Helical Field Research Articles

Unstable twisted modes in interpenetrating space plasmas containing superthermal species

The electrostatic twisted modes with orbital angular momentum and associated kinetic instability are studied in a permeating space plasma containing streaming particle species. The plasma containing superthermal electrons and ions is modeled by using a non-gyrotropic Kappa distribution function which penetrates through a relatively slow moving (static) plasma and gives rise to dispersion, damping and growth of ion-acoustic mode under various conditions. Using the Vlasov-Poisson model, the solutions of twisted modes are defined by Laguerre-Gaussian mode functions, which decompose the plasma distribution function and electric field into components characterized by the axial and azimuthal wave numbers. The dielectric constant is derived and analyzed for threshold condition of wave dispersion and instability in the presence of helical electric field with illustrations. The wave excitations due to penetration of solar wind into cometary clouds or interstellar electron-ion plasmas is examined.

Explicit Prediction of Continental Convection in a Skillful Variable‐Resolution Global Model

AbstractWe present a new global‐to‐regional model, cfvGFS, able to explicitly (without parameterization) represent convection over part of the Earth. This model couples the Geophysical Fluid Dynamics Laboratory Finite‐Volume Cubed‐Sphere Dynamical Core (FV3) to the Global Forecast System physics and initial conditions, augmented with a six‐category microphysics and a modified planetary boundary layer scheme. We examine the characteristics of cfvGFS on a 3‐km continental U. S. domain nested within a 13‐km global model. The nested cfvGFS still has good hemispheric skill comparable to or better than the operational Global Forecast System, while supercell thunderstorms, squall lines, and derechos are explicitly represented over the refined region. In particular, cfvGFS has excellent representations of fine‐scale updraft helicity fields, an important proxy for severe weather forecasting. Precipitation biases are found to be smaller than in uniform‐resolution global models and competitive with operational regional models; the 3‐km domain also improves upon the global models in 2‐m temperature and humidity skill. We discuss further development of cfvGFS and the prospects for a unified global‐to‐regional prediction system.

Interpreting magnetic helicity flux in solar flux emergence

Magnetic helicity flux gives information about the topology of a magnetic field passing through a boundary. In solar physics applications, this boundary is the photosphere and magnetic helicity flux has become an important quantity in analysing magnetic fields emerging into the solar atmosphere. In this work we investigate the evolution of magnetic helicity flux in magnetohydrodynamic (MHD) simulations of solar flux emergence. We consider emerging magnetic fields with different topologies and investigate how the magnetic helicity flux patterns correspond to the dynamics of emergence. To investigate how the helicity input is connected to the emergence process, we consider two forms of the helicity flux. The first is the standard form giving topological information weighted by magnetic flux. The second form represents the net winding and can be interpreted as the standard helicity flux less the magnetic flux. Both quantities provide important and distinct information about the structure of the emerging field and these quantities differ significantly for mixed sign helicity fields. A novel aspect of this study is that we account for the varying morphology of the photosphere due to the motion of the dense plasma lifted into the chromosphere. Our results will prove useful for the interpretation of magnetic helicity flux maps in solar observations.

