Magnetic Field Asymmetry Research Articles

Correction of the axial asymmetry of the poloidal magnetic field in the Globus-M spherical tokamak

The toroidal inhomogeneity of the poloidal magnetic field—the so-called error fields that arise due to imperfections in manufacturing and assembling of the electromagnetic system-was measured in the Globus-M spherical tokamak. A substantial inhomogeneity corresponding to the n = 1 mode, which gave rise to a locked mode and led to discharge disruption, was revealed. After compensation of this inhomogeneity with the help of special correction coils, the discharge duration increased and the global plasma parameters improved substantially. A technique for determining and compensating the n = 1 mode inhomogeneity is described, the measured dependences of the penetration threshold of the m = 2/n = 1 mode on the plasma parameters are given, and results of experiments in which record parameters for the Globus-M tokamak were achieved after correction of the poloidal magnetic field are presented.

Magnetic-field asymmetry of nonlinear mesoscopic transport in channels coupled to a single metallic gate

The nonlinear transport of asymmetric quantum wires has been studied under the influence of an external magnetic field. The investigated structures were based on a modulation doped GaAs/AlGaAs heterostructure with a two-dimensional electron gas approximately 80 nm below the surface. The in-plane asymmetry of the quantum wire was realized by two different side walls, one defined via an etched channel and the other by a metallic top gate. Magnetic fields were applied perpendicular to the sample surface. The conductance of the quantum wires was studied at a temperature of 4.2 K. It was found that such asymmetric quantum wires show a pronounced magnetic-field asymmetry, defined as the change in the conductance induced by a change in the magnetic-field sign. The asymmetric conductance increases linearly with the bias voltage. These findings could be explained by a magnetic-field asymmetry of nonlinear mesoscopic transport.

Magnetic reconnection at 3D null points: effect of magnetic field asymmetry

The aim of this paper is to investigate the properties of magnetic reconnection at a 3D null point, with respect to their dependence on the symmetry of the magnetic field around the null. In particular we examine the rate of flux transport across the null point with symmetric/asymmetric diffusion regions, as well as how the current sheet forms in time, and its properties. Mathematical modelling and finite difference resistive MHD simulations are used. It is found that the basic structure of the mode of magnetic reconnection considered is unaffected by varying the magnetic field symmetry, that is, the plasma flow is found cross both the spine and fan of the null. However, the peak intensity and dimensions of the current sheet are dependent on the symmetry/ asymmetry of the field lines. As a result, the reconnection rate is also found to be strongly dependent on the field asymmetry.

A new dynamo pattern revealed by solar helical magnetic fields

Abstract A previously unobservable mirror asymmetry of the solar magnetic field – a key ingredient of the dynamo mechanism which is believed to drive the 11-year activity cycle – has recently been measured. This was achieved through systematic monitoring of solar active regions carried out for more than 20 years at observatories in Mees, Huairou and Mitaka. In this Letter we report on detailed analysis of vector magnetic field data, obtained at Huairou Solar Observing Station in China. Electric current helicity (the product of current and magnetic field components in the same direction) was estimated from the data and a latitude–time plot of solar helicity during the last two solar cycles has been produced. We find that like sunspots helicity patterns propagate equatorwards, but unlike sunspot polarity helicity in each solar hemisphere does not change sign from cycle to cycle, thus confirming the theory. There are, however, two significant time–latitudinal domains in each cycle when the sign briefly inverts. Our findings shed new light on stellar and planetary dynamos and are yet to be included in the theory.

Spin-orbit coupling effects in two-dimensional circular quantum rings: Elliptical deformation of confined electron density

We study electron states confined in two-dimensional circular quantum rings in the presence of spin-orbit coupling due to both structure and crystal inversion asymmetry in the external magnetic field. It is demonstrated that the confined electron density loses the circular symmetry of the confinement potential provided that both Rashba and Dresselhaus coupling constants are nonzero, with the exception of a special case of equal coupling constants and absence of the spin Zeeman interaction. An elliptical deviation from the circular symmetry---present already for a single confined electron---is for two electrons strengthened by the Coulomb repulsion. We discuss signatures of the charge-density deformation in the experimentally accessible quantities: magnetization and charging properties of the ring. Relevance of the results of one-dimensional ring models for description of spin-orbit coupling effects is also discussed.

