Magnetized Edge Research Articles

Electrically induced net magnetization in FePSe3 nanoribbons: the role of edge reconstructions.

Magnetized edge states of nanoribbon systems open a new path for designing functional spintronic devices. Here, we introduce a general mechanism for electrically generating nonzero net magnetization in antiferromagnetic (AFM) semiconducting nanoribbons. In the proposed spin configuration, in which the empty and occupied edge states of one side close to the Fermi energy are in the same spin channel, the Zeeman-type spin splitting between the states of opposite edges arising from the electric field allow the system to be tuned from the AFM semiconducting phase to the ferromagnetic (FM) metallic phase, yielding nonzero net magnetization. Our ab initio calculations show that this strategy is realizable in the example of the FePSe3 nanoribbon, in which self-passivation-driven reconstruction at the Se termination edge gives rise to the key spin configuration. Moreover, we demonstrate that an electric field could trigger a series of electronic phase transitions among AFM semiconductor, AFM half-metal, and FM metal phases, based on which we were able to design an electronically controllable versatile spintronics device.

Single field-of-view tomographic imaging of 3D impurity emission distribution in magnetized edge plasma of LHD.

A new tomographic scheme is proposed for reconstructing three dimensional (3D) impurity emission distributions from two dimensional (2D) measurements with a single field-of-view in the magnetized edge plasma in a Large Helical Device (LHD). The 2D image is obtained with a multi-channel fiber array spectrometer, which views the entire region of the edge stochastic magnetic layer of LHD, including divertor plates, divertor legs, the stochastic layer, and the last closed flux surface. The scheme introduces new regularization terms in the Lagrangian function, based on the transport feature in magnetized plasma that the transport parallel to the magnetic field lines is much faster than the transport across the magnetic field, thus assuming smooth distribution in the parallel direction. The scheme is benchmarked with the test data of 3D distribution in the measurement volume, where the effectiveness of the various regularization terms is surveyed and feasibility of the scheme is confirmed. The new scheme is applied to the experimental data in LHD for carbon impurity emissions of C1+ and C3+, where the obtained distributions are discussed taking into account the plasma wall interaction and charge dependence of ionization potentials.

Sublattice engineering and voltage control of magnetism in triangular single and bi‐layer graphene quantum dots

magnified imageWhen a Dirac electron is confined to a triangular graphene quantum dot with zigzag edges, its low‐energy spectrum collapses to a shell of degenerate states at the Fermi level leading to a magnetized edge. The shell degeneracy and the total magnetization are proportional to the edge size and can be made macroscopic. In this review, we start with a general discussion of magnetic properties of graphene structures and its relation to broken sublattice symmetry. Then, we discuss single electronic properties of single and bilayer triangular graphene quantum dots, focusing on the nature of edge states. Finally, we investigate the role of electronic correlations in determining the nature of ground state and excitation spectra of triangular graphene quantum dots as a function of dot size and filling fraction of the shell of zero‐energy states. The interactions are treated by a combination of tight‐binding, Hartree–Fock and configuration interaction methods. We show that the spin polarization of the triangular graphene quantum dots can be controlled through gating, i.e., by adding or removing electrons. In bilayer graphene dots, the relative filling of edge states in each layer and the magnetization can be tuned down to single localized spin using an external vertical electrical field. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

Progress with the COGENT Edge Kinetic Code: Implementing the Fokker‐Planck Collision Operator

AbstractCOGENT is a continuum gyrokinetic code for edge plasma simulations being developed by the Edge Simulation Laboratory collaboration. The code is distinguished by application of a fourth‐order finite‐volume (conservative) discretization, and mapped multiblock grid technology to handle the geometric complexity of the tokamak edge. The distribution function F is discretized in v‖ – μ (parallel velocity – magnetic moment) velocity coordinates, and the code presently solves an axisymmetric full‐f gyro‐kinetic equation coupled to the long‐wavelength limit of the gyro‐Poisson equation. COGENT capabilities are extended by implementing the fully nonlinear Fokker‐Plank operator to model Coulomb collisions in magnetized edge plasmas. The corresponding Rosenbluth potentials are computed by making use of a finite‐difference scheme and multipole‐expansion boundary conditions. Details of the numerical algorithms and results of the initial verification studies are discussed. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The Madison plasma dynamo experiment: A facility for studying laboratory plasma astrophysics

