Magnetic Field Limit Research Articles

Limits of strong magneto-convective fluctuations in liquid metal flow in a heated vertical pipe affected by a transverse magnetic field

It is known from experimental and numerical studies that in channel flows of liquid metals, strong buoyancy forces, provided by temperature gradients, generate in magnetic field specific structures, which manifest themselves in the form of large-scale magneto-convective fluctuations (MCFs), accompanied by temperature fluctuations of anomalously large amplitude. The specific structure of MCFs depends on the properties of the liquid metal, on the channel configuration and its orientation to gravity and to the magnetic field, and on the combination of governing parameters of the flow. In this work we made the first attempt to define the domain of existence of MCFs in the parameter space for one specific configuration of the heated magnetohydrodynamic (MHD) flow. We studied experimentally a downward flow of mercury in the one-sided heated pipe in a transverse magnetic field, which can be considered as a first approximation to channels within a blanket module of a tokamak. We found that for this configuration, the weak buoyancy limit is defined by the critical Richardson number Rimin≈0.08, which indicates the value of required buoyancy, which should be strong enough to manifest itself in MCFs. The weak magnetic field limit is defined by the critical Stuart number Nmin≈1.5, which indicates the value of the required magnetic field that affects the developed turbulence and provides the transition from homogeneous turbulence to MCFs. The high magnetic field limit, which determines the boundary of MFs suppression by a strong magnetic field, depends on the flow rate and on the heating rate and has a clearer configuration in terms of a Reynolds number Rh, defined through the Hartmann layer thickness. At moderate Richardson numbers, the boundary is confined by a line Ri=6/Rh. With further increase of Ri, the border line rises more steeply and tends to an absolute upper limit Rh−1=2/25, above which the magnetic field completely laminarizes the flow for any buoyancy forces. We have shown, that MCFs strongly affect the heat transport, being able to essentially enhance it.

Parallel Propagating Waves in Weakly Magnetized Relativistic Electron Plasma

The parallel propagating waves (Langmuir, R and L waves) are studies in different plasma environments by choosing the different values of $\eta =({mc^{2}}/{k_{B}T})$ and $({\omega ^{2}_{\mathrm{ pe}}}/{\omega ^{2}_{\mathrm{ ce}}})$ . The dispersion relations of these waves are derived under a weak magnetic field limit for a fully relativistic plasma environment. The analytical solution of the integrals in the dispersion relations is not possible, so we use a numerical quadrature approach for their solution. We observe that in both relativistic and weakly relativistic plasmas, these modes shifted to lower values of frequency. The cutoff points of R- and L-waves shifted to further lower values in high-density plasma compared to low density.

Thermal transport in a weakly magnetized hot QCD medium

The thermal transport coefficients in a weakly magnetized quark-gluon plasma have been investigated within the ambit of a quasiparticle model to encode the effects of the realistic equation of state. The presence of a weak magnetic field leads to the Hall-type conductivity associated with thermal transport in the medium. An effective covariant kinetic theory has been employed to quantify the thermal dissipation while incorporating the mean field contributions in the medium. The interplay of thermal transport and electric charge transport in the weakly magnetized medium has been explored in terms of the Wiedemann-Franz law. Strong violation of the Wiedemann-Franz law has been observed in temperature regimes near to the transition temperature. The behaviour of thermal conductivity in the strong magnetic field limit has also been studied. It is observed that both the magnetic field and equation of state have a significant impact on the thermal dissipation in the medium.

Asymptotics of the three-dimensional Vlasov equation in the large magnetic field limit

We study the asymptotic behavior of solutions to the Vlasov equation in the presence of a strong external magnetic field. In particular we provide a mathematically rigorous derivation of the guiding-center approximation in the general three dimensional setting under the action of large inhomogeneous magnetic fields. First order corrections are computed and justified as well, including electric cross field, magnetic gradient and magnetic curvature drifts. We also treat long time behaviors on two specific examples, the two dimensional case in carte-sian coordinates and a poloidal axi-symmetric geometry, the former for expository purposes. Algebraic manipulations that underlie concrete computations make the most of the linearity of the stiffest part of the system of characteristics instead of relying on any particular variational structure.