Pair Tunneling Resonance in the Single-Electron Transport Regime

We predict a new electron pair tunneling (PT) resonance in nonlinear transport through quantum dots with positive charging energies exceeding the broadening due to thermal and quantum fluctuations. The PT resonance shows up in the single-electron transport (SET) regime as a peak in the derivative of the nonlinear conductance, d(2)I/dV(2), when the electrochemical potential of one electrode matches the average of two subsequent charge addition energies. For a single level quantum dot (Anderson model) we find the analytic peak shape and the dependence on temperature, magnetic field, and junction asymmetry and compare with the inelastic cotunneling peak which is of the same order of magnitude. In experimental transport spectroscopy the PT resonance may be mistaken for a weak SET resonance judging only by the voltage dependence of its position. Our results provide essential clues to avoid such erroneous interpretation.

Magnetic-field asymmetry of nonlinear transport in narrow channels with asymmetric hybrid confinement

The authors realized narrow constrictions in a two-dimensional electron gas (2DEG) with an in-plane asymmetry by means of a hybrid confinement. Unequal sidewalls of the constrictions were formed by combination of a single etched trench in a modulation doped GaAs/AlGaAs heterostructure which cuts through the 2DEG with an adjacent metallic top-gate, which depletes the underlying 2DEG. The two-terminal conductance was analyzed in magnetic fields applied perpendicular to the sample surface. In nonlinear transport, the conductance shows pronounced differences for an inversion of the magnetic field. The asymmetric conductance is related to scattering asymmetries caused by the different sidewalls in the asymmetric constriction.

Plasma rotation driven by static nonresonant magnetic fields

Recent experiments in high temperature DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 64 (2002)] plasmas reported the first observation of plasma acceleration driven by the application of static nonresonant magnetic fields (NRMFs), with resulting improvement in the global energy confinement time. Although the braking effect of static magnetic field asymmetries is well known, recent theory [A. J. Cole et al., Phys. Rev. Lett. 99, 065001 (2007)] predicts that in some circumstances they lead instead to an increase in rotation frequency toward a “neoclassical offset” rate in a direction opposed to the plasma current. We report the first experimental confirmation of this surprising result. The measured NRMF torque shows a strong dependence on both plasma density and temperature, above expectations from neoclassical theory. The consistency between theory and experiment improves with modifications to the expression of the NRMF torque accounting for a significant role of the plasma response to the external field and for the beta dependence of the plasma response, although some discrepancy remains. The magnitude and direction of the observed offset rotation associated with the NRMF torque are consistent with neoclassical theory predictions. The offset rotation rate is about 1% of the Alfven frequency or more than double the rotation needed for stable operation at high βN above the n=1 no-wall kink limit in DIII-D.

NEW OBSERVATION OF FAILED FILAMENT ERUPTIONS: THE INFLUENCE OF ASYMMETRIC CORONAL BACKGROUND FIELDS ON SOLAR ERUPTIONS

Failed filament eruptions not associated with a coronal mass ejection (CME) have been observed and reported as evidence for solar coronal field confinement on erupting flux ropes. In those events, each filament eventually returns to its origin on the solar surface. In this Letter, a new observation of two failed filament eruptions is reported which indicates that the mass of a confined filament can be ejected to places far from the original filament channel. The jetlike mass motions in the two failed filament eruptions are thought to be due to the asymmetry of the background coronal magnetic fields with respect to the locations of the filament channels. The asymmetry of the coronal fields is confirmed by an extrapolation based on a potential field model. The obvious imbalance between the positive and negative magnetic flux (with a ratio of 1:3) in the bipolar active region is thought to be the direct cause of the formation of the asymmetric coronal fields. We think that the asymmetry of the background fields can not only influence the trajectories of ejecta, but also provide a relatively stronger confinement for flux rope eruptions than the symmetric background fields do.

