Magnetic Fields In Excess Research Articles

Transient response of single-domain Y-Ba-Cu-O rings to pulsed magnetic fields

Shielding current limits and magnetic diffusion characteristics have been measured at 77 K for a set of YBCO single-domain rings. These were fabricated from melt-textured cylindrical YBCO monoliths that were densified to nearly 100%, and then oriented from a single seed. The rings were surrounded by a drive coil that can, under pulse conditions, achieve applied magnetic fields in excess of 1 T and induce currents in excess of 50 kA. Simultaneous magnetic characterization with a Rogowski coil and Hall probe was used to determine the induced current in the sample and the magnetic field in the center of the sample. Magnetic fields trapped in the samples were mapped with a scanning Hall probe. When compared with similar measurements on multidomain c-axis-oriented YBCO rings, the flux penetration is faster and more uniform around the circumference of the ring. The observed critical current density, /spl ap/ 15,000 A/cm/sup 2/ at 77 K, is suitable for application in penetration-type fault current limiters. Separate measurements of the trapped magnetic field and critical current density in the rings are compared with results obtained by analysis of magnetic diffusion characteristics.

High magnetic field studies of the hidden order transition in URu2Si2.

We studied in detail the low temperature/high magnetic field phases of URu2Si2 single crystals with specific heat, magnetocaloric effect, and magnetoresistance in magnetic fields up to 45 T. Data obtained down to 0.5 K, and extrapolated to T=0, show a suppression of the hidden-order phase at H0(0)=35.9+/-0.35 T and the appearance of a new phase for magnetic fields in excess of H1(0)=36.1+/-0.35 T observed only at temperatures lower than 6 K. In turn, complete suppression of this high field state is attained at a critical magnetic field H2(0)=39.7+/-0.35 T. No phase transitions are observed above 40 T. We discuss our results in the context of itinerant versus localized f electrons.

State-of-the-art superconducting accelerator magnets

With the LHC the technology of NbTi-based accelerator magnets has been pushed to the limit. By operating in superfluid helium, magnetic fields in excess of 10 T have been reached in various one meter-long model magnets while full scale magnets, 15 meter-long dipoles, have demonstrated possibility of safe operation in the 8.3-9 tesla range, with the necessary, very tight, field accuracy. The paper reviews the key points of the technology that has permitted the construction of the largest existing superconducting installations (Fermilab, Desy and Brookhaven), highlighting the novelties of the design of the LHC dipoles, quadrupoles and other superconducting magnets. All together the LHC project will need more than 5000 km of fine filament superconducting cables capable of 14 kA @ 10 T, 1.9 K.

Measurements of the inverse Faraday effect from relativistic laser interactions with an underdense plasma.

Magnetic fields in excess of 7 MG have been measured with high spatial and temporal precision during interactions of a circularly polarized laser pulse with an underdense helium plasma at intensities up to 1x10(19) W cm(-2). The fields, while of the form expected from the inverse Faraday effect for a cold plasma, are much larger than expected, and have a duration approaching that of the high intensity laser pulse ( <3 psec). These observations can be explained by particle-in-cell simulations in 3D. The simulations show that the magnetic field is generated by fast electrons which spiral around the axis of the channel created by the laser field.

Transient response of 50 KiloAmp Y-Ba-Cu-O rings and ring pairs to pulsed magnetic fields

Shielding current limits and magnetic diffusion characteristics have been measured at 77 K in large-grain YBCO rings with oriented microstructures. The samples are surrounded by a drive coil that can achieve magnetic fields in excess of 1 Tesla and induce currents in excess of 50 kA when driven by a current pulse of a few msec duration. Simultaneous magnetic measurements with a Rogowski coil and a Hall probe allow determination of the induced current in the sample and the field in the center of the sample. These measurements show that field penetration occurs in a complex way that includes delays and transient effects caused by magnetic diffusion and sample heating. Dramatic threshold effects are observed that are probably related to a creep-flow transition coupled with local heating effects. Geometric effects are investigated using a single drive coil and a pair of YBCO rings with various spacings. A test geometry equivalent to a simple penetration-type inductive fault current limiter is used; the experimental results are therefore of interest for design and characterization of these devices.

