On some consequences of an external magnetic field applied to HTS coils

A computational model which enables to evaluate the distribution of the critical currents, electric fields and the voltage in the winding of a solenoidal high temperature superconducting (HTS) magnets subjected to an external magnetic field parallel with the magnet axis, was developed. The model comes out from the well-known power law between the electric field and the transport current of the HTS tape short sample. It allows to predict the voltage–current V(I) characteristics of both the pancake coils and the complete magnet. The model was applied to the magnet system consisting of 22 pancake coils made of multifilamentary Bi(2223)/Ag tape at 20 K, which is subjected to an external uniform magnetic field parallel with the coil axis. A rather unexpected behavior of the magnet at different operating conditions (operating current and external magnetic field strength) is predicted, analyzed and reported together with a theoretical explanation. On one hand, the external uniform magnetic field parallel with the coil axis increases the resulting magnetic field strength, however, on the other hand it simultaneously decreases the angle between the resulting magnetic field and the tape surface. Thus, the effect of higher magnetic loading caused by the presence of an external magnetic field strength which is acting on individual turns located close to the coil’s flanges is compensated by more favorable orientation of the tape with respect to the resulting magnetic field. As a result, increase in the critical currents of these turns is expected. Further, the results indicate, that in case of the high field HTS insert coils the anisotropy in the I_c(B) characteristic does not play a substantial role. As a consequence, the technology of the production of the tapes for high field insert HTS coils should concentrate rather on the tapes having the current carrying capacity as high as possible, than on the attempt how to decrease the anisotropy in the I_c(B) by changing the architecture of the filaments in the tape.     

Magnetic fluid droplet deformation in electrostatic field

The stability of the ferrofluid (FF) subjected to the electric field is crucial for the application in high voltage (HV) technology. There are several cases where the fluid interacts with solid interfaces. We examined experimentally FF drop on a glass surface. The drop was exposed to the steady electric field. During tests the suspended particles started to aggregate. Changing drop's shape was recorded during a time period. It was observed that the further deformation development depends on aggregates shape and location. The results are compared with the behaviour of pure carrier fluid. Understanding the phenomenon associated with FF drop deformation can help more reliable HV component design.     

Nonlinear response to external magnetic fields

The purpose of this paper is to study conditions under which a system of itinerate spin-$\text{1}\text{2}$ fermions might exhibit a macroscopic linear response to external magnetic fields after long times. Exact expressions are obtained for the nonlinear response of the magnetization and the total energy. We find that for a constant field there is no response (our model contains no mechanism for the relaxation of spins). For an oscillatory field there is a response in which secular terms (in the time) appear which are associated both with nonlinear terms in the external field and with contributions from the background medium. The secular terms involving the magnetic field would not be seen if one used the usual approximations of microscopic linear response theory. They give rise to new conditions which must be satisfied if the system is to exhibit a macroscopic linear response in the long-time limit.     

Magnetic ordering in (110) Eu films in an applied magnetic field

Below its ordering temperature (T _N = 90 K), bulk bcc Eu has a helical magnetic state with propagation vectors along the three equivalent 〈100〉 directions. In contrast, epitaxial (110)Eu films exhibit a unique magnetic ordering: the domain with a magnetic helix propagating along the in-plane [001] direction vanishes on cooling, at the expense of other domains with helices propagating along [100] and [010]. This paper is devoted to the study of the stability of the magnetic domains in an external magnetic field using neutron scattering experiments and macroscopic magnetization measurements. The helix propagating along the [001] direction can be restored by the application of an external field along this direction. On the contrary, when a magnetic field is applied along an intermediate direction, specifically [10], the domain with a helix propagating along [001] is suppressed. Both effects depend on the film thickness. They are explained if one considers that, because of the low magnetic anisotropy of Eu, a helix with a propagation vector parallel to (or close to) the applied magnetic field is energetically more favourable than cycloidal structures with unchanged propagation vectors. Finally, the amplitudes of the propagation vectors and their directions (that are modified in films compared to bulk) do not vary under magnetic field.     

论静电场和静磁场的相对性

电场和磁场形式不同但本质相同,在不同的惯性系下,二者可以相互转变,转变的规律遵循最基本的力学原理.结合狭义相对论和基本的力学定律,可以更加直观地表达电场与磁场的相互转变过程和转变形式,从而更深刻地理解电场与磁场同宗同源的本质.     

Magnetic excitations and phase transition of CsFeBr3 in an external magnetic field

In CSFeBr₃ the Fe²⁺ ion with effective spin one has locally a singlet ground state (m=0). The antiferromagnetic interactions between neighbouring Fe-ions are too weak as compared with the anisotropy constant to introduce long range order in the absence of an external field. By inelastic neutron scattering we studied the magnetic excitations in an external magnetic field up to 5 Tesla applied along thec-axis. A linear Zeeman splitting was observed with a Landé factorg=2.4. The field renormalizes the dispersion curves in such a way that the exchange interaction has decreasing influence with increasing field. Theoretical calculations according to the excitonic model of Lindgård describe the experimental results very well. At 4.1 Tesla a phase transition appears to a commensurate long range order with a 120° arrangement of the spins in the hexagonal plane. Within the limits of experimental observation this phase transition has no influence on the dynamical behaviour. No critical phenomena could be observed. The dynamical structure factor |G _j()|² of the lower Zeeman split modes decreases with increasing field.     

