Magnetic Field Shielding Research Articles

Alternating Magnetic Field Shielding of YBCO Tubes in Static Field

The alternating magnetic field shielding efficiency of a YBCO tube was measured in a static field of up to 1 kG, bearing the possibility of its application as a field regulator in mind. The shielding efficiency is higher when the sample is field-cooled than when it is zero-field-cooled, as predicted by the critical state model. In the case of field cooling, the tube still shields about 25% of ac field of 1 kHz even in 500 G dc field. Shielding efficiency monotonously increases with increasing frequency from 10 Hz to 1 kHz.

Microstructure and levitation properties of floating zone melted YBCO samples

Radiation zone melting has been used to produce texture in sintered YBCO cylindrical samples. Microstructural analysis by electron microscopy and pole figure measurements reveals that the production process gives rise to a preferential orientation within large domains. DC transport measurements show that changes in alignment orientation can result in the inability to carry a transport current. Both AC magnetic field shielding and levitation properties are substantially improved by the floating zone melting, the levitation force being increased with the textured domain size growth. >

Magnetic shielding of stray magnetic fields in the scanning electron microscope

A stray magnetic field troubles all designers of electron microscopes. The influence of this field is most critical when an ultrahigh-vacuum low-voltage scanning microscope is to be designed. Transversal components of the stray magnetic field deflect the primary beam, but a stray field of the same value acting along the optical axis does not influence the primary beam significantly. We designed a shielding for our low-voltage scanning electron microscope which suppresses the influence of the transversal stray magnetic field by two orders but does not influence the field of the focusing lens. Therefore, unlike the full shielding of a small diameter, the segmental shielding proposed does not impair resolution.

Device-structure dependence of shift in SQUID characteristics by flux trapping

Shifts in voltage-flux characteristics of a SQUID by flux trapping have been measured to study effectiveness of guard ring structure on shielding of magnetic field. The measurements are made under controlled magnetic field. Magnitude of the shift depends on the device structure. It is found that there exists a threshold field for the flux trapping, and the field is reduced by introducing the guard-ring in the SQUID. Comparing to the SQUID without the structure, the SQUID with it needs higher-grade shielding to prevent the flux trapping during cooling down.

Metallurgical reactions and enhanced magnetic field shielding in YBCO-PbS superconductors

The metallurgical reactions occurring during sintering of PbS-doped YBCO superconductors have been investigated using thermal analysis, XRD patterns and SEM micrographs. It is shown that Pb preferentially reacts with Ba to form BaPbO 3. Addition of PbS is also found to lower the temperature of partial melting of Y 1–2–3 phase to 970°C, ensuring less porous microstructure. Moreover, the highly conductive second-phase BaPbO 3 fills the remaining vacant spaces among superconducting grains, providing better electrical contacts among grains and still dense microstructure. These materials are found to have highly promising magnetic shielding properties and increased current densities. The unique features of these materials described above are responsible for these improved characteristics.

Thermal instability, magnetic field shielding and trapping in single-grain YBa2Cu3O7−δ bulk materials

We report results on the temperature dependence of the trap field BT and the thermal instability (flux jump) field Hfj in single-grain YBa2Cu3O7−δ disks. Above 45 K, BT is limited by the critical current density Jc and BT=BT0(1−T/Tc)q with q≊1.67. At 4.2 K, the first flux jump field Hfj of ∼3 T is about an order of magnitude larger than theoretical prediction based on the adiabatic thermal instability model. We also find that the occurrence of large flux avalanches probably causes severe microcracking damages to the superconducting disks.

Magnetic shielding properties of YBa2Cu3O7-x tubes

The magnetic shielding properties of polycrystalline YBa2Cu3O7-x, tubes have been investigated at 77 K in weak magnetic fields. Based on a detailed analysis of a grain-boundary weak-link picture of granular superconductors, their transport, magnetic field penetration, shielding and trapping behaviour are interpreted in terms of a critical state model. The correlation between properties and material structure is discussed.

