External Vertical Field Research Articles

Retraction Note to: Effects of Alfvenic Poloidal Flow and External Vertical Field on Plasma Position in IR-T1 Tokamak

Retraction Note to: Effects of Alfvenic Poloidal Flow and External Vertical Field on Plasma Position in IR-T1 Tokamak

Interface contact and modulated electronic properties by external vertical strains and electric fields in graphene/MoS2 heterostructure

Based to the first-principles calculations, we study the electronic properties of graphene/MoS2 heterostructure by modulating the vertical strains and applying external electric field. Graphene/MoS2 heterostructure is a van der Waals heterostructure (vdWH) with the interlayer spacing is 3.2 Å for the equilibrium state, and the contact property of the interface is n-type Schottky contact. The Schottky barrier height (SBH) changes with vertical strains which induces a change of charge transfer between graphene and MoS2 layer. In addition, with strain or without strain, the applied positive electric field can effectively promote the charge transfer from graphene to MoS2, while the negative electric field has the opposite effect. These findings support for the design of field effect transistors based on graphene vdWHs.

Graphene/GeTe van der Waals heterostructure: Functional Schottky device with modulated Schottky barriers via external strain and electric field

Vertical heterogeneous combinations of two-dimensional (2D) materials have been evident as an effective strategy to design novel photovoltaic and optoelectronic devices. Herein, we perform the first-principles calculations about electronic and optical properties of graphene/GeTe van der Waals heterostructures (vdWHs), accompanied by the efficient modulations about Schottky barriers via an external strain and electric field. A slightly opened band gap of 2.0 meV is obtained for graphene/GeTe vdWH in the equilibrium configuration, and the Dirac point is well preserved at K point. The built-in electric field caused by the charge transfer is devoted to its visible contribution to the efficient hindrance for photoexcited electron-hole pair recombination, consequently increasing the number of carriers and extending their lifetimes. The graphene/GeTe vdWH possesses enhanced optical absorption in the visible region in contrast to the separate GeTe monolayer, indicating its significant potential applications in optoelectronic devices. Besides, the graphene/GeTe vdWH at the equilibrium state exhibits an n-type Schottky contact with n-type Schottky barrier height (SBH) of 0.684 eV. By applying suitable external vertical strains or electric fields, the SBHs can be efficiently tuned, making it of significant possibility to realize the transportations between p-type Schottky contact and n-type Schottky contact, and between Schottky contact and Ohmic contact at the graphene/GeTe interface. These findings together predict the graphene/GeTe vdWH nanocomposite as a competitive candidate in next-generation optoelectronics and Schottky devices.

Controlled immobilization of IgG1 on carboxymethyl and amino-dextran SPR chips under external vertical electric fields

Aiming at producing surface plasmon resonance (SPR) sensor chips of improved performance, a novel approach for controlled immobilization of antibody (Ab) molecules has been developed through the implementation of surface functionalization and external vertical electric field (EVEF) assisted immobilization techniques. Carboxymethyl dextran (CMD) chips were reacted with ethylenediamine to create surface amino groups and subsequently reacted with the activated carboxyl group of the fragment crystallisable (Fc) region of Ab. An external vertical electric field was applied to control the movement of antibody molecules toward the chip and their orientation on the surface. The EVEF imparted a current density of 4.17 μA cm−2 through the solution. Immobilization was achieved via a carbodiimide reaction as the antibody molecules reached the functionalized surface. Physical characterization was performed through ATR-FTIR, AFM and surface plasmon resonance (SPR) analyses. Results show that the orientation of antibody molecule on the amino dextran (AmDex) surface is better than on the CMD surface in terms of antibody accessibility for interaction with toxin molecule. Moreover, applying EVEF during the antibody immobilization stage leads to an increased density of the antibody layer and enhanced accessibility of antibody binding domain due to improved orientation of antibody in the direction of the electric field. Discrete-Time-Fourier-Transformation analysis of the chip surface line profiles obtained by AFM analysis clearly show an increased order of Ab molecules after implementation of EVEF. The new findings obtained in this study are expected to be useful for controlling Ab molecule orientation on the immobilizing surface.