Magnetic property of pyrolytic carbon prepared on diamond powder in applied magnetic field

We have prepared pyrolytic carbon samples from triethylamine on diamond powder in applied magnetic field of 6 T and investigated their magnetic properties. A ferromagnetic sample was obtained from the pyrolysis products even at room temperature, with spontaneous magnetization of 6.4 A m²/kg at 300 K, which is twice as large as that of the sample prepared without diamond powder. Therefore, the diamond powder seems to promote the formation of a three-dimensional ferromagnetic structure in pyrolytic carbon.     

Magnetic fluids in an external field

Within mean field approximation we investigate the phase diagrams of magnetic fluids in presence of a magnetic field. In a finite field the magnetic phase transition is absent, but instead a line of first order liquid-liquid transitions ending in a critical point occurs for a magnetic interaction, which is sufficiently strong. Varying the magnetic field these critical points extend from the tricritical point at H=0 to a critical endpoint. For a fluid with Ising spins we calculate the critical lines and several tricritical exponents analytically. For Heisenberg fluids we obtain the phase diagrams from a numerical solution of the mean field equations of state. Received 20 March 1998     

The self-consistent response of the electron density of a crystal copper film to an external electrostatic field

The response of the electrons of an fcc copper (001) film to an external electrostatic field is calculated. In order to determine the distribution of the screening charge, the equations of the electron density-functional theory are solved self-consistently by an original method. The position of the “image plane,” which is involved in a correct asymptotic expression for the exchange-correlation potential in the vacuum region, is first determined when performing a quantum-mechanical calculation for an anisotropic crystal film. The nonlinear electron response is characterized by the evolution of the “center of gravity” of the induced charge, which is also investigated. The calculations take into account the crystal structure of the film, and the results differ essentially from the predictions of the “jelly” model.     

Disintegration of a drop in an external electrostatic field

Regular features of the disintegration of both a drop of a perfectly conducting liquid and a drop of a dielectric liquid into two or three parts in an external uniform electric field are studied using the principle of minimizing the potential energy of the final state of a closed system with spontaneous processes.     

Effective field theory approach to light propagation in an external magnetic field

The recent PVLAS experiment observed rotation of polarization and ellipticity when a linearly polarized laser beam passes through a transverse magnetic field. The phenomenon cannot be explained in conventional QED. We attempt to accommodate the result by employing an effective theory for the electromagnetic field alone. No new particles with a mass of order the laser frequency or below are assumed. To quartic terms in the field strength, a parity-violating term appears besides the two ordinary terms. The rotation of polarization and ellipticity are computed for parity-asymmetric and -symmetric experimental set-ups. While rotation occurs in an ideal asymmetric case and has the same magnitude as ellipticity, it disappears in a symmetric set-up like PVLAS. This would mean that we have to appeal to some low-mass new particles with nontrivial interactions with photons to understand the PVLAS result. PACS 12.20.-m; 12.20.Fv; 42.25.Lc; 42.25.Ja     

Magnetic field effects in the presence of electrostatic interactions

Magnetic field effects on ion radical pair (IRP) recombination reactions are considered. In the diffusion approximation simple analytical expressions for the magnitudes of these effects are obtained. They are reduced to those obtained within the simple cage approximation in the case, when the intramolecular spin interactions are strong enough and to those obtained within the free radical approximation in the opposite case. The applicability of both simple approximations are shown to be closely connected with peculiarities of the IRP decay kinetics. Namely, the cage approximation is applicable for the strong spin interactions because the IRP decays exponentially at small times. The free radical approximation is valid in the opposite limit of weak interactions since at large times the IRP decays as a free radical pair.     

Behavior of electron spin in an external axisymmetric electrostatic field

The self-orientation of the spin of an ultrarelativistic electron in an external axisymmetric focusing electrostatic field is investigated qualitatively. The conditions under which a collective phenomenon of this kind occurs and its nature are clarifiedTranslated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 14–17, February, 1991.     

Dirac oscillator in an external magnetic field

We show that (2+1)-dimensional Dirac oscillators in an external magnetic field is mapped onto the same with reduced angular frequency in absence of magnetic field. This can be used to study the atomic transitions in a radiation field. Relativistic Landau levels are constructed explicitly. Several interesting features of this system are discussed.     

Mobility edge shift in external magnetic field

It is shown that magnetoconductivity in the critical region near the metal-insulator transition is equal to $\text{A}{}_{\mathrm{l}}\text{H}$ and the coefficient A_l does not depend on the proximity to the mobility edge. The magnetic field leads to the mobility edge shift, which is proportional to $\text{H}{}^{\mathrm{<mtext>1</mtext><mtext>(2v</mtext><mtext>)</mtext>}}$, where v is the conductivity critical index.     

Instability of a charged spherical drop moving parallel to an external electrostatic field

It is shown that the pressure of electrostatic fields induced by the self-charge of a drop and by the polarization charge and aerodynamic pressure of a laminar gas flow around a moving charged drop acting simultaneously reduce the critical instability conditions for the surface of the drop. For these conditions, the spectrum of capillary oscillations of the drop is calculated. It is found that, at various values of the charge, field strength, and velocity of the drop, the vibrational instability of the drop surface develops through the interaction of different oscillation modes, namely, second and third, second and fourth, and third and fifth.