Perpendicular Flux Research Articles

Experimental and numerical characterization of the turbulence in the scrape-off layer of MAST

Numerical simulations of interchange turbulence in the scrape-off layer are performed in a regime relevant for a specific L-mode Mega Ampere Spherical Tokamak (MAST) discharge. Such a discharge was diagnosed with a reciprocating arm equipped with a Gundestrup probe. A detailed comparison of the average and statistical properties of the simulated and experimental ion saturation current is performed. Good agreement is found in the time-averaged radial profile, in the probability distribution functions and in qualitative features of the signals such as the shape, duration and separation of burst events. These results confirm the validity of the simple interchange model used and help us to identify where it can be improved. Finally, the simulated data are used to assess the importance of the temperature fluctuations on plasma potential and radial velocity measurements acquired with Langmuir probes. It is shown that the correlation between the actual plasma quantities and the signal of the synthetic diagnostics is poor, suggesting that accurate measurements of the temperature fluctuations are needed in order to obtain reliable estimates of the perpendicular fluxes.

Numerical investigation of Scrape Off Layer anomalous particle transport for MAST parameters

Numerical simulations of L-mode plasma turbulence in the Scrape Off Layer of MAST are presented. Relevant features of the boundary plasma, such as the thickness of the density layer or the statistics of the fluctuations are related to the edge density and temperature, plasma current and parallel connection length. It is found that the density profile is weakly affected by the edge density, it broadens when the current or the temperature are decreased while the connection length has the opposite effect. The statistics of the turbulence is relatively insensitive to variations of all the edge parameters and show a certain degree of universality. Effective transport coefficients are calculated for several plasma conditions and display a strong nonlinear dependence on the parameters and on the radial variable. Finally, it is shown how the perpendicular particle fluxes in the Scrape Off Layer are related to the edge parameters.

3D harmonic model of the end region for turbogenerators

PurposeThe large size of models and long computing time prevent the creation of full‐scale, three‐dimensional models of end region of turbogenerators. Only exact three‐dimensional model can illustrate complex phenomena of end region losses. Also some methods of decreasing such losses cannot be simulated in two‐dimensional models. The purpose of this paper is to focus on a method of creating three‐dimensional models of turbogenerators' end regions for calculations of eddy current losses.Design/methodology/approachTime‐stepping is the most expensive part of computation. A harmonic model would be free from that disadvantage and it can provide a tool to make an accurate, fully three‐dimensional model of a steady state for different loads and provide results in a reasonable time.FindingsThe research focuses on the method of creating three‐dimensional models of turbogenerators end region for calculations of eddy current losses. By using two‐dimensional, time‐stepping models and empirical loss functions for a main flux and three‐dimensional models for eddy current losses from a perpendicular flux of an end connections, it is found that fast analysis of that complex part of a machine can be achieved.Originality/valueThe approach proposed in the paper is a universal and novel method of calculation losses of turbogenerators' end regions. Combining two‐dimensional and three‐dimensional models provides advantages of both known methods: fast computation time from simplified models and good representation of complex geometry of a machine.

Fluid and drift-kinetic description of a magnetized plasma with low collisionality and slow dynamics orderings. II. Ion theory

The ion side of a closed, fluid and drift-kinetic theoretical model to describe slow and macroscopic plasma processes in a fusion-relevant, low collisionality regime is presented. It follows the ordering assumptions and the methodology adopted in the companion electron theory [Ramos, Phys. Plasmas 17, 082502 (2010)]. To reach the frequency scale where collisions begin to play a role, the drift-kinetic equation for the ion distribution function perturbation away from a Maxwellian must be accurate to the second order in the Larmor radius. The macroscopic density, flow velocity and temperature are accounted for in the Maxwellian, and are evolved by a fluid system which includes consistently the gyroviscous part of the stress tensor and second-order contributions to the collisionless perpendicular heat flux involving non-Maxwellian fluid moments. The precise compatibility among these coupled high-order fluid and drift-kinetic equations is made manifest by showing that the evolution of the non-Maxwellian part of the distribution function is such that its first three velocity moments remain equal to zero.