Action of 50 Hz magnetic fields on neurite outgrowth in pheochromocytoma cells

This study tests the capacity of 50 Hz magnetic and electric fields to stimulate neurite outgrowth in PC-12D cells, a cell line which originated from a pheochromocytoma in rat adrenal medulla. The cells were plated on collagen-coated, plastic petri dishes and exposed to sinusoidal 50 Hz magnetic fields for 22 h in a 5% CO2 incubator at 37 degrees C. Two 1,000 turn coils, 20 cm in diameter, were assembled in a Helmholtz configuration to generate a magnetic field in a vertical orientation, thereby inducing a companion electric field in the dish with intensity proportional to radius. A magnetic-field shield housed the control samples in the same incubator. Total cells and number of cells with neurites at least as long as one cell diameter or having a growth cone were counted within a radius of 0.3 cm of the dish center and within an annulus of 1.7-1.8 cm radii in 60 mm dishes, at 3.6 cm radius in 100 mm dishes, and between 1.9 and 2.1 cm radii in the outer well of organ cultured dishes, which are physically separated into two concentric wells. Sham exposure demonstrated no difference in percentage of cells with neurites between the exposed and control locations in the incubator. Exposures were done at 4.0, 8.9, 22, 29, 40, 120, 236, and 400 milliGauss (mG). At dish radii of 1.7-1.8 cm in the 60 mm dishes these magnetic flux densities induced electric fields of 1.1, 2.5, 5.9, 8.1, 11, 33, 65, and 110 microV/m, respectively, while within a radius of 0.3 cm, the induced electric fields were less than 0.2, 0.4, 1.0, 1.5, 1.9, 6.0, 11, and 19 microV/m, respectively. For other dishes, the larger radii produced proportionally larger induced electric fields. At each field strength, there were two control dishes and four to nine exposed dishes; 100 or more cells were counted at each location on the dishes. The results demonstrate that magnetic fields stimulate neurite outgrowth in a flux-density-dependent manner between 22 and 40 mG, reaching an apparent stimulation plateau between 40 and 400 mG; no effects were seen at 8.9 mG or lower. There was no apparent neurite stimulation due to the electric field. Although relatively low intensity (> or = 22 mG) magnetic fields alone can stimulate a morphological response in a cell which is normally stimulated by nerve growth factor molecules binding to membrane receptors, the chemical basis of this response is unknown.

Shielding effectiveness of a metal sheet parallel to the axis of a circular loop

The equation of the magnetic shielding effectiveness for a configuration made of a circular (transmitting) coil in a semi-half space on one side of a metal sheet and a magnetic sensor in the space on the other side of the sheet has been derived by Moser for the case where the axis of the coil is perpendicular to the sheet. One of the objectives of the present paper is to derive an equation for the magnetic shielding effectiveness in the case where the coil axis is parallel to the sheet. This paper shows that this effectiveness is smaller by a certain amount than the case where a magnetic dipole is placed at the center of the transmitting coil instead of the coil. The amount of reduction is a function of the ratio of the coil radius to the distance from the center of the coil to the observation point. Another objective of this paper is to determine what kind of quantities can be obtained by the measurement method by the MIL-STD-285 which is used for the measurement of the shielding effectiveness of the wall of the shielded room. In this measurement, the magnetic field shielding effectiveness is derived by measurement of the induced voltage of the receiving coil when the transmitting and receiving coils with a diameter of 12 inches are placed on both sides of the sheet at a distance of 12 inches, respectively, from the sheet in a plane perpendicular to the sheet. It is shown that the result of such a measurement gives rise to a value almost equal to (or smaller by about 0.4 dB than) the magnetic shielding effectiveness in a hypothetical case where a magnetic dipole is placed at the center of the transmitting coil and a magnetic sensor is placed at the center of the receiving coil.