The Madison plasma dynamo experiment (MPDX) is a novel, versatile, basic plasma research device designed to investigate flow driven magnetohydrodynamic (MHD) instabilities and other high-$\beta$ phenomena with astrophysically relevant parameters. A 3 m diameter vacuum vessel is lined with 36 rings of alternately oriented 4000 G samarium cobalt magnets which create an axisymmetric multicusp that contains $\sim$14 m$^{3}$ of nearly magnetic field free plasma that is well confined and highly ionized $(>50\%)$. At present, 8 lanthanum hexaboride (LaB$_6$) cathodes and 10 molybdenum anodes are inserted into the vessel and biased up to 500 V, drawing 40 A each cathode, ionizing a low pressure Ar or He fill gas and heating it. Up to 100 kW of electron cyclotron heating (ECH) power is planned for additional electron heating. The LaB$_6$ cathodes are positioned in the magnetized edge to drive toroidal rotation through ${\bf J}\times{\bf B}$ torques that propagate into the unmagnetized core plasma. Dynamo studies on MPDX require a high magnetic Reynolds number $Rm > 1000$, and an adjustable fluid Reynolds number $10< Re <1000$, in the regime where the kinetic energy of the flow exceeds the magnetic energy ($M_A^2=($v$/$v$_A)^2 > 1$). Initial results from MPDX are presented along with a 0-dimensional power and particle balance model to predict the viscosity and resistivity to achieve dynamo action.

Magnetic bucket for rotating unmagnetized plasma

A new experiment is described which generates flow in unmagnetized plasma. Confinement is provided by a cage of permanent magnets, arranged to form an axisymmetric, high-order, multipolar magnetic field. This field configuration-sometimes called a "magnetic bucket"-has a vanishingly small field in the core of the experiment. Toroidal rotation is driven by J × B forces applied in the magnetized edge. The cross-field current that is required for this forcing flows from anodes to thermionic cathodes, which are inserted between the magnet rings. The rotation at the edge reaches 3 km/s and is viscously coupled to the unmagnetized core plasma. We describe the conditions necessary for rotation, as well as a 0-dimensional power balance used to understand plasma confinement in the experiment.

Stirring Unmagnetized Plasma

A new concept for spinning unmagnetized plasma is demonstrated experimentally. Plasma is confined by an axisymmetric multicusp magnetic field and biased cathodes are used to drive currents and impart a torque in the magnetized edge. Measurements show that flow viscously couples momentum from the magnetized edge (where the plasma viscosity is small) into the unmagnetized core (where the viscosity is large) and that the core rotates as a solid body. To be effective, collisional viscosity must overcome the ion-neutral drag due to charge-exchange collisions.

Applications of the wave kinetic approach: from laser wakefields to drift wave turbulence

AbstractNonlinear wave-driven processes in plasmas are normally described by either a monochromatic pump wave that couples to other monochromatic waves, or as a random phase wave coupling to other random phase waves. An alternative approach involves a random or broadband pump coupling to monochromatic and/or coherent structures in the plasma. This approach can be implemented through the wave-kinetic model. In this model, the incoming pump wave is described by either a bunch (for coherent waves) or a sea (for random phase waves) of quasi-particles. This approach has been applied to both photon acceleration in laser wakefields and drift wave turbulence in magnetized plasma edge configurations. Numerical simulations have been compared to experiments, varying from photon acceleration to drift mode-zonal flow turbulence, and good qualitative correspondences have been found in all cases.

Applications of the wave kinetic approach: From laser wakefields to drift wave turbulence

Nonlinear wave-driven processes in plasmas are normally described by either a monochromatic pump wave that couples to other monochromatic waves or as a random phase wave coupling to other random phase waves. An alternative approach involves a random or broadband pump coupling to monochromatic and/or coherent structures in the plasma. This approach can be implemented through the wave kinetic model. In this model, the incoming pump wave is described by either a bunch (for coherent waves) or a sea (for random phase waves) of quasiparticles. This approach has been applied to both photon acceleration in laser wakefields and drift wave turbulence in magnetized plasma edge configurations. Numerical simulations have been compared to experiments, varying from photon acceleration to drift mode-zonal flow turbulence, and good qualitative correspondences have been found in all cases.