Magnetic Field Distribution Around Superconducting Coils in Ferromagnetic Environment

There are two kinds of high temperature superconducting motor (HTS) according to the angle of the superconductor's shape, one with the superconducting bulk and the other with the superconducting tapes. Using the superconducting tapes, HTS motors, whose superconducting magnets are located in the rotor, have been researched by many researchers and institutes. This paper research a novel HTS structure that the superconducting coils are placed in the slots of the stator made of ferromagnetic materials. This paper researches the magnetic field distribution of the superconducting coil in the slots and teeth of the stator. The electromagnetic field distribution under the influence of rotating magnetic field and ferromagnetic material is presented firstly. Then, considering the effect of coupling of AC current that flow in the superconducting coils, the distribution of magnetic field is further analyzed. Finally, in order to reduce the alternating current loss, a ferromagnetic slab is added beside the superconducting coils and the improved magnetic field distribution is researched. The simulation result shows that this method is instrumental in harvesting a decrease of the perpendicular flux density from the 0.34 T to 0.06 T.

Two-dimensional Bloch electrons under strong magnetic modulation

The band structure of a high-mobility two-dimensional electron gas patterned with a square lattice of holes (antidots) is studied theoretically under the influence of a magnetic modulation consisting of perpendicular magnetic flux tubes with the same period and nonzero net flux per unit cell. The magnetic field pierces the system through the patterned holes only, so that the coupling with the electrons is purely quantum mechanical. The model takes implicitly into account the coupling between the different Bloch bands. The flux-dependent energy structure exhibits a Hofstadter butterfly-type spectrum. Such a structure is repeated indefinitely without distortion with a period of one magnetic flux quantum through a lattice hole. Rectangular deviations from the square lattice are also studied. It is found that the number and width of the magnetic gaps decrease, and even disappear for large antidot filling fractions.

Combined Aharonov–Bohm and Zeeman spin-polarization effects in a double quantum dot ring

A mesoscale Aharonov–Bohm (AB) ring with a quantum dot (QD) embedded in each arm is computationally modeled for unique transmission properties arising from a combination of AB effects and Zeeman splitting of the QD energy levels. A tight-binding Hamiltonian is solved, providing analytical expressions for the transmission as a function of system parameters. Transmission resonances with spin-polarized output are presented for cases involving either a perpendicular field, or a parallel field, or both. The combination of the AB-effect with Zeeman splitting allows sensitive control of the output resonances of the device, manifesting in spin-polarized states which separate and cross as a function of applied field. In the case with perpendicular flux, the AB-oscillations exhibit atypical non-periodicity, and Fano-type resonances appear as a function of magnetic flux due to the flux-dependent shift in the QD energy levels via the Zeeman effect.

Spin splitter regime of a mesoscopic Rashba ring

Using the non-equilibrium Greens' function formalism we calculate the spin currents in a one-dimensional ring coupled to three leads and in the presence of perpendicular magnetic flux Φ and Rashba spin–orbit coupling. A finite bias is applied between the input lead and the other two output leads. We show that the spin–orbit coupling allows one to operate this system as a spin splitter, i.e. the output leads deliver spin-polarized currents with different orientations. We find that the spin splitter operation can be tuned at integer multiples of Φ / Φ 0 . Its efficiency depends not only on the value of the Rashba coupling but also on the bias applied between the input and output leads. The selected spin orientation of the output leads can be reversed by a slight change of their contact position. We discuss as well the connection between the spin splitter operation and the spectral properties of the ring.

The heat flux pattern on HT-7 lower toroidal limiter

An expression for the heat flux on the lower toroidal limiter of HT-7 is derived according to the cosine law, taking into account of the ripple fields. The relationship between the heat flux pattern and the safety factor at LCFS is described. The self-shadow effect and perpendicular heat deposition are also taken into account. The perpendicular heat flux onto the limiter roof is analysed on the basis of the funnel effect, and a quantative result is obtained. In addition, the detailed 3D temperature distribution is simulated with COMSOL (FEMLAB).