ASYMMETRY OF HELICITY INJECTION FLUX IN EMERGING ACTIVE REGIONS

Observational and modeling results indicate that typically the leading magnetic field of bipolar active regions (ARs) is often spatially more compact, while more dispersed and fragmented in following polarity. In this paper, we address the origin of this morphological asymmetry, which is not well understood. Although it may be assumed that, in an emerging Ω-shaped flux tube, those portions of the flux tube in which the magnetic field has a higher twist may maintain its coherence more readily, this has not been tested observationally. To assess this possibility, it is important to characterize the nature of the fragmentation and asymmetry in solar ARs and this provides the motivation for this paper. We separately calculate the distribution of the helicity flux injected in the leading and following polarities of 15 emerging bipolar ARs, using the Michelson Doppler Image 96 minute line-of-sight magnetograms and a local correlation tracking technique. We find from this statistical study that the leading (compact) polarity injects several times more helicity flux than the following (fragmented) one (typically 3-10 times). This result suggests that the leading polarity of the Ω-shaped flux tube possesses a much larger amount of twist than the following field prior to emergence. We argue that the helicity asymmetry between the leading and following magnetic field for the ARs studied here results in the observed magnetic field asymmetry of the two polarities due to an imbalance in the magnetic tension of the emerging flux tube. We suggest that the observed imbalance in the helicity distribution results from a difference in the speed of emergence between the leading and following legs of an inclined Ω-shaped flux tube. In addition, there is also the effect of magnetic flux imbalance between the two polarities with the fragmented following polarity displaying spatial fluctuation in both the magnitude and sign of helicity measured.

Magnetic-Field Asymmetry of Electron Wave Packet Transmission in Bent Channels Capacitively Coupled to a Metal Gate

We study the electron wave packet moving through a bent channel. We demonstrate that the packet transmission probability becomes an asymmetric function of the magnetic field when the electron packet is capacitively coupled to a metal plate. The coupling occurs through a nonlinear potential which translates a different kinetics of the transport for opposite magnetic-field orientations into a different potential felt by the scattered electron.

Ring current asymmetry during a magnetic storm

The ring current dynamics during the magnetic storm has been studied in the work. The response of the magnetospheric current systems to the external influence of the solar wind, specifically, resulting in the development of the asymmetric ring current component, has been calculated using the magnetic field paraboloid model. The asymmetric ring current has been considered as a family of spatial current circuits in the Northern and Southern hemispheres, composed of the zones of the partial ring current in the geomagnetic equator plane, which close through the system of field-aligned currents into the ionosphere. The value of the total partial ring current has been estimated by comparing the calculated asymmetry of the magnetospheric magnetic field at the geomagnetic equator with the value of the Asym-H geomagnetic index. The variations in the symmetric and asymmetric components of the ring current magnetic field have been calculated for the magnetic storm of November 6–14, 2004. The contributions of the magnetospheric current systems to the Dst and AU geomagnetic indices have been calculated.

Characteristics and variability of Titan's magnetic environment

The structure and variability of Saturn's magnetic field in the vicinity of Titan's orbit is studied. In the dawn magnetosphere, the magnetic field presents a significant radial component directed towards Saturn, suggesting that Titan is usually located below the planet's warped and dynamic magnetodisc. Also, a non-negligible component along the co-rotation direction suggests that Saturn's magnetic field lines close to the magnetodisc are being swept back from their respective magnetic meridians. In the noon sector, Titan seems to be closer to the magnetodisc central current sheet, as the field lines in this region seem to be more dipolar. The distance between the central current sheet and Titan depends mainly on the solar wind pressure. Also, delta|B|/|B| approximately 0.5 amplitude waveforms at periods close to Saturn's kilometric radiation period are present in the background magnetic field. This modulation in the field is ubiquitous in Saturn's magnetosphere and associated with the presence of a rotating asymmetry in the planet's magnetic field.

Tunable magnetic-field asymmetry of nonlinear mesoscopic transport: Field-effect controlled backscattering in a quantum wire

The nonlinear conductance of quantum wires has been studied in magnetic fields by applying an in-plane electric field via side gates. It has been found that magnetic-field asymmetries, defined as the change in the conductance induced by a change in the magnetic-field sign, become more pronounced the larger the gate voltage is and increase linearly with the bias voltage up to several millivolts. With an in-plane electric field the asymmetry can be tuned, the symmetry recovered, and also the asymmetry reversed, which is associated with a field-effect controlled backscattering of electrons.

Long-term variations of asymmetry of the solar magnetic field and geometry of the interplanetary magnetic field

A comparative analysis of variations of the mean solar magnetic field and the interplanetary magnetic field over all history of their measurements is performed. Asymmetry of the solar magnetic field and its manifestation in the heliosphere is investigated. Long-term variations of the solar magnetic field and the heliosphere, which manifest themselves in alternation of dominating magnetic polarities of different sign, are discovered. On the basis of the analysis of cumulative sums of the IMF components, long-term variations of the IMF geometry and of the solar wind spiral angle are found. The cumulative sum of the IMF B z component perpendicular to the ecliptic plane also shows long-term variations. Time intervals are revealed, in which negative values of the IMF B z component dominate, and an increased geomagnetic activity is observed.