Wall modes in magnetoconvection at high Hartmann numbers

Three-dimensional turbulent magnetoconvection at a Rayleigh number of $Ra=10^{7}$ in liquid gallium at a Prandtl number $Pr=0.025$ is studied in a closed square cell for very strong external vertical magnetic fields $B_{0}$ in direct numerical simulations which apply the quasistatic approximation. As $B_{0}$, or equivalently the Hartmann number $Ha$, are increased, the convection flow, which is highly turbulent in the absence of magnetic fields, crosses the Chandrasekhar linear stability limit for which thermal convection ceases in an infinitely extended layer and which can be assigned a critical Hartmann number $Ha_{c}$. Similar to rotating Rayleigh–Bénard convection, our simulations reveal subcritical sidewall modes that maintain a small but finite convective heat transfer for $Ha>Ha_{c}$. We report a detailed analysis of the complex two-layer structure of these wall modes, their extension into the cell interior, and a resulting sidewall boundary layer composition that is found to scale with the Shercliff layer thickness.

Tunable electric properties of bilayer InSe with different interlayer distances and external electric field

Using density functional theory we explore the band structure of bilayer Indium selenide (InSe), and we find that the van der Waals interaction has significant effects on the electric and optical properties. We then explore the tuning electronic properties by different interlayer distances and by an external vertical electric field. Our results demonstrate that the band gaps of bilayer InSe can be continuously tuned by different interlayer coupling. With decreasing interlayer distances, the tunable band gaps of bilayer decrease linearly, owing to the enhancement of the interlayer interaction. Additionally, the band structure of bilayer InSe under external vertical fields is discussed. The presence of a small external electric field can make a new spatial distribution of electron–hole pairs. A well separation based on the electrons and holes, localized in different layers can be obtained using this easy method. These properties of bilayer InSe indicates potential applications in designing new optoelectronic devices.

Electronic and transport properties for Ti3C2O2 under the influence of a vertical electric field and stacking number

The effect of a vertical electric field on the structural, electronic, and transport properties of Ti3C2O2 with varying numbers of layers was systematically investigated by means of the first-principles method. This result demonstrated that the related properties of Ti3C2O2 could be effectively tuned by the stacking number and the vertical electric field. When the vertical electric field increased to 2 V/Å, the energy bands of the monolayer and multilayers of the Ti3C2O2 were separated obviously. However, the external vertical field could not open a band gap in the two dimensional Ti3C2O2. Its transmission coefficients and thermoelectric power factors increased linearly with the increase of the stacking number. It revealed that the trilayer Ti3C2 had the potential of being a thermoelectric material. The transport properties for a dual-gated Ti3C2O2 model were examined using non-equilibrium Green’s functions. The results clearly confirmed that the vertical electric field could manipulate the transport properties of Ti3C2O2, and the corresponding mechanisms are also discussed in detail.

Behavior of Conducting Liquid within an Arc Furnace in a Vertical Magnetic Field

In a vertical magnetic field, bulk electromagnetic forces arise in conducting melt within the bath of a dc arc furnace. As a result, the melt is set in motion. The flow of slag and metal in the furnace bath may lead to effective mixing but may also have negative consequences, such as increased lining wear in the region of the hearth electrode. There has been little research on conductive mixing in the bath of a dc arc furnace. Theoretical concerns include the character of the flow in the bath under the action of magnetic fields of specific magnitude; practical considerations include the lack of simple and reliable sources of magnetic fields. In the present work, the utility of a transparent physical model in studying the flow of conducting liquid in an external vertical magnetic field is investigated. The applicability of the modeling results to processes in the 5-t bath of an industrial dc arc furnace is analyzed. It proves possible in principle to study the flow of conducting melt in external vertical magnetic fields on models based on transparent nonmetallic conducting liquids. The use of an aqueous solution of table salt permits assessment of the liquid velocity at its free surface and close to the hearth electrode by video recording. By physical modeling of the flow of conducting fluid in the bath under the action of an external vertical magnetic field, with different switching sequences of the hearth electrode and different currents in the bath, it is possible to establish the character of the liquid flow when the hearth electrode is at the center of the bath or somewhat displaced. The mean rate of liquid rotation in the horizontal plane is increased when the hearth electrode is at some distance from the bath axis. The strength of the vertical magnetic field producing conductive motion in the metal bath of a 5-t dc arc furnace is estimated: around 5 kA/m.