Observational signatures of magnetic field structure in relativistic AGN jets

Context. Active galactic nuclei (AGN) launch highly energetic jets sometimes outshining their host galaxy. These jets are collimated outflows that have been accelerated near a supermassive black hole located at the centre of the galaxy. Their, virtually indispensable, energy reservoir is either due to gravitational energy released from accretion or due to the extraction of kinetic energy from the rotating supermassive black hole itself. In order to channel part of this energy to the jet, though, the presence of magnetic fields is necessary. The extent to which these magnetic fields survive in the jet further from the launching region is under debate. Nevertheless, observations of polarised emission and Faraday rotation measure confirm the existence of large scale magnetic fields in jets.Aims. Various models describing the origin of the magnetic fields in AGN jets lead to different predictions about the large scale structure of the magnetic field. In this paper we study the observational signatures of different magnetic field configurations that may exist in AGN jets in order to asses what kind of information regarding the field structure can be obtained from radio emission, and what would be missed.Methods. We explore three families of magnetic field configurations. First, a force-free helical magnetic field corresponding to a dynamically relaxed field in the rest frame of the jet. Second, a magnetic field with a co-axial cable structure arising from the Biermann-battery effect at the accretion disk. Third, a braided magnetic field that could be generated by turbulent motion at the accretion disk. We evaluate the intensity of synchrotron emission, the intrinsic polarization profile and the Faraday rotation measure arising from these fields. We assume that the jet consists of a relativistic spine where the radiation originates from and a sheath containing thermalised electrons responsible for the Faraday screening. We evaluate these values for a range of viewing angles and Lorentz factors. We account for Gaussian beaming that smooths the observed profile.Results. Radio emission distributions from the jets with dominant large-scale helical fields show asymmetry across their width. The Faraday rotation asymmetry is the same for fields with opposing chirality (handedness). For jets which are tilted towards the observer the synchrotron emission and fractional polarization can distinguish the field’s chirality. When viewed either side-on or at a Blazar type angle only the fractional polarization can make this distinction. Further this distinction can only be made if the direction of the jet propagation velocity is known, along with the location of the jet’s origin. The complex structure of the braided field is found not to be observable due to a combination of line of sight integration and limited resolution of observation. This raises the possibility that, even if asymmetric radio emission signatures are present, the true structure of the field may still be obscure.

Band Structures of Quasi-One-Dimensional Incommensurate Helical Systems: A Case Study of Infinite Chromium Extended Metal Atom Chain.

Extended metal atom chains (EMACs) are promising candidates for molecular wires but their band structures remain to be explored. As a quasi-one-dimensional (Q1D) system, the incommensurate helical nature of EMACs hinders such calculations. In this work, we resolved this issue via explicit implementation of helical symmetry. Moreover, the pattern of metal d bands was rationalized by a systematic investigation on a series of related Q1D helical systems. Two critical factors, helical ligand field and chemically asymmetric ligand field, are proposed and identified. We found that the symmetry and ligand fields of the system dominate the pattern of the metal d bands, instead of specific chemical composition of ligands. The presented method and rationale are applicable to not only EMACs but also related Q1D helical systems.

Unified formulation for helicity and continuous spin fermionic fields

We propose a unified BRST formulation of general massless fermionic fields of arbitrary mixed-symmetry type in d-dimensional Minkowski space. Depending on the value of the real parameter the system describes either helicity fields or continuous spin fields. Starting with the unified formulation we derive a number of equivalent descriptions including the triplet formulation, Fang-Fronsdal-Labastida formulation, light-cone formulation and discuss the unfolded formulation.

Gauge field and fermion production during axion inflation

We study the dual production of helical Abelian gauge fields and chiral fermions through the Chern-Simons (CS) coupling with a pseudo-scalar inflaton in the presence of a chiral anomaly. Through the CS term, the motion of the inflaton induces a tachyonic instability for one of the two helicities of the gauge field. We show that the resulting helical gauge field necessarily leads to the production of chiral fermions by deforming their Fermi sphere into discrete Landau levels. The population of the lowest Landau level leads to a chiral asymmetry as inferred from the chiral anomaly, while the higher levels are populated symmetrically through pair production. From the backreaction of the fermions on the gauge field production we derive a conservative but stringent upper bound on the magnitude of the gauge fields. Consequently, we find that the scalar perturbations sourced by these helical gauge fields, responsible for enhanced structure formation on small scales, get reduced significantly. We also discuss the fate of the primordial chiral asymmetry and of the helical gauge fields after inflation, and show that the instability in the chiral plasma tends to erase these primordial asymmetries. This result may impact scenarios where the baryon asymmetry of the Universe is connected to primordial magnetic fields.