Thermal conductivity of the spin-Peierls compoundCuGeO3

The thermal conductivity of the Spin-Peierls (SP) compound CuGeO_3 was measured in magnetic fields up to 16 T. Above the SP transition, the heat transport due to spin excitations causes a peak at around 22 K, while below the transition the spin excitations rapidly diminish and the heat transport is dominated by phonons; however, the main scattering process of the phonons is with spin excitations, which demonstrates itself in an unusual peak in the thermal conductivity at about 5.5 K. This low-temperature peak is strongly suppressed with magnetic fields in excess of 12.5 T.

The critical state in YBa 2Cu 3O 7 − x high temperature superconductor

A practical method was devised for measurements of the diamagnetic response of high temperature superconductor specimens. The method is useful for the study of the temperature and field dependencies of the local magnetic induction and its two-dimensional visualisation B( T, H a, x, y) with a resolution of 0.1 gauss and spatial resolution of 0.1 mm in the plane of the specimen surface. The proposed method facilitates quite simple and quick measurements at liquid nitrogen temperature and in applied H a magnetic field in excess of 1 T. Cross-shaped miniature Hall probes (HPs) of thin film n-InSb on semi-insulating substrate i-GaAs with a small magnetically sensitive zone of 0.1 × 0.1 mm 2 were used as measuring transducers. The high degree of doping of n-InSb layers ( n ~ 10 18 cm −3) resulted in a sensitivity of 5–7 μV mT −1 (with nominal current up to 10 mA through HPs) and thermosensitivity of less than 0.03% K −1, and made it possible to operate without the need for thermal stabilization of transducers. This method enabled us to study anisotropy of the critical current densities ( J c) of elt-grown YBa 2Cu 3O 7 − x pellets. Also, we have been able to determine local cracks and grain connectivity in the YBa 2Cu 3O 7 − x pellet samples.

Experimental quantum chemistry at ultrahigh magnetic fields: Some opportunities

We discuss the opportunities for experiments in quantum chemistry and molecular physics at magnetic fields in excess of 100 Tesla. In a multinational collaboration, scientists at Los Alamos National Laboratory have led a campaign of experiments using explosively driven generators with peak fields between 130 and 1000 Tesla. Most of these experiments explored quantum-limit phenomena. We expect to conduct a similar campaign in the spring of 1997. We are also planning experiments with several reusable laboratory facilities: (1) the Pegasus high-energy-density facility, an operating 4 MJ fast capacitor bank system, which should be capable of reaching fields of 600 Tesla in practical experimental volumes; (2) the 100 Tesla nondestructive magnet, which is expected to be in operation by 1998; and (3) the Atlas facility, 38 MJ fast capacitor bank system, which should be capable of reaching fields of 1800 Tesla in practical experimental volumes. Atlas should be in operation by 1999. We also discuss some of the experimental techniques developed for both physics and chemistry experiments at high magnetic fields. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 64: 619–628, 1997

Population hole burning on the [formula omitted] zero-phonon transition in MBE films of CaF 2:Eu 2 +

Spectral holes due to the redistribution of population among the ground state hyperfine levels have been detected on the 413 nm zero-phonon transition in CaF 2:Eu 2 + MBE films grown on Si(1 1 1), in magnetic fields in excess of 3 T. These are the first long-lived (up to minutes) population holes reported for a paramagnetic ion. The hole lifetime results from phonon-induced transitions among hyperfine levels of the Eu 2 + ground electron spin states. The central hole linewidth of 40 MHz is probably determined by the EuF superhyperfine interaction which leads to spectral diffusion due to F nuclear spin flips. The hole pattern contains a narrow central hole (40 MHz) and side holes and antiholes which result from the hyperfine splittings in the ground and excited states.

A compact vibrating-sample magnetometer with variable permanent magnet flux source

An automated vibrating-sample magnetometer is described where the variable magnetic field in excess of ±1.1 T is produced in a 26 mm bore by an assembly of nested, rotating permanent magnet cylinders. The homogeneity is better than 0.3%±4 mT in 1 cc. The instrument has dimensions 200×420×700 mm, and its mass is 20 kg. Sensitivity is better than 0.4 μJ/T (4×10−4 emu), which permits study of the susceptibility, magnetization, and hysteresis of a wide range of magnetic materials, including ferromagnetic thin films.