Finite size effects on magnetic flux penetration into YBCO/LSMO hybrids

The attractive idea to create artificial superconductor/ferromagnet heterostructures (SC/FM) for easy control of the superconductor properties by magnetic field is widely considered last decade. Of a special interest for applications are the HTSC/FM heterostructures, particularly the YBCO/LSMO, where the magnetization value of LSMO could be adjusted by doping, by variation of oxygen content, and magnetic domain structure could be controlled by reasonable magnetic field. We concentrate on the in-plane field penetration into the YBCO/LSMO hybrid film, which is of practical interest as the in-plane field easier saturates the magnetic film. The study is performed by the magneto-optic visualization technique at T down to 7 K. We found a striking transformation of the in-plane external field into a wave of alternating perpendicular flux, the particular features of which depended on the temperature and magnetic prehistory at temperature above superconducting transition. To shed light on the mechanism of the effect, we have investigated the magnetic domain pattern of manganite film and it's transformations due to variation of temperature and the field. The results are discussed taking into account the finite size of the hybrid structure and the magnetostatic field distribution.

Logarithmic scaling of Lyapunov exponents in disordered chiral two-dimensional lattices

We analyze the scaling behavior of the two smallest Lyapunov exponents for electrons propagating on two-dimensional lattices with energies within a very narrow interval around the chiral critical point at $E=0$ in the presence of a perpendicular random magnetic flux. By a numerical analysis of the energy and size dependence we confirm that the two smallest Lyapunov exponents are functions of a single parameter. The latter is given by $\text{ln}\text{ }L/\text{ln}\text{ }\ensuremath{\xi}(E)$, which is the ratio of the logarithm of the system width $L$ to the logarithm of the correlation length $\ensuremath{\xi}(E)$. Close to the chiral critical point and energy $|E|⪡{E}_{0}$, we find a logarithmically divergent energy dependence $\text{ln}\text{ }\ensuremath{\xi}(E)\ensuremath{\propto}{|\text{ln}({E}_{0}/|E|)|}^{1/2}$, where ${E}_{0}$ is a characteristic energy scale. Our data are in agreement with the theoretical prediction of Fabrizio and Castelliani [Nucl. Phys. B 583, 542 (2000)] and resolve an inconsistency of previous numerical work.

Electron acceleration signatures in the magnetotail associated with substorms

We present Cluster multisatellite observations of accelerated electrons in the near‐Earth magnetotail associated with substorms. We found that the hardest electron energy spectra appear in the earliest stage of substorm expansion in the near‐Earth tail region and that they gradually become softer during the events. Enhancement of the high‐energy electron flux occurs generally associated with the bulk acceleration of ions (fast flow) and electrons. It is also shown that the high‐energy electrons sometimes show preferential perpendicular acceleration associated with the temporal enhancement of the normal component of the magnetic field, and then the anisotropic distribution quickly becomes isotropic. During the dipolarization interval, in which no convection signature is observed, perpendicular flux drops to less than the initial value, and the parallel flux is more than the perpendicular flux. The results suggest that the electron acceleration mechanism is mostly consistent with adiabatic betatron acceleration, while Fermi acceleration is not clear in the high‐energy part. The effect of the pitch angle scattering is also important. The dispersive signature of the high‐energy electron flux indicates fast dawnward drift loss, namely, the three‐dimensional effect of the limited plasma acceleration region.