Physical Parameters of a Reactor-Stellarator with Small Ripples of the Helical Magnetic Field

The paper describes the calculation data on the physical parameters of a reactor-stellarator, where the nonuniformities of the helical field are smaller than the toroidal magnetic field nonuniformities: eh et case, in the present calculations, these coefficients were assumed to be in proportion to the first power of the collision frequency, Di ∝ ν for νeff < 2ωE, and to Di ∝ ν−1 for the inverse inequality. Here, ωE is the rotation frequency of plasma in the radial electric field. As before, the plasma electrons corresponded to the mode of De ∝ ν−1. As initial parameters for numerical calculations, a reactor with R = 8 m, rp = 2 m, and B0 = 5 Т was taken. A numerical code was used to solve the set of equations that describes the plasma space−time behavior in the reactor-stellarator under the conditions of equal diffusion fluxes. The start of reactor operation in the mode of thermonuclear burning was provided by heating sources with a power of several tens of megawatts. Steady-state operating conditions of a self-sustained thermonuclear reaction were attained by maintaining the plasma density through DT fuel pellet injection into the plasma.

Orbital motion of a particle levitated in a standing-vortex acoustical trap

Stable acoustical tweezers and particle manipulation using vortex beams have recently been accomplished in air [Nature Commun. 6: 8661 (2015)] and in water [Phys. Rev. Lett. 116: 024301 (2016)]. In both configurations, a particle is trapped along the propagation axis of a focused vortex, where the acoustic field exhibits a pressure node. More recently, a stable orbital motion of a particle trapped off-axis was achieved by alternating between two counter-rotating vortices [Phys. Rev. Lett. 120: 044301 (2018)]. However, this kind of orbital motion had not been possible so far with a steady helical field having a single angular momentum state. In this work, we present the first experimental evidence of the simultaneous 3D trapping and stable orbiting of millimeter-sized particles in air due to angular momentum transfer in a standing wave trap created by the interference of two vortices. In contrast with previous work, the two vortices have the same helicity with respect to the laboratory reference frame, and thus the angular momentum of this field is well-defined. A description of the particle kinematics as a function of different control parameters is presented, along with a comparison between experimental measurements of the field and theoretical simulations.

All-dielectric transformation medium mimicking a broadband converging lens.

Radio waves carrying orbital angular momentum (OAM) may potentially increase spectrum efficiency and channel capacity based on their extra rotational degree of freedom. However, due to their divergence characteristics, vortex waves are not suitable to transmit over a long distance in the radio frequency (RF) and microwave domains. In this paper, a transformation optics (TO) based all-dielectric converging lens is proposed. The beam divergence angle of the vortex wave passing through the lens can be decreased from 25° to 9°. The transformed material parameters of the converging lens are determined by solving Laplace's equation subject to specific boundary conditions. Far-field antenna radiation patterns as well as near-field helical phase and electric field amplitude distributions obtained from numerical simulations are reported, demonstrating the broadband characteristics of the proposed microwave lens. Moreover, the all-dielectric compact lens design comprised by a graded permittivity profile can be fabricated by additive manufacturing technology, which greatly facilitates the potential development and application of vortex wave based wireless communications.

Relaxation models for single helical reversed field pinch plasmas at low aspect ratio

A recent study [R. Paccagnella, Phys. Plasmas 23, 092512 (2016)] about the scaling with the aspect ratio (i.e., the ratio between the major and minor radius of the torus) of the dominant mode in single helical reverse field pinch plasmas has shown, at intermediate aspect ratio, that the dominant toroidal mode number in the helical states can be interpreted as the result of a relaxation process. In this work, the same theoretical model is compared and validated with the experimental data obtained in the low aspect ratio RELAX device [S. Masamune et al., J. Phys. Soc. Jpn. 76, 123501 (2007).]. Some difficulties have emerged in this comparison that may be interpreted either as related to a residual level of magnetic chaos in the RELAX plasmas or possibly to genuine toroidal effects not analyzed in this paper, where a cylindrical approximation is considered.

Polymer stabilized cholesteric liquid crystal particles with high thermal stability

Cholesteric liquid crystals (CLCs) spontaneously organizing into a periodic helical structure with a twist axis perpendicular to the local director can reflect selected wavelengths of circularly polarized light. The structural color can be easily controlled by manipulating the helical structure, temperature, electric field, and magnetic field. Despite the unique structure and superior performances, free-standing CLCs with confined fluidity, stimuli response, and high structure stability at high temperatures still remain a challenge. Herein, we report a simple and controllable preparation of a novel type of free-standing 3D confined polymer stabilized cholesteric liquid crystal particles (PSCLCPs) based on the microfluidic emulsification, interfacial polymerization, and UV curing. The size of PSCLCPs can be precisely controlled by adjusting the flow rates of the injected fluids in a microfluidic chip. The fluidity of CLCs is effectively restricted within the physical confinement of the polymer layer, the PSCLCPs present reversible thermal response between the cholesteric phase and the isotropic phase. The CLC domains in microcapsules possess superior microstructure stability at high temperatures (near 220 °C). The stand-alone PSCLCPs with confined fluidity, stimuli response and high structure stability at high temperatures will provide ever better performances in their tremendous applications in the field of smart photonic and electro-optical devices.