Scattering cross-section under external magnetic field using the optical theorem

The cross-section for the lowest order $2\rightarrow2$ elastic scattering between two charged scalars under external magnetic field mediated via a neutral scalar, has been computed in strong as well as weak magnetic field limits. This has been done by applying the optical theorem where the cross-section is expressed in terms of the imaginary parts of different one-loop graphs contributing to the forward scattering amplitudes. The modification in the amplitudes due to the external magnetic field has been done by means of replacing the charged scalar propagators with the Schwinger proper-time ones. Significant modifications of the cross-sections with respect to the vacuum cross-section are observed due to the external magnetic field.

Thermal corrections to the gluon magnetic Debye mass

We compute the gluon polarization tensor in a thermo-magnetic environment in thestrong magnetic field limit at zero and high temperature. The magnetic field effects are introduced using Schwinger's proper time method. Thermal effects are computed in the HTL approximation. At zero temperature, we reproduce the well-known result whereby for a non-vanishing quark mass, the polarization tensor reduces to the parallel structure and its coefficient develops an imaginary part corresponding to the threshold for quark-antiquark pair production. This coefficient is infrared finite and simplifies considerably when the quark mass vanishes. Keeping always the field strength as the largest energy scale, in the high temperature regime we analyze two complementary hierarchies of scales: $q^2<< m_f^2<< T^2$ and $m_f^2<< q^2<< T^2$. In the latter, we show that the polarization tensor is infrared finite as $m_f$ goes to zero. In the former, we discuss the thermal corrections to the magnetic Debye mass.

Pion pressure in a magnetic field

While the partition function for QCD in a magnetic field $H$ has been calculated before within chiral perturbation theory up to two-loop order, our investigation relies on an alternative representation for the Bose functions which allows for a clear-cut expansion of thermodynamic quantities in the chiral limit. We first focus on the pion-pion interaction in the pressure and show that -- depending on magnetic field strength, temperature and pion mass -- it may be attractive or repulsive. We then analyze the thermodynamic properties in the chiral limit and provide explicit two-loop representations for the pressure in the weak magnetic field limit $|qH| \ll T^2$.

Modelling of RFX-mod2 tokamak equilibria with DEMO-like shape conditions and negative triangularity

In this paper, DEMO-like shaped plasma equilibria are defined for RFX-mod2 tokamak operations by using the Inverse Equilibrium Tool (IET code). IET allows for the computation of the coil currents needed to obtain a predetermined plasma shape with well defined plasma global parameters (i.e. total plasma current and total poloidal magnetic flux at the boundary) by solving a constrained minimization problem. The new shape conditions would allow achieving higher plasma current and plasma density values at the same toroidal magnetic field and safety factor limits of previous RFX-mod tokamak operations. The feasibility of these new equilibria is explored in terms of coil current requirements and vertical stability analysis. This study shows that RFX-mod2 is a flexible device, able to perform DEMO-like shaped tokamak operations with low requirements on both magnitude and distribution of active coil currents.

Modeling of spherical dust charging in collisionless magnetized plasmas with DiMPl

Determining the equilibrium charge of conducting spheres in plasmas is important for interpreting Langmuir probe measurements, plasma surface interactions, and dust particle behavior. The Monte Carlo code Dust in Magnetised Plasmas (DiMPl) has been developed for the purpose of determining the forces and charging behavior of conducting spheroids under a variety of conditions and benchmarked against previous numerical results. The floating potentials of spheres in isothermal, collisionless, hydrogen plasmas as a function of magnetic field strength and size relative to the Debye length are studied using DiMPl and compared with new results from the N-body tree code (pot) and recent particle in cell measurements. The results of all three simulations are similar, identifying a small range at modest ion magnetization parameters over which the electron current is reduced relative to the ion current. The potential as a function of magnetic field strength is found to be relatively insensitive to dust size for dust smaller than the Debye length. The potential of large dust is found to depend less strongly on flow speed for modest magnetic field strengths and to decrease with increasing flow speed in the presence of strong magnetic fields for smaller dust. A semi-empirical model for the potential of small dust in a collisionless plasma as a function of magnetic field strength is developed, which reproduces the expected currents and potentials in the high and low magnetic field limit.