Magnetic phase diagram for the quasi-two-dimensional organic conductor α-(BEDT-TTF) 2RbHg(SCN) 4

We have studied the magnetoresistance (MR) of the low-dimensional organic charge transfer salt α-(BEDT-TTF) 2RbHg(SCN) 4 at low temperatures and at high dc magnetic fields. Electrical conduction occurs via both the quasi-two-dimensional closed hole orbit portion of the Fermi surface (FS), as well as the quasi-one-dimensional open orbit FS. An antiferromagnetic ordering transition occurs in this material below 10.5 K, which results in a density wave ground state. The consequences of this ordering can be seen in the MR background, which measures the quasi-1D FS. This ordering can be broken with the application of magnetic fields in excess of 32 T for T < 4 K, or with applied pressures of about 8 kbar.

First performance test of the 12 T split coil test facility SULTAN III

The test facility SULTAN III at PSI is primarily devoted to qualification of full-size conductors developed for the next generation of fusion experimental devices. Very recently, the facility generating magnetic fields in excess of 11 T was completed and successfully tested. The split coil arrangement with a radial access of 94 mm*144 mm is ready to perform tests of superconductors carrying currents up to 50 kA at temperatures between 4.5 K and 7 K. First results on a fusion dedicated prototype conductor are presented. The main features of the SULTAN III facility are summarized.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Andreev exclusion of bulk state quasiparticles from a surface in 3He-A at the low temperature limit: implications for transport properties

Abstract We present preliminary measurements of the thermal boundary resistance between 3 He-A and sintered silver, and also of the thermal damping force on a vibrating wire. The A-phase is stabilised by a magnetic field in excess of about 0.35 T. Vibrating wire resonators in 3 He-B “contacts” serve as thermometers. The magnitudes of both the boundary resistance and the thermal damping force are found to be comparable to the corresponding B-phase values, and an exponential temperature dependence is evident, in spite of the T 3 dependence of the quasiparticle number density. We propose that Andreev scattering from a non-uniform A-phase texture is responsible.

Considerations for practical conductor design of Chevrel phase wires

It is noticed that the application of PbMo/sub 6/S/sub 8/ (PMS) wires for the generation of steady-state magnetic fields in excess of 20 T requires that several conditions be fulfilled in order to be realistic. Practical design considerations for PbMo/sub 6/S/sub 8/ wires are reported. Besides the availability of wires with sufficient lengths and uniform high critical current density J/sub c/, thermal and mechanical stability have to be achieved. The most severe criterion for thermal stability is based on the prevention of flux jumps in the presence of screening currents in the adiabatic limit. This results in filament diameters between 710 mu m and 90 mu m for critical current densities of 5*10/sup 8/ A/m/sup 2/ and 2*10/sup 9/ A/m/sup 2/, respectively. A dynamic stabilization of a PMS wire in an epoxy-impregnated coil is not very efficient. The behavior of the J/sub c/ of PbMo/sub 6/S/sub 8/ wires under tensile and compressive stress is discussed.

The superdiamagnetic effect of magnetic fields on one and two component multilamellar liposomes

For multilamella vesicles of DMPC, DPPC, DSPC, binary mixtures of DMPC-DPPC, DMPC-DSPC, DMPC-DPPE, DOPC and egg lecithin, the optical turbidity decreases significantly on the application of a magnetic field in excess of about 0.2 T, provided that the temperature is above the pretransition value. The turbidity reaches a limiting value for magnetic fields of about 2 T. The effect is attributed to augmentation of the diamagnetic anisotropy of the lipid molecules by clustering within the bilayer, with consequent orientation of either the individual ‘superdiamagnetic’ clusters or the whole liposome. It is suggested that, since most animal cell membranes are largely in the liquid crystalline phase, it is possible that homogeneous magnetic fields as low as 0.2 T may cause biologically significant changes within the membrane.

Depression of the liquid crystal-isotropic transition temperature by a magnetic field

A reduction in the temperature of the transition to the biphasic and clearing points by the application of a magnetic field in excess of the critical unwinding field has been observed in a cholesteric liquid crystal. This observation qualitatively is in accordance with the phenomenological expansion of the excess free energy in the order parameter S. Inclusion of a twist term in the gradients of S leads to a rescaling of T* which is inversely proportional to the equilibrium pitch P0. When the pitch of the cholesteric liquid crystal is increased or removed by a magnetic field, a reduction in the transition temperature is expected. The observed effect is apparently insignificant in thermotropic liquid crystals but may be more pronounced in the lyotropic liquid crystal of this study due to its very small latent heat.