Age-related disruption of circadian rhythms: Possible relationship to memory impairment and implications for therapy with magnetic fields

Disorganization of circadian rhythms, a hallmark of aging, may be related causally to the progressive deterioration of memory functions in senescence and possibly Alzheimer's disease (AD). In experimental animals, disruption of circadian rhythms produces retrograde amnesia by interfering with the circadian organization of memory processes. The circadian system is known to be synchronized to external 24 h periodicities of ambient light by a neural pathway extending from the retina to the suprachiasmatic nucleus (SCN) of the anterior hypothalamus. There is also evidence that the earth's magnetic field is a time cue ("Zeitgeber") of circadian organization and that shielding of the ambient magnetic field leads to disorganization of the circadian rhythms in humans. Since aging is associated with a delay of the circadian rhythm phase, and since light, which phase advances circadian rhythms, mimics the effects of magnetic fields on melatonin secretion, we postulate that application of magnetic fields might improve memory functions in the elderly as a result of resynchronization of the circadian rhythms. Moreover, since the circadian rhythm organization is more severely disrupted in patients with AD, it is possible that magnetic treatment might prove useful also in improving memory functions in these patients. If successful, application of magnetic fields might open new avenues in the management of memory disturbances in the elderly and possibly in AD.

Magnetic fields and seasonality of affective illness: implications for therapy.

Seasonal affective disorder is characterized by recurrent winter depression associated with hypersomnia, overeating, and carbohydrate craving. The severe form of winter depression affects about 5% of the general population and is believed to be caused by light deficiency. About 70%-80% of patients with winter depression experience attenuation of symptoms when exposed to bright light therapy. Hypotheses pertaining to the pathogenesis of winter depression implicate the effects of light on different characteristics of circadian rhythms. One of the environmental factors which may be implicated, in addition to light, in the pathophysiology of winter depression is the geomagnetic field. There is strong indication that the pineal gland is a magnetosensitive system and that changes in the ambient magnetic field alter melatonin secretion and synchronize the circadian rhythms. In man, shielding of the ambient magnetic field significantly desynchronizes circadian rhythms which could be gradually resynchronized after application of magnetic fields. The strength of the environmental magnetic field diminishes during the winter months, leading to increased susceptibility for desynchronization of circadian rhythms. Thus, since the acute application of magnetic fields in experimental animals resembles that of acute exposure to light with respect to melatonin secretion (i.e., suppression of melatonin secretion), magnetic treatment might be beneficial for patients with winter depression. In addition, since the environmental light and magnetic fields, which undergo diurnal and seasonal variations, influence the activity of the pineal gland, we propose that a synergistic effect of light and magnetic therapy in patients with winter depression would be more physiological and, therefore, superior to phototherapy alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Direct-overwrite system for magnetooptic memories with laser-controlled magnetic-field shielding

The principles of a novel direct-overwrite system which exploits the shielding power of a soft-magnetic layer are elucidated. The field supplied by one extra layer with permanent magnetization can be purposefully switched on and off in that region of the memory layer that is heated above its write threshold by varying the power of the laser beam. Results of a numerical analysis are presented which predict good reliability of the system and a write field of about 16 kA/m (200 Oe). >

Electromagnetic shielding behavior of wire-mesh screens

The electromagnetic shielding behavior of bounded-junction wire-mesh screens is discussed in the frequency range where the individual meshes are electrically small. The screen, whose meshes are assumed to be square, is described by a equivalent sheet impedance operator. The plane-wave shielding effectiveness of a planar-mesh screen is derived. The problem of coupling between antennas on either side of the planar mesh is addressed, and its applicability to the measurement of mesh properties is described. The low-frequency electric and magnetic shielding afforded by mesh enclosures is considered. Because of the reactive character of the mesh surface, the plane-wave shielding effectiveness decreases with increasing frequency. The enclosure's magnetic-field shielding effectiveness increases with increasing frequency, but saturates at a maximum value that depends on the geometry of the enclosure, the mesh size, and the mesh wire radius. The enclosure electrostatic-field shielding effectiveness depends only on the enclosure and mesh geometries.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Vibrational circular dichroism theory: formulation defining magnetic dipole atomic polar tensors and vibrational nuclear magnetic shielding tensors

The theory of vibrational circular dichroism is formulated in terms of magnetic dipole atomic polar tensors, an extension of infrared atomic polar tensors to magnetic dipole transitions. Relations are provided to define a vibrational nuclear magnetic shielding tensor that describes electronic shielding of an external magnetic field for a nucleus.