Lorenz gauge formulation for time-dependent density functional theory

We describe the inclusion of electrodynamic fields in Time-Dependent Density Functional Theory (TDDFT) by incorporating both the induced scalar and vector potentials within the time-dependent Kohn-Sham equation. The Hamiltonian is described in both the Coulomb and Lorenz gauges, and the advantages of the latter are outlined. Integral expressions are defined for the retarded potentials of each gauge and a methodological approach to evaluate these nontrivial expressions with low computational cost is adopted. Various molecular structures of relatively small sizes are studied, including water, benzene, and conductive carbon chains. Absorption cross sections resulting from both pulse and boost excitations suggest a preserved gauge-invariance of the proposed formal approach to TDDFT in the weak magnetic field limit.

Limit of Thermal-Vibration Noise in Magnetic Field Detection with Magnetoelectric-Composite Cantilevers

The fundamental thermal limit of magnetic field sensing using the resonance-enhanced magnetoelectric (ME) effect in strain-coupled ferroic composite cantilevers is investigated theoretically with results applied to $({\mathrm{Fe}}_{90}{\mathrm{Co}}_{10}{)}_{78}{\mathrm{Si}}_{12}{\mathrm{B}}_{10}/\mathrm{Si}/\mathrm{Al}\mathrm{N}$ using experimental material parameters. An analytic theory of the ME response and noise from thermal vibrations, Johnson-Nyquist noise from dielectric losses in $\mathrm{Al}\mathrm{N}$, and amplifier electronics is given to yield the magnetic field detection limit for the first bending mode. A systematic investigation of scaling behavior of ME response, sensitivity, noise components, and detection limit with cantilever size, layer thicknesses, quality factor ${Q}_{f}$, and resonance frequency $f$ is presented emphasizing the low-frequency small-size regime essential for biomagnetic sensing applications. Thermal vibrations are found to dominate all investigated noise sources for practical composite sizes except at low quality factors. Detection limit scaling inversely with functional layer thickness, as $1{/\mathrm{size}}^{2}$ for constant $f$ and ${Q}_{f}$, and as ${Q}_{f}^{\ensuremath{-}1/2}$ (constant size and $f$) is found. Achievable detection limits below $100\phantom{\rule{0.2em}{0ex}}\mathrm{fT}/\mathrm{H}{\mathrm{z}}^{1/2}$ are predicted for $({\mathrm{Fe}}_{90}{\mathrm{Co}}_{10}{)}_{78}{\mathrm{Si}}_{12}{\mathrm{B}}_{10}/\mathrm{Si}/\mathrm{Al}\mathrm{N}$ composite cantilevers at sizes below $10\phantom{\rule{0.2em}{0ex}}\mathrm{mm}$ (and even lower without size constraint) with $f=1\phantom{\rule{0.2em}{0ex}}\mathrm{kHz}$, ${Q}_{f}=1000$, and $({\mathrm{Fe}}_{90}{\mathrm{Co}}_{10}{)}_{78}{\mathrm{Si}}_{12}{\mathrm{B}}_{10}$ layers of $50\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{m}$. The scaling behavior implies the absence of an absolute detection limit for thick ferroic layers within the range of validity of the experimental material parameters observed at $\ensuremath{\sim}2\dots{}4\phantom{\rule{0.1em}{0ex}}\ensuremath{\mu}\mathrm{m}$.

Magneto-vortical effect in strongly coupled plasma

Based on a holographic model incorporating both the chiral anomaly and the gravitational anomaly, we study the effect of magneto-vortical coupling on the transport properties of a strongly coupled plasma. The focus of present work is on the generation of a vector charge density and an axial current, as response to vorticity in a magnetized plasma. The transport coefficients parameterizing the vector charge density and axial current are calculated both analytically (in the weak magnetic field limit) and also numerically (for general values of the magnetic field). We find the generation of vector charge receives both non-anomalous and anomalous contributions, with the non-anomalous contribution dominating in the limit of a strong magnetic field and the anomalous contribution sensitive to both chiral anomaly and gravitational anomaly. On the contrary, we find the axial current is induced entirely due to the gravitational anomaly, thus we interpret the axial current generation as chiral vortical effect. The corresponding chiral vortical conductivity is found to be suppressed by the magnetic field. By the Onsager relation, these transport coefficients are responsible for the generation of a thermal current due to a transverse electric field or a transverse axial magnetic field, which we call the thermal Hall effect and the thermal axial magnetic effect, respectively.