Two hot, low-field magnetic DA white dwarfs

view Abstract Citations (49) References (24) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Two hot, low-field magnetic DA white dwarfs. Liebert, J. ; Schmidt, G. D. ; Green, R. F. ; Stockman, H. S. ; McGraw, J. T. Abstract The authors report the discovery of two hot DA white dwarf stars from the Palomar Green Survey, one of which is proved to be, and the other likely to be, a star with surface magnetic field in excess of 106 gauss. PG 1658+441 has an energy distribution fitted by a 30,000 K pure hydrogen atmospheric model. Circular spectropolarimetry revealed longitudinal Zeeman components to the three lowest Balmer lines, with an indicated longitudinal field strength (Be) at ≡0.7 megagauss. Moreover, the high-quality intensity spectra showed apparent splitting of Hβ and Hα into triplets by a mean surface field (Bs) of approximately 2.3 megagauss. PG 0136+251 is a hotter star with weaker lines; its energy distribution implies a 40,000-50,000 K hydrogen-rich atmosphere. Spectropolarimetry revealed unconvincing evidence for magnetic line splitting. However, the anomalously large widths of the Balmer line cores suggest Bs ≥ 1.3 megagauss. It is argued that two other possible broadening agents - an abnormally high surface gravity or very rapid rotation - cannot account for the shapes of its Balmer lines. Publication: The Astrophysical Journal Pub Date: January 1983 DOI: 10.1086/160593 Bibcode: 1983ApJ...264..262L Keywords: Magnetic Stars; Stellar Magnetic Fields; Stellar Spectra; Stellar Temperature; White Dwarf Stars; Balmer Series; Lyman Alpha Radiation; Polarimetry; Spectral Line Width; Visible Spectrum; Astrophysics full text sources ADS | data products SIMBAD (6) MAST (1)

Superconducting magnetometer for quality control of nuclear fuel rods

A superconducting magnetometer is used to assay gadolinium in uranium dioxide nucear fuel rods. Less than 10 percent by weight gadolinium is present in solid solution in the fluorite phase to provide power flattening and to extend the time between refueling of nuclear reactors. The magnetometer is part of an automated rod scanner which provides quality inspection of more than 50 000 fuel rods per year. The magnetometer can resolve a single out-of-specification gadolinia pellet (10 g of UO2 with a maximum of 10% Gd) with as much as 0.2 percent by weight ferromagnetic impurity. The measurement techniques considered during development of the scanner were neutron methods, x-ray fluorescence spectroscopy, modulated-field magnetometry, and moving sample magnetometry. The latter technique was applied by moving the fuel rod at speeds of about 5 cm/sec through two regions of different static magnetic field in excess of 20 000 oersteds. Each region contains a sampling coil which detects changes in magnetization within a volume 1 cm3. Combination of the two signals permits separation of the gadolinium magnetization from the uranium magnetization and ferromagnetic magnetization. The choice of the moving-sample magnetometric method has resulted in a low maintenance, economical, stable inspection device which has performed reliably in the manufacturing environment since August 1977.

Detection of microwave emission from both components of the red dwarf binary EQ Pegasi

The detection at 4.9 GHz of the late main sequence binary EQ Pegasi (dM3.5e + dM4.5e) with the VLA is reported. Both components were detected, as flux levels of 0.69 mJy and 0.4 mJy, respectively. Thermal gyroresonance emission from the quiescent coronae of these stars appears to explain observations of the authors, as it does those of Gary and Linsky (1981) for chi Ori and UV Ceti, provided coronal magnetic fields in excess of 300 gauss exist over a region that has a length scale of at least twice the radii of these stars. Support for this model is provided by the unlikelihood of both stars flaring simultaneously, and by the fact that the emission was confined to each star within the observational uncertainty of a few AU.

One-photon and two-photon annihilation of positronium in strong magnetic fields

In the presence of a strong magnetic field (such as those believed to be characteristic of neutron stars:B≳-1012 Gauss) positronium may annihilate through the emission of a single photon, the magnetic field providing the photon momentum. We report on calculations of the one-photon and two-photon annihilation rates for the ground state of positronium, for magnetic fields in the range (1–44)×1012 Gauss, and give, in the two-photon case, the minimum energy half-width of the emission line due to the momentum contributions from the magnetic field. We find that unless neutron stars have magnetic fields in excess of 1013 Gauss, it is unlikely that the one-photon process will be observable.