Pre-ionization by AC Ohmic coil operation in the TST-2 spherical tokamak

AC Ohmic coil operation experiments with frequencies up to 10 kHz were performed on the TST-2 spherical tokamak device, and the pre-ionization process was studied in detail. The minimum loop voltage for pre-ionization was 0.4 V, which corresponds to 0.5 V at the inboard limiter. Dependences of growth rate and saturation level of the process on various parameters were obtained, and they are compared with a time-dependent 0-dimensional model based on Townsend avalanche and loss along field lines. Most of the dependences are reproduced qualitatively by the model, and quantitative differences are within a factor of several. However, the external vertical field dependence of the appearance time, which is defined as the time to observe a plasma, and the isotope effect cannot be reproduced by the model. An ambipolar diffusion state which is predicted theoretically but mitigated experimentally is discussed. It was found that secondary electron emission at the limiter surfaces is a candidate mechanism to mitigate the state.

What happens to the vortex structures when the rising bubble transits from zigzag to spiral?

It has been demonstrated by many experiments carried out over the last 60 years that in certain liquids a single millimetre-sized bubble will rise within an unstable path, which is sometimes observed to transit from zigzag to spiral. After performing several groups of direct numerical simulations, the present work gives a theoretical explanation to reveal the physical mechanism causing the transition, and the results are presented in two parts. In the first part, in which a freely rising bubble is simulated, equal-strength vortex pairs are observed to shed twice during a period of the pure zigzag path, and this type of motion is triggered by the amounts of streamwise vorticities accumulated on the bubble interface, when a critical value is reached. However, when the balance between the counter-rotating vortices is broken, an angular velocity is induced between the asymmetric vortex pairs, driving the bubble to rise in an opposite spiral path. Therefore, although there is no preference of the spiral direction as observed in experiments, it is actually determined by the sign of the stronger vortex thread. In the second part, external vertical magnetic fields are imposed onto the spirally rising bubble in order to further confirm the relations between the vortex structures and the unstable path patterns. As shown in our previous studies (Zhang & Ni, Phys. Fluids, vol. 26 (10), 2014, 102102), the strength of the double-threaded vortex pairs, as well as the imbalance between them, will be weakened under magnetic fields. Therefore, as the vortex pairs become more symmetric, the rotating radius of the spirally rising bubble is observed to decrease. We try to answer the question, put forward by Shew et al. (2005, Preprint, ENS, Lyon), ‘what caused the bubble to transit from zigzag to spiral naturally?’

Shift in principal equilibrium current from a vertical to a toroidal one towards the initiation of a closed flux surface in ECR plasmas in the LATE device

In toroidal electron cyclotron resonance (ECR) plasmas under a weak external vertical field a part of the pressure driven vertical charge separation current returns along the helical field lines, generating a toroidal current. The rest circulates via the conducting vacuum vessel. Only the toroidal current contributes to the production of a closed flux surface. Both the toroidal and vertical currents are an equilibrium current that provides a radial force by the interaction with the vertical field and the toroidal field, respectively, to counter-balance the outward pressure ballooning force.We have done experiments using 2.45 GHz microwaves in the low aspect ratio torus experiment (LATE) device to investigate in what way and how much the toroidal current is generated towards the initiation of a closed flux surface. In steady discharges by kW under various both the pressure and the toroidal current become large with . When G, a toroidal current of 290 A is generated and the vertical field is reduced to 1.2 G inside the current channel, being close to the initiation of a closed flux surface. In this plasma the return current does not obey Ohm’s law. Instead, the return current flows so that the electric force on the electron fluid is balanced with the pressure gradient along the field lines. Near the top and bottom boundaries superthermal electrons flow beyond the potential barrier onto the walls along the field lines. In another discharge by the low power of kW under G, both the toroidal current and the pressure steadily increase for an initial duration of 1.1 s and then abruptly jump, generating an initial closed flux surface. While the counter force from the vertical current is initially dominant, that from the toroidal current gradually increases and becomes four times larger than that from the vertical current just before the initiation of a closed flux surface. The results suggest that the conversion ratio to the return current along the field lines from the vertical charge separation current increases as the electron temperature increases. Upon initiation of a closed flux surface the counter force from the toroidal current also jumps by three times and becomes 10 times larger than that from the vertical current.