Effect of magnetic field environment on the performance of 3/2 solenoid valve

• 3/2 solenoid valve functionality has been checked under external magnetic field environment. • External magnetic field limit has been defined below which 3/2 solenoid valve functions well. • Magnetic shielding of the 3/2 solenoid valve has been proposed. ITER, an experimental fusion reactor uses solenoid valves to close cryogenic control valves instantly in case of any incidental event in the ITER cryogenic system takes place. Many of these solenoid valves are functioning with the magnetic field environment of the order of ∼25 m T. Since a solenoid valves is an electrically activated valve, presence of the environment magnetic field can lead to partial or non-closing of the valve. In this paper, the effect of magnetic field environment on the performance of 3/2 solenoid valves through simulation and experiment is studied. The variable external magnetic field is generated by supplying variable power from Switched Mode Power Supply (SMPS) to the magnetic coil and magnetic field readings are recorded with a tesla meter. The validation of the magnetic field profile of the magnetic coil is done by comparing the E-mag analysis simulation performed on ANSYS® platform and the experimental results. An experiment setup, inside ITER-India cryogenic laboratory, has been assembled to see the functionality of the 3/2 solenoid valve in the presence of an external magnetic field. A 3/2 solenoid valve is placed in the external magnetic field which has been generated with the help of an air core inductor magnetic coil. The external magnetic field from the coil is varied from 10 m T to 35 m T with the help of a variable power supply and checked the functionality of the solenoid valve in different orientation and different external magnetic fields strengths. The outcome of the study shows that the 3/2 solenoid valve stopped functioning over the external magnetic field strength of 14.29 m T. The paper also concludes the requirement of the magnetic shielding and correct orientation of the solenoid valves to be installed in order to nullify the effect of external magnetic field on the functionality of the 3/2 solenoid valves.

Observation of a thermoelectric Hall plateau in the extreme quantum limit

The thermoelectric Hall effect is the generation of a transverse heat current upon applying an electric field in the presence of a magnetic field. Here, we demonstrate that the thermoelectric Hall conductivity αxy in the three-dimensional Dirac semimetal ZrTe5 acquires a robust plateau in the extreme quantum limit of magnetic field. The plateau value is independent of the field strength, disorder strength, carrier concentration, or carrier sign. We explain this plateau theoretically and show that it is a unique signature of three-dimensional Dirac or Weyl electrons in the extreme quantum limit. We further find that other thermoelectric coefficients, such as the thermopower and Nernst coefficient, are greatly enhanced over their zero-field values even at relatively low fields.

Numerical simulation of the wave turbulence on the surface of a ferrofluid in a horizontal magnetic field

The turbulence of capillary waves on the surface of a ferrofluid with a high permeability in a horizontal magnetic field is considered in the framework of a one-dimensional weakly nonlinear model. In the limit of a strong magnetic field, the surface waves under study can propagate without distortions along or against the direction of external field, i.e., similar to Alfvén waves in a perfectly conducting fluid. The interaction of counter-propagating nonlinear waves leads to the development of wave turbulence on the surface of the liquid. The computational data show that the spectrum of turbulence is divided into two parts: a low-frequency dispersionless region, where the magnetic forces dominate and a high-frequency dispersive one, in which the influence of capillary forces becomes significant. In the first region, the spectrum of the surface elevation has the same exponent in k and ω domains and its value is close to -3.5, what is in a good agreement with the estimation obtained from the dimensional analysis of the weak turbulence spectra. At the high frequencies, the computed spatial spectrum of the surface waves is close to k-5/2 which corresponds to ω-5/3 in terms of the frequency. This spectrum does not coincide with the Zakharov-Filonenko spectrum obtained for pure capillary waves. A possible explanation of this fact is in the influence of coherent structures (like shock waves) usually arising in media with weak dispersion.