Finite size effect of dust charging in the magnetized edge plasma

A newly developed 3-dimensional particle–particle particle-mesh (P3M) code is applied to study the charging process of micrometer size dust grains confined in the magnetized plasma near the material wall of a tokamak. Plasma particles (electrons and ions) are treated kinetically (particle-in-cell with Monte Carlo collisions (PIC-MCC)), which allows to self-consistently resolve the electrostatic sheath in front of the material wall. In order to accurately resolve the plasma particles’ motion close to the dust grain, the PIC technique is supplemented with molecular dynamics (MD), employing an analytic electrostatic potential for the interaction with the dust grain. The dynamics of charging of dust grains confined in the magnetized edge plasma and its dependence on the size of the dust is investigated. The results are compared with the case of unmagnetized plasma.

Drift‐ordered fluid equations for modelling collisional edge plasma

AbstractThe fluid description of collisional magnetized plasma by Braginskii is based on an ordering assuming the species flow velocities to be on the order of the ion thermal speed. This large flow assumption is normally not valid for plasma fusion devices in general and tokamak edge plasmas in particular. To remove this shortcoming Mikhailovskii and Tsypin developed a plasma fluid description assuming the flow velocities to be on the order of the diamagnetic drift velocities, which are smaller than the ion thermal speed. However, when deriving their equations Mikhailovskii and Tsypin missed important terms in their expressions for the species collisional viscous stress tensors. Starting from corrected Mikhailovskii‐Tsypin fluid equations, we derive a system of non‐linear reduced equations describing field‐aligned fluctuations in low‐beta collisional magnetized edge plasma; namely, equations for plasma density, electron and ion energies (or, equivalently, temperatures), parallel ion flow velocity, parallel current, vorticity (or, equivalently, electrostatic potential), perturbed parallel electromagnetic potential, and perturbed magnetic field. Our equations locally conserve particle number and total energy, and insure that perturbed magnetic field and total plasma current are divergence‐free. In addition, while intended primarily for modelling plasma turbulence, they contain the neoclassical results for plasma current, parallel ion flow velocity, and parallel gradients of equilibrium electron and ion temperatures. (© 2004 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Poloidal Rotation Induced by Injecting Lower Hybrid Waves in Tokamak Plasma Edge

The poloidal rotation of the magnetized edge plasma in tokamak driven by the ponderomotive force which is generated by injecting lower hybrid wave(LHW) electric field has been studied. The LHW is launched from a waveguide in the plasma edge, and by Brambilla's grill theory, analytic expressions for the wave electric field in the slab model of an inhomogeneous cold plasma have been derived. It is shown that a strong wave electric field will be generated in the plasma edge by injecting LH wave of the power in MW magnitude, and this electric field will induce a poloidal rotation with a sheared poloidal velocity.

Surface erosion instabilities driven by edge plasma

The interaction of magnetized edge plasma with the material surface in the divertor region of a fusion device is analytically investigated. It is shown that the process of surface erosion is unstable against small perturbations of the surface shape, which leads to a gradual roughening of the initially smooth target surface. Two distinct reasons are studied for possible drivers of such erosion instabilities: modification of ion deposition flux and surface temperature variation. The wavenumbers of the unstable perturbations are determined from a linear approximation of the surface erosion equation. For the plasma parameters relevant to fusion reactors the estimated mean time of surface roughening is about a few hours. Consequently, the erosion instability is of importance for long pulse and steady state operation of future reactor-grade fusion devices

Magnetic domain structure in ultrathin cobalt films

The magnetic domain structures in ultrathin fcc cobalt films, epitaxially grown on Cu(100), have been investigated by means of a scanning electron microscope with spin polarization analysis of the secondary electrons. In the ‘as-grown’ state the films show a single domain configuration with small oppositely magnetized edge domains. Demagnetization of the films creates multidomain structures. In all films, ranging from 3 to 19.5 monolayer thickness, qualitatively the same domain structure has been found with the {110} directions as the easy axes of magnetization. The domain boundaries exhibit an irregular shape which is explained by a decreasing magnetostatic energy contribution to the Néel wall energy. Large domains with lateral extensions of some hundred microns dominate. A coinciding second phase with considerably smaller domains has been found occasionally in the films.

Magnetic domain structure in ultrathin ferromagnetic films

We have studied the magnetic domain structure of epitaxially grown cobalt on Cu(100) by means of a scanning microscope with spin polarization analysis of the secondary electrons. In the ‘as grown’ state the films show a single domain configuration with small oppositely magnetized edge domains. Demagnetizing the films creates multi-domain structures. The domain structure and shape is qualitatively the same for all thicknesses studied (3–19.5 monolayers). The magnetization of the domains is aligned with the 〈110〉 in-plane directions. By comparing the spin polarization data to Kerr effect measurements we determine the ‘magnetic probing depth’ to be about 3 monolayers.