Investigation of 27Al‐NMR chemical shifts in zeolites of the faujasite type

The chemical bond of aluminum in aluminosilicates is related to the structure of these solids and particularly, for zeolites it affects their catalytic properties. In order to interpret the 27Al-NMR-spectra of solids one has to consider the quadrupole interaction between the nuclear quadrupole moment and the electric field gradient at the nucleus site, the chemical shift of the resonance due to the shielding of the external magnetic field by the electron shell and the dipole-dipole interaction between the nuclei. In our work we want to get some information about the aluminium distribution in A-zeolites by measuring the mean value of the resonance shift which is possible if the contribution of the quadrupole interaction and of the dipole-dipole interaction to the line width can be reduced to a sufficiently low value. Concerning the structure of zeolites see e.g. BARRER (especially about faujasites PFELFER). The ,'Al-NMR in zeolite powder samples (regarding influence of the quadrupole interaction on the NMR- line shape in zeolites see LECHERT) is characterized by a single line which corresponds to the transitions between the energy levels with magnetic quantum numbers + 1/2 and -1/2 and which is strongly broadened by the quadrupole interaction to second order. The line width 6v1,, (width at half heigh) is given by

Shielding of longitudinal magnetic fields with thin, closely spaced, concentric cylinders of high permeability material

Formulae for the longitudinal shielding effectiveness of N thin, closely spaced, concentric cylinders of high permeability material have been developed and experimentally tested. For shields which cannot be oriented, or which change their orientation in the ambient field, the shielding effectiveness for longitudinal fields is generally the limiting criterion and no design formulae have previously been published for more than two shields. A simple diagrammatical method of writing the shielding formula is presented. Use of these equations is demonstrated by application to the design of magnetic shields for hydrogen maser atomic clocks. Examples of design tradeoffs such as size, weight, and material thickness are discussed. Experimental data on three sets of shields fabricated by three manufacturers are presented.

Problems with turbomolecular pumps in magnetic fields

With the increase of the application of turbomolecular pumps in plasma physics the problem of strong magnetic fields in the vicinity of the turbomolecular pumps comes up more and more frequently. If the rotor of a turbomolecular pump turns with high speed in a magnetic field eddy currents in the rotor will cause undesirable heating of the pump. The maximal magnetic flux density specific for each turbomolecular pump is determined by experiment for a series of pumps. For higher densities of the magnetic field shielding elements must be applied, because excessive heating of the pump rotor would cause destruction of the pump.

The shielding of asynchronous magnetic fields in the superconducting synchronous generator with electromagnetic shields

The shielding effects of the single and double electromagnetic shields was investigated in terms of the field winding mechanical shield, the superconductor location, thickness, and conductivity. The eddy current loss and electromagnetic shield were evaluated in terms of the diffusion time constant.

Ultra-low magnetic field apparatus for a cryogenic gyroscope

We have completed the construction of an apparatus for continued Earth-based testing of a gyroscope system to be used in a satellite test of general relativity. The immediate goal is a readout capable of measuring the direction of the gyroscope spin axis to an angular resolution of one arcsecond over a limited range. A combination of SQUID magnetometers and persistent current loops are used to measure the London moment of the spinning superconducting rotor, which is levitated electrostatically. The apparatus makes use of a new magnetic field shielding technique for obtaining large superconductor shielded regions below 10 −7 gauss to obtain a trapped flux signal in the gyroscope sufficiently smaller than the London moment signal. A recently prepared shield provides a gyroscope environment of 2.3 × 10 −7 gauss; a factor of nearly 100 improvement over our previous ability using conventional μ-metal shielding.

A high accuracy gyroscope readout test facility for the relativity gyroscope experiment

We are building an apparatus for Earth-based testing of a gyroscope system to be used in a satellite test of general relativity. The immediate goal is a readout capable of measuring the direction of the gyroscope spin axis to an angular resolution of one arcsecond over a limited range. A combination of SQUID magnetometers and persistent current loops are used to measure the London moment of the spinning superconducting rotor levitated electrostatically. To obtain a trapped flux signal in the gyroscope sufficiently smaller than the London moment signal, the apparatus makes use of a new magnetic field shielding technique for obtaining large superconductor shielded regions below 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> gauss.