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)

Influence of Mo capping layer on magnetic anisotropy of MgO/CoFeB/Mo

In this paper, the influence of Mo capping layer on magnetic anisotropy of MgO/CoFeB/Mo with varying thickness is studied. It is found that Mo capping layer shows more saturated magnetic moments than Ta capping layer. The direction of the external magnetic field has a great influence on the magnetic anisotropy. The MgO/CoFeB/Mo sample prepared in an applied magnetic field parallel to the plane shows in-plane magnetic anisotropy (IMA). IMA becomes weak as the CoFeB thickness decreases, and it still exists when the thickness decreases to 1.1 nm. At the same time, the saturation field vertical to the plane decreases. When the thickness of CoFeB layer decreases to 0.9 nm or less, the IMA disappears. In our study, the saturated magnetization and magnetic dead layer are 1600 emu/cm3 and 0.26 nm at the annealing temperature 200 ℃, and the interface anisotropy is 0.91 erg/cm2, which is smaller than previous research results. Increasing the annealing temperature helps the sample keep the saturated state under a small magnetic field vertical to the plane, and makes IMA weak and transform into PMA. The variation of the Mo capping layer thickness affects the saturation magnetic moment of the sample. The magnetic moment shows a sharp downtrend when the Mo layer is between 1.2 and 1.6 nm, then it turns stabler with Mo capping layer thickening. Meanwhile, when the Mo capping layer is 1.6 nm, the external vertical saturation field becomes smallest. However under the parallel magnetic field, changing the thickness or annealing temperature, or changing both leads to no PMA occurring. When the magnetic field direction changes from parallel to vertical direction, some of the samples show PMA after the annealing process. The magnetic anisotropy of MgO/CoFeB/Mo varies with the thickness of Mo capping layer. IMA is present when the Mo capping layer is 1 nm or less while PMA is present when the Mo capping layer is between 1.2 and 5 nm. The sample coercive force in the vertical direction varies with thickness, and its magnetic hysteresis loss is much larger when the thickness of Mo capping layer is 1.4 nm.

Open field equilibrium current and cross-field passing electrons as an initiator of a closed flux surface in EC-heated toroidal plasmas

A model for the non-inductive initiation of a closed flux surface observed in electron cyclotron (EC) heated toroidal plasmas is presented. First, a pressure-driven equilibrium toroidal current develops under a weak external vertical field so as to counter balance the pressure-ballooning and current-hoop forces. When the self-field from the current almost cancels out the external vertical field, a forward energetic part of electrons in the velocity space begins to make cross-field passing (CFP) orbits. The CFP electrons are generated by the EC heating of bulk electrons and subsequent pitch-angle scattering, which is analyzed using the Fokker–Planck equation. They provide an additional current that closes the field lines. The model is examined for experiments in the small low aspect ratio device of LATE and in the large conventional device of JT-60U with a search for appropriate modes of EC heating. Simultaneous coincidence of the model with these two experiments is obtained in terms of microwave power and driven current. The results predict that initiation of a closed flux surface requires more and more EC power as the plasma major radius increases. In particular, careful injection of high N‖ EC waves is needed for large devices, both for initiation of a closed flux surface and for subsequent enlargement of the flux surface by the usual EC current drive onto the closed flux area.