Connections between plasma sheet transport, Region 2 currents, and entropy changes associated with convection, steady magnetospheric convection periods, and substorms

Here we describe how energy‐dependent magnetic drift in the presence of a pressure gradient in the direction of the drift leads to a divergence of perpendicular particle flux, and that this violates conservation of flux tube particle content. We address how, within the plasma sheet, this divergence of particle flux should be expected to lead simultaneously to the divergence of perpendicular current that drives the Region 2 (R2) current system and to significant violation of entropy conservation. The modeling results of Wang et al. (2004b) show that the above violation of entropy conservation due to magnetic drift, when taken together with magnetic field stretching, offers a resolution to the pressure crises question. On the basis of our argument that the same energy‐dependent magnetic drift effect leads to the perpendicular divergence that drives the R2 field‐aligned current system and the violation of entropy conservation, we suggest that the existence of the R2 current system can by itself be viewed as a signature of violation of entropy conservation. Finally, we propose that observational evidence suggests that an enhanced rate of entropy reduction and R2 currents resulting from particle divergence within the vicinity of the Harang reversal may be a critical aspect of the substorm expansion phase.

Exciton Storage in a Nanoscale Aharonov-Bohm Ring with Electric Field Tuning

We study analytically the optical properties of a simple model for an electron-hole pair on a ring subjected to perpendicular magnetic flux and in-plane electric field. We show how to tune this excitonic system from optically active to optically dark as a function of these external fields. Our results offer a simple mechanism for exciton storage and readout.

Physics‐based formula representations of high‐latitude ionospheric outflows: H+ and O+ densities, flow velocities, and temperatures versus soft electron precipitation, wave‐driven transverse heating, and solar zenith angle effects

Extensive systematic dynamic fluid kinetic (DyFK) model simulations are conducted to obtain advanced simulation‐based formula representations of ionospheric outflow parameters, for possible use by global magnetospheric modelers. Under F10.7 levels of 142, corresponding to solar medium conditions, we obtain the H+ and O+ outflow densities, flow velocities, and perpendicular and parallel temperatures versus energy fluxes and characteristic energies of soft electron precipitation, wave spectral densities of ion transverse wave heating, and F region level solar zenith angle in the high‐latitude auroral region. From the results of hundreds of DyFK simulations of auroral outflows for ranges of each of these driving agents, we depict the H+ and O+ outflow density and flow velocity parameters at 3 RE altitude at the ends of these 2‐h simulation runs in spectrogram form versus various pairs of these influencing parameters. We further approximate these results by various distilled formula representations for the O+ and H+ outflow velocities, densities, and temperatures at 3 RE altitude, as functions of the above indicated four “driver” parameters. These formula representations provide insight into the physics of these driven outflows, and may provide a convenient set of tools to set the boundary conditions for ionospheric plasma sources in global magnetospheric simulations.

Particle transfer in edge transport barrier with stochastic magnetic field

Charged particle losses at the plasma edge affected by resonant magnetic perturbations (RMP) are considered by taking into account the electron and ion flows both parallel and perpendicular to perturbed field lines. Calculations are done for H-mode plasmas of low collisionality, i.e., under conditions where significant pump out of particles has been observed in experiments on the DIII-D tokamak [J. Luxon, Nucl. Fusion 42, 614 (2002)] with RMP from the I-coils. It is demonstrated that the perpendicular ion flux, arising by magnetic field stochastization due to the deviation of poloidal rotation from the neoclassical one, is of more importance than the parallel ion flow. With both loss contributions included, computations provide a pump out level in agreement with observations if the screening of RMP by the plasma rotation is taken into account. The impact of possible enhancement in the perpendicular electron transport due to fluctuations observed with RMP in the edge transport barrier is assessed.

Particle and field characteristics of broadband electrons observed by the FAST satellite during geomagnetic storms: A multievent study