Coronal Mass Ejections, Magnetic Fields, and the Green Corona in Cycle 23

We study a time – latitudinal distribution of CMEs observed by the SOHO spacecraft, their projected speeds and associated magnetic fields, as well as the north – south (N – S) asymmetry of solar surface magnetic fields, and the coronal green line intensities. We have found that (a) there exists an intricate relation between the average projected velocity of CMEs and the mean value of large-scale magnetic fields; (b) there exists a pronounced N – S asymmetry in both the distribution and the number of CMEs; (c) this asymmetry is in favor of the northern hemisphere at the beginning of the cycle, and of the southern hemisphere from 2001 onward, being, in fact, (d) closely related with the N – S asymmetry in the distribution of large-scale magnetic fields and the coronal green line intensities.

Current Helicity and Twist as Two Indicators of the Mirror Asymmetry of Solar Magnetic Fields

A comparison between the two tracers of magnetic field mirror asymmetry in solar active regions – twist and current helicity – is presented. It is shown that for individual active regions these tracers do not possess visible similarity but averaging by time over the solar cycle, or by latitude, reveals similarities in their behavior. The main property of the data set is antisymmetry over the solar equator. Considering the evolution of helical properties over the solar cycle we find signatures of a possible sign change at the beginning of the cycle, though more systematic observational data are required for a definite confirmation. We discuss the role of both tracers in the context of solar dynamo theory.

Validity and Breakdown of Onsager Symmetry in Mesoscopic Conductors Interacting with Environments

We investigate magnetic-field asymmetries in the linear transport of a mesoscopic conductor interacting with its environment. Interestingly, we find that the interaction between the two systems causes an asymmetry only when the environment is out of equilibrium. We elucidate our general result with the help of a quantum dot capacitively coupled to a quantum Hall conductor and discuss the asymmetry dependence on the environment bias and induced dephasing.

Resistive wall mode stabilization in slowly rotating high beta plasmas

DIII-D experiments show that the resistive wall mode (RWM) can remain stable in high β scenarios despite a low net torque from nearly balanced neutral beam injection heating. The minimization of magnetic field asymmetries is essential for operation at the resulting low plasma rotation of less than 20 krad s−1 (measured with charge exchange recombination spectroscopy using C VI emission) corresponding to less than 1% of the Alfvén velocity or less than 10% of the ion thermal velocity. In the presence of n = 1 field asymmetries the rotation required for stability is significantly higher and depends on the torque input and momentum confinement, which suggests that a loss of torque-balance can lead to an effective rotation threshold above the linear RWM stability threshold. Without an externally applied field the measured rotation can be too low to neglect the diamagnetic rotation. A comparison of the instability onset in plasmas rotating with and against the direction of the plasma current indicates the importance of the toroidal flow driven by the radial electric field in the stabilization process. Observed rotation thresholds are compared with predictions for the semi-kinetic damping model, which generally underestimates the rotation required for stability. A more detailed modeling of kinetic damping including diamagnetic and precession drift frequencies can lead to stability without plasma rotation. However, even with corrected error fields and fast plasma rotation, plasma generated perturbations, such as edge localized modes, can nonlinearly destabilize the RWM. In these cases feedback control can increase the damping of the magnetic perturbation and is effective in extending the duration of high β discharges.

Locked mode thresholds on the MAST spherical tokamak

In a tokamak, misalignments of field coils and coil connections, introduced in the construction of the device, lead to small asymmetries in magnetic field known as error fields. Even at small levels these can stop the plasma rotating, driving resistive tearing modes that lead to plasma termination. Recently, a set of four error field correction coils has been installed on the MAST spherical tokamak. These coils have been used to correct for the error field, allowing MAST to operate in previously inaccessible regimes. With error field correction, MAST can operate at densities at least 30% lower than previously attained. The error field coils have also been used to apply large error fields to the plasma to explore the phenomenon of mode locking. It was found that the locked mode threshold scales with line averaged electron density, safety factor and toroidal field in a similar way to that found from conventional aspect ratio tokamaks. This scaling law has been used to estimate the locked mode threshold for a proposed power plant and a component test facility based on the spherical tokamak concept.