Chiral kinetic theory from Landau level basis

We derive a chiral kinetic theory with Landau level basis, which is valid for slow-varying magnetic field with arbitrary magnitude. We apply the new chiral kinetic theory to calculate the electric conductivity transverse to the magnetic field in a magnetized QED and QCD plasma. Under the lowest Landau level approximation and relaxation time approximation, we find the transverse conductivity approaches a constant in the large magnetic field limit and is inversely proportional to the relaxation time. We also obtain a frequency-dependent transverse conductivity in response to a time-dependent electric field. We find a high frequency enhancement in this conductivity.

Prediction and Implementation of Graphene and Other Two-Dimensional Material Based Superconductors: A Review

This article comprehensively reviews the theoretical attempts toward prediction and subsequent realization of superconductivity in graphene and other two-dimensional (2-D) elemental materials and their compounds. From the large family of 2-D materials, a potential set of candidates were categorized by the researchers employing first principle study, mostly with density functional theory as a tool. The effect of different external chemical and physical parameters such as charge doping (through intercalation, substitution, electric field, etc.), pressure, and strain on superconducting transition of these materials was also summarized. Due to severe difficulty in the implementation techniques, only a handful of experimental authentications were reported by researchers so far. Experimental investigations on the effect of influencing external parameters mentioned above were also in a nascent state. Researchers are in search of “unconventional superconductivity” and “Ising effect” in graphene and other 2-D materials, which can eventually lead to higher critical temperature and higher limit of in-plane magnetic field. However, the implementation across the whole spectrum is still a crucial challenge.

Thermal diffusion and quantum chaos in neutral magnetized plasma

We calculate the thermal diffusion constant $D_T$ and butterfly velocity $v_B$ in neutral magnetized plasma using holographic magnetic brane background. We find the thermal diffusion constant satisfies Blake's bound. The constant in the bound $D_T2\pi T/v_B^2$ is a decreasing function of magnetic field. It approaches one half in the large magnetic field limit. We also find the existence of a special point defined by Lyapunov exponent and butterfly velocity on which pole-skipping phenomenon occurs.

High-speed shear-driven dynamos. Part 2. Numerical analysis

This paper aims to numerically verify the large Reynolds number asymptotic theory of magneto-hydrodynamic (MHD) flows proposed in the companion paper Deguchi (J. Fluid Mech., vol. 868, 2019, pp. 176–211). To avoid any complexity associated with the chaotic nature of turbulence and flow geometry, nonlinear steady solutions of the viscous resistive MHD equations in plane Couette flow have been utilised. Two classes of nonlinear MHD states, which convert kinematic energy to magnetic energy effectively, have been determined. The first class of nonlinear states can be obtained when a small spanwise uniform magnetic field is applied to the known hydrodynamic solution branch of plane Couette flow. The nonlinear states are characterised by the hydrodynamic/magnetic roll–streak and the resonant layer at which strong vorticity and current sheets are observed. These flow features, and the induced strong streamwise magnetic field, are fully consistent with the vortex/Alfvén wave interaction theory proposed in the companion paper. When the spanwise uniform magnetic field is switched off, the solutions become purely hydrodynamic. However, the second class of ‘self-sustained shear-driven dynamos’ at the zero external magnetic field limit can be found by homotopy via the forced states subject to a spanwise uniform current field. The discovery of the dynamo states has motivated the corresponding large Reynolds number matched asymptotic analysis in the companion paper. Here, the reduced equations derived by the asymptotic theory have been solved numerically. The asymptotic solution provides remarkably good predictions for the finite Reynolds number dynamo solutions.

An Integrated Coil Form Test Coil Design for High Current REBCO DC Solenoids

REBCO high-temperature superconductors (HTS) are being utilized to extend the limits of dc solenoidal magnetic fields to 32 T in a user magnet. It is reasonable to expect that these field limits will continue to be surpassed resulting in higher stored energy coils. These larger coils will require kilo-amp level currents to reduce their inductance to manageable levels. Lower inductance coils will be necessary to eliminate unacceptably long ramp times and expensive high voltage isolation hardware by reducing inductive charging and quench voltages. A novel high-current coil concept, using an integrated coil form (ICF), is described here. The coil concept is being developed in combination with a high-current flux pump at the University of Cambridge. The first test coil will be charged to 5.6 kA and will demonstrate the ICF coil winding technique, current lead connections, layer transitions, and coil terminations, as well as ramping and quench performance. The test coil design is described.