic field, the convection structure consists of a few large-scale retrograde flows in the azimuthal direction and localized thin sheet-like plumes. A detailed term-by-term analysis of the magnetic field amplification processes shows that the magnetic field is amplified through stretching of magnetic lines, which occurs typically through four types of flow: the retrograde azimuthal flow near the outer boundary, the downwelling flow of the sheet plume, the prograde azimuthal flow near the rim of the tangent cylinder, and the cylindrical-radially alternating flows of the plume cluster. The current loop structure emerges as a result of stretching the magnetic lines along the magnetic field by the flow acceleration. The most remarkable effects of the generated magnetic field on the flow come from the strong azimuthal (toroidal) magnetic field. Similarities of the present model in the convection and magnetic field structures to previous studies at larger and even smaller Ekman numbers suggest universality of the dynamo mechanism in rotating spherical dynamos.

Propagation of energetic electrons in a hollow plasma fiber

Transport of energetic electrons in a hollow plasma fiber is investigated. The high-current electron beam induces in the fiber strong radial electric fields and azimuthal magnetic fields on the inner and outer surfaces of the hollow fiber. The hot electrons are pushed out by the surface magnetic field and returned into the fiber by the sheath electric field. Imbalance of the latter fields can drive chaotic oscillations of electrons around the fiber wall. Intense thin return-current layers inside both the inner and outer wall surfaces are observed. This enhances local joule heating around both surfaces by the return current.

Laser second harmonic generation in a rippled density plasma in the presence of azimuthal magnetic field

AbstractThe strong azimuthal magnetic fields, observed in many laser plasma experiments, are shown to be a potential source of second harmonic generation. The laser imparts oscillatory velocity to electrons and exerts a longitudinal pondromotive force on them. This force in conjunction with the azimuthal magnetic field and density ripple produces a second harmonic current with significant transverse component. The latter produces resonant second harmonic radiation.

Stimulated Raman scattering of a laser beam in a plasma with azimuthal magnetic field

A strong azimuthal magnetic field localizes the lower hybrid waves radially in laser produced plasmas. The laser pump parametrically excites a lower hybrid wave and a backscattered electromagnetic sideband wave. The density perturbation due to the lower hybrid wave couples with the oscillatory velocity of electrons due to the pump wave, to produce a nonlinear current driving the sideband. The pump and sideband waves exert a ponderomotive force on electrons driving the lower hybrid wave. The local effects reduce the growth rate of stimulated Raman scattering. The fundamental radial eigenmode (p=0) of the lower hybrid wave is the maximally growing mode. The scattering process can be used as a diagnostic for the azimuthal magnetic field.

Strong field anisotropic diffusion models for the Earth’s core

The vector spherical harmonic equations and toroidal–poloidal spectral interactions are developed for the strong azimuthal field viscous and thermal diffusion tensors given respectively by D ν=2ρν 0 I+ρν φφ sin 2 θ 1 φ 1 φ and D κ=κ 0 I+κ φφ sin 2 θ 1 φ 1 φ , where I is the unit tensor, θ is the co-latitude, 1 φ is the easterly unit vector and the coefficients ν φφ and κ φφ are spherically symmetric. These diffusion tensors model anisotropic diffusion in the presence of a strong azimuthal magnetic field. Physically, these models represent enhanced or inhibited diffusion along the magnetic field lines in the Earth’s core. Mathematically, these models are restricted to be analytic along the rotation axis. The spectral interactions are derived for the momentum equation for the body force, ∇· τ , where τ = D ν ·(∇ v ) s . The subscript s denotes the trace-free symmetric part of the velocity gradient. There are four viscous interaction terms, ( φφtt n ), ( φφst n ), ( φφts n ), ( φφss n ). The spectral interactions for the transpose and symmetric part of τ are also derived. The spectral interactions may be used in dynamically-consistent pseudo-spectral geodynamo codes.

Stimulated Raman Scattering of a Laser in a Magnetic Plasma Channel

A depressed density plasma channel, in the presence of a strong azimuthal magnetic field, supports localized lower hybrid modes of finite azimuthal mode number. A high amplitude laser propagating through the channel undergoes stimulated Raman scattering off a lower hybrid mode producing a back propagating electromagnetic sideband wave. The pump beats with the sideband to exert a ponderomotive force on the electrons driving the lower hybrid wave. The density perturbations associated with the lower hybrid wave couple with the oscillatory velocity due to the pump, producing a nonlinear current, driving the sideband. The radial profile of the sideband and the frequency shift have signatures of magnetic field and can be used as a diagnostics for azimuthal magnetic field.

Relativistic laser guiding in an azimuthal magnetic field in a plasma

Strong azimuthal magnetic field, Bs θ̂, generated in an intense laser plasma interaction, has a focusing effect on the laser. The rays having E⊥Bs propagate as an X mode with a refractive index maximum on the axis and are focused. However, rays with E∥Bs propagate unaffected by the magnetic field. The relativistic mass nonlinearity reinforces additional focusing on the laser. Under the combined effect of both focusing mechanisms a linearly polarized laser emerges with a reduced spot size and elliptical cross section. This process may provide a diagnostic means for measuring the azimuthal magnetic field.