Analytical Technique for Determination of Toroidal Plasma Displacement

An analytical technique for the determination of toroidal plasma displacement is presented. First, the plasma horizontal position is calculated from the external vertical field coil characteristics. The calculation is made focusing on the external vertical field coil current and voltage changes due to a horizontal displacement of plasma column. Also for comparison of result, a set of magnetic probes were designed, constructed and used. The results from these two techniques are compared and discussed.

Study of Electron Orbits for Formation of Toroidal Closed Flux Surface by ECH

Electron orbits under the external vertical field (BV) and the self poloidal field of the toroidal plasma current in a toroidal geometry are investigated by using analytic model fields in order to search for the conditions of occurence of appropriate confinement asymmetry of fast electrons along the field lines. This asymmetry efficiently and quickly generates a toroidal current, which may close the field lines in ECH-started plasmas. It is analytically shown that the characteristics of confinement asymmetry depend on the product of the major radius and the strength of the external vertical field. The results suggest that in large devices BV should be lowered and also some artificial means to speed up the pitch angle scattering for the fast electrons at a higher energy range are beneficial to generate the toroidal current in order to close the filed lines.

Determination of the plasma position for its real-time control in the COMPASS tokamak

An efficient horizontal and vertical stabilization of the plasma column position are essential for a reliable tokamak operation. Plasma position is generally determined by plasma current, plasma pressure and external vertical and horizontal magnetic fields. Such fields are generated by poloidal field coils and proper algorithm for the current control have to by applied, namely, in case of fast feedback loops. This paper presents a real-time plasma position reconstruction algorithms developed for the COMPASS tokamak. Further, its implementation in the MARTe (Multithreaded Application Real-Time executor) is described and the first results from test of the algorithm for real-time control of horizontal plasma positions are presented.

Characteristics of MHD Equilibrium and Related Issues on LHD

In the vacuum of the Large Helical Device (LHD), we can change the plasma volume, the aspect ratio, the ellipticity, the rotational transform, and the height of the magnetic hill through the control of the vertical and the qudrupole components of the magnetic field and the helical coil pitch parameter. The two effects of the finite beta on the magnetohydrodynamic (MHD) configuration, the magnetic surface torus outward shift and the invasion of the stochastic region into the plasma core, are discussed. The former is qualitatively the same as that by the external vertical field control. According to the comparison between a theoretical prediction in the finite beta and the vacuum field calculation in the vertical field control, the latter effect is strongly affected by the nonaxisymmetric component of the equilibrium current. A theoretical prediction suggests that an MHD equilibrium beta limit different from the conventional one exists due to the lack of the equilibrium force balance in the stochastic region. The key parameters to improve the accuracy of the identification of the MHD equilibrium configuration are shown to be the identification of the toroidal current profile, the anisotropic pressure effect, and the identification of the plasma boundary shape.

RETRACTED ARTICLE: Effects of Alfvenic Poloidal Flow and External Vertical Field on Plasma Position in IR-T1 Tokamak

We analyzed dynamic equilibrium properties of a large aspect ratio and low Beta tokamaks, in particular deriving a modified relation for the Shafranov shift in the presence of poloidal flow and external vertical field, and demonstrate it experimentally on the IR-T1 tokamak. Poloidal flow can produce modifications in the equilibrium properties. By increasing Alfvenic Mach number from zero, flow produce outward force, and plasma shifted in outward direction. If the poloidal Alfvenic Mach number equal to one, singularity will observe in the solution of generalized Grad-Shafranov equation. Also inversion of Shafranov shift in the transition of flow speed between sub-Alfvenic to super-Alfvenic speeds can be observed due to inward force produced by flow.

Current Ramp-up by Microwave Power in the LATE Device

In the low aspect ratio torus experiment (LATE) device, plasma current is initiated and ramped up to 20 kA solely by microwave power at the electron cyclotron (EC) range of frequency with a ramp of the external vertical field Bv for the radial equilibrium of plasma torus at larger currents. Measurements suggest that an energetic electron tail in the energy range of about 200 keV carries the current. The line averaged electron density is higher than the plasma cutoff density, suggesting that tail electrons might be driven by electron Bernstein waves mode-converted from the launched electromagnetic waves.