Return of grand unified theory baryogenesis: Source of helical hypermagnetic fields for the baryon asymmetry of the universe

It has been considered that baryogenesis models without a generation of $B$-$L$ asymmetry such as the GUT baryogenesis do not work since the asymmetry is washed out by the electroweak sphalerons. Here, we point out that helical hypermagnetic fields can be generated through the chiral magnetic effect with a chiral asymmetry generated in such baryogenesis models. The helical hypermagnetic fields then produce baryon asymmetry mainly at the electroweak symmetry breaking, which remains until today. Therefore, the baryogenesis models without $B$-$L$ asymmetry can still be the origin of the present baryon asymmetry. In particular, if it can produce chiral asymmetry mainly carried by right-handed electrons of order of $10^{-3}$ in terms of the chemical potential to temperature ratio, the resultant present-day baryon asymmetry can be consistent with our Universe, although simple realizations of the GUT baryogenesis are hard to satisfy the condition. We also argue the way to overcome the difficulty in the GUT baryogenesis. The intergalactic magnetic fields with $B_0\sim 10^{-16 \sim 17}$ G and $\lambda_0 \sim 10^{-2\sim3}$ pc are the smoking gun of the baryogenesis scenario as discussed before.

Laminar and Turbulent Dynamos in Chiral Magnetohydrodynamics. II. Simulations

Abstract Using direct numerical simulations (DNS), we study laminar and turbulent dynamos in chiral magnetohydrodynamics with an extended set of equations that accounts for an additional contribution to the electric current due to the chiral magnetic effect (CME). This quantum phenomenon originates from an asymmetry between left- and right-handed relativistic fermions in the presence of a magnetic field and gives rise to a chiral dynamo. We show that the magnetic field evolution proceeds in three stages: (1) a small-scale chiral dynamo instability, (2) production of chiral magnetically driven turbulence and excitation of a large-scale dynamo instability due to a new chiral effect (α μ effect), and (3) saturation of magnetic helicity and magnetic field growth controlled by a conservation law for the total chirality. The α μ effect becomes dominant at large fluid and magnetic Reynolds numbers and is not related to kinetic helicity. The growth rate of the large-scale magnetic field and its characteristic scale measured in the numerical simulations agree well with theoretical predictions based on mean-field theory. The previously discussed two-stage chiral magnetic scenario did not include stage (2), during which the characteristic scale of magnetic field variations can increase by many orders of magnitude. Based on the findings from numerical simulations, the relevance of the CME and the chiral effects revealed in the relativistic plasma of the early universe and of proto-neutron stars are discussed.

Probing the B-fields of AGN jets on kiloparsec scales - NGC 6251

AbstractBy constructing images of the Faraday rotation measure (RM) of large scale astrophysical jets, the line-of-sight magnetic field component and electron density in the region of Farady rotation can be investigated. A significant gradient in the RM transverse to the jet direction may indicate a corresponding gradient in the line-of-sight magnetic field, implying a toroidal or helical magnetic field, which would, in turn, imply the presence of an associated electrical current in the jet. The detection of such large scale gradients can reliably demonstrate that helical or toroidal fields can persist to large distances from the central AGN. We present a kiloparsec-scale Faraday rotation map of NGC 6251 that shows statistically significant transverse RM gradients across its kiloparsec scale jet structure that imply an outward current.