During geomagnetic storms, remarkable electron‐flux enhancements (>1012 eV cm−2 s−1) over a broadband energy range (∼0.05–10 keV) are sometimes observed near the equatorward edge of the auroral oval. We call such electron flux enhancements broadband electrons (BBEs). In this paper, we identified 12 BBE events from the electron‐energy spectra obtained by the Fast Auroral Snapshot (FAST) satellite during 81 large geomagnetic storms (minimum Dst index <−90 nT) between September 1996 and March 2004. Ground‐based magnetic field data show that the BBEs are observed ∼6–28 min after the onset of substorms during the main phase of large storms. During these events, the pitch angle distributions of electrons show isotropic features at a higher energy range above ∼1 keV, except for an upward loss cone. At a lower energy range below ∼1 keV, field‐aligned downward electron fluxes are most intense, and perpendicular fluxes are weakest. These results imply that a higher‐energy part of BBEs originates from higher altitudes in the inner magnetosphere and that a lower‐energy part is accelerated parallel to the local magnetic field at lower altitudes near the satellite. Intense fluctuations of electric and magnetic fields and enhanced lower‐frequency (0.1 Hz to 16 kHz) waves are observed during the BBEs. These characteristics are common for all BBE events. We compared lower‐energy (<1 keV) electron fluxes with field‐aligned Poynting fluxes of the waves at 0.06–4 Hz during the BBEs. The Poynting fluxes are systematically downward with intensity comparable to that of the lower‐energy electron fluxes, suggesting downward acceleration of lower‐energy electrons by the observed low‐frequency waves.

Eddy Current Analysis Considering Lamination for Stator Core Ends of Turbine Generators

In this paper, we introduce a calculation method for in-plane eddy currents produced by perpendicular flux at stator core-ends of turbine generators. In the method, the laminated core is subdivided into each electrical steel sheet by 3-D finite elements. The insulated layers of the sheets are modeled by the gap elements, which represent the magnetic resistance of the layers. The combination method of double nodes and anisotropic conductivity is proposed to consider the insulation between the steel sheets. It is especially required for the stator teeth with slits. The validity of the calculation method is verified by comparing the measured and calculated results in the case of a simple model that simulates the end part of the turbine generator. The method is also applied to the real machine to clarify the eddy current distribution.

Study on inverter fault-tolerant operation of PMSM DTC

This paper presents an investigation of inverter fault-tolerant operation for a permanent magnet synchronous motor (PMSM) direct torque control (DTC) system under various inverter faults. The performance of a faulty standard 6-switch inverter driven PMSM DTC system is analyzed. To avoid the loss or even disaster caused by the inverter faults, a topology-modified inverter with fault-tolerant capability is introduced, which is reconfigured as a 3-phase 4-switch inverter. The modeling of the 4-switch inverter is then analyzed and a novel DTC strategy with a unique nonlinear perpendicular flux observer and feedback compensation scheme is proposed for obtaining a continuous, disturbance-free drive system. The simulation and experimental results demonstrate that the proposed inverter fault-tolerant PMSM DTC system is able to operate stably and continuously with acceptable static and pretty good dynamic performance.

3D numerical simulations of energy transport in the stochastic boundary of TEXTOR-DED with a finite difference method

The effect of magnetic field line ergodization that eliminates magnetic surfaces (either by a resonant magnetic perturbation like in TEXTOR-DED or by intrinsic plasma effects like in W7-X) imposes the need for plasma transport models being able to describe this properly. To handle the ergodicity the concept of local magnetic coordinates allowing a correct discretization of the transport equations with minimized numerical errors is used. For the simulation of plasma transport in perturbed volume, a numerical method based on the finite difference concept has been developed, using a custom-tailored unstructured grid in local magnetic coordinates. This grid is generated by field line tracing to guarantee complete separation of the large parallel transport along B and that perpendicular to B and the ergodicity of the magnetic field does not limit applicability of the method in contrast to the methods based on finite volume ansatz. Perpendicular and parallel fluxes can be effectively separated in our approach and treated independently in the numerical method which has been implemented in the FINDIF code.The finite difference code FINDIF is used to investigate the energy transport in the complex 3D TEXTOR-DED tokamak geometry, where the plasma structures and transport are closely related to the structure of the magnetic field lines. Numerical grids have been prepared in order to simulate 12/4 and 6/2 modes of the DED operation, respectively. In particular, the question, what is the role of long and short magnetic field lines in the heat transfer from the core plasma to divertor surface, is addressed. Simulation results are compared with experimentally determined temperature profiles and heat fluxes at the target.