The influence of the Hall term on the development of magnetized laser-produced plasma jets

We present 2D axisymmetric simulation results describing the influence of the Hall term on laser-produced plasma jets and their interaction with an applied magnetic field parallel to the laser axis. Bending of the poloidal B-field lines produces an MHD shock structure surrounding a conical cavity, and a jet is produced from the convergence of the shock envelope. Both the jet and the conical cavity underneath it are bound by fast MHD shocks. We compare the MHD results generated using the extended-MHD code Physics as an Extended-MHD Relaxation System with an Efficient Upwind Scheme (PERSEUS) with MHD results generated using GORGON and find reasonable agreement. We then present extended-MHD results generated using PERSEUS, which show that the Hall term has several effects on the plasma jet evolution. A hot low-density current-carrying layer of plasma develops just outside the plume, which results in a helical rather than a purely poloidal B-field, and reduces magnetic stresses, resulting in delayed flow convergence and jet formation. The flow is partially frozen into the helical field, resulting in azimuthal rotation of the jet. The Hall term also produces field-aligned current in strongly magnetized regions. In particular, we find the influence of Hall physics on this problem to be scale-dependent. This points to the importance of mitigating the Hall effect in a laboratory setup, by increasing the jet density and system dimensions, in order to avoid inaccurate extrapolation to astrophysical scales.

Design of a Short-Period Helical Permanent Magnet Undulator

The design of a helical permanent magnet undulator with a period length of 24 mm for the production of circularly and linearly polarized radiation will be discussed. This undulator consists of four helically shaped magnet arrays, which is expected to produce adjustable helical fields in a cylindrical gap. The helical fields were calculated with the 3-D OPERA computer code producing vertical and horizontal peak fields of 1.29 T for a 24-mm-period length within a bore diameter of 5 mm. The sizes and location of the end poles were optimized to minimize the first and second field integrals. Furthermore, on-axis horizontal or vertical magnetic fields can be generated independently by shifting the relative positions of diagonal pairs of magnet arrays in this device. It leads to the generation of circular and linear polarized radiation in one undulator. The undulator phase shifter can be controlled by mechanical changes to adjust field strength and to tune the helical and linear photon energy. This paper describes magnetic field simulations and mechanical design of the short-period helical permanent magnet undulator.

Excitation of high wavenumber fluctuations by externally-imposed helical fields in edge pedestal plasmas

Two-step mode coupling analyses for nonlinear excitation of the ballooning mode (BM) in pedestal plasma by external helical magnetic field perturbation [Resonant Magnetic Perturbations (RMP)] are presented. This technique allows calculating the effect of higher harmonic sidebands generated by interaction of long scale RMP pump and BM. It is shown that RMP field perturbations can modify the BM growth rate and frequency through nonlinear Reynolds stress and magnetic stress. In particular, it is shown that both stresses can efficiently excite high wavenumber BM fluctuations which, in turn, can enhance the transport in the pedestal. Another notable feature of this analysis is the existence of short scale (high-ky) nonlinear instability at Alfven time scale near the ideal BM threshold boundary.

The propagation of ion-acoustic waves carrying orbital angular momentum in the electron–positron–ion plasmas

Ion-acoustic (IA) waves carrying orbital angular momentum (OAM) are investigated in an unmagnetized, uniform, and collisionless electron–positron–ion (e–p–i) plasma system. Employing the hydrodynamic theory, the paraxial equation in term of ion perturbed number density is derived and discussed about its Laguerre–Gaussian (LG) beam solutions. Obtaining an approximate solution for the electrostatic potential, the IA wave characteristics including helical electric field structure, energy density, and OAM density are theoretically studied. Based on the numerical analysis, the effects of positron concentration, radial and angular mode number as well as beam waist on the obtained potential profile are investigated. It is shown that the depth (height) and width of the LG potential profile wells (barriers) are considerably modify by the variation of positron concentration.

The limit of vanishing viscosity for the incompressible 3D Navier–Stokes equations with helical symmetry

In this paper, we are concerned with the vanishing viscosity problem for the three-dimensional Navier–Stokes equations with helical symmetry, in the whole space. We choose viscosity-dependent initial u0ν with helical swirl, an analogue of the swirl component of axisymmetric flow, of magnitude O(ν) in the L2 norm; we assume u0ν→u0 in H1. The new ingredient in our analysis is a decomposition of helical vector fields, through which we obtain the required estimates.