Showing papers on "Shields published in 1995"

Geometrical aspects of magnetic shielding at extremely low frequencies

Abstract: The magnetic shielding effectiveness for closed and open shield structures is studied at extremely low frequencies. Analytical solutions are used for simple geometries, while more complex structures are evaluated using a finite-element method. Both highly conductive and ferromagnetic materials are studied, and their different shielding behavior is shown. Ferromagnetic shields give good results for small and closed shields and they also give a large field attenuation at close range to the source for open shield geometries. Highly conductive materials, on the other hand, are found to be suitable for large shield sizes. The attenuation is, however, reduced in the close vicinity of the source. Comparisons of numerical results with analytical calculations and measurements confirmed the high accuracy of the finite-element model. >

Principles of quasistatic magnetic shielding with cylindrical and spherical shields

Abstract: The basic principles that underlie materials-based quasistatic magnetic shielding are described. Shielding mechanisms are identified and shown in specific examples that involve long cylindrical and spherical shield geometries. Analytic results are given both for shields that enclose the shielded region and for shields that enclose the source. The two configurations are reciprocal in the sense that identical shields have identical shielding factors for a uniform externally applied field and for a dipole source within the shield. Flux shunting and induced current shielding mechanisms are each described quantitatively and qualitatively in shielding situations where only one mechanism is involved. Situations where the mechanisms are simultaneously involved are then analyzed, and the relevances of the descriptions that involve only one mechanism are identified as regimes within the general descriptions.

Application of superconducting shields in current-limiting and special-purpose transformers

Abstract: The principles of operation and the design of power transformers based on superconducting shields have been considered. The laboratory prototypes of fault current-limiting and special-purpose (arc welding/furnace) transformers have been tested. The application of both low-T/sub c/ and high-T/sub c/ superconducting shields has been studied. >

Synthetic dielectric material for broadband-selective absorption and reflection

Abstract: This invention relates to material for broadband-selected absorption and reflection of electromagnetic waves for all frequencies below optical (hereinafter referred to as RF) using dilute concentrations of metallic particles suspended in a lightweight dielectric for such applications as radar cross section suppressors, microwave heat exchangers, plasma generators, transformation of RF energy into thermal optical energy, shields for RF discharge devices, etc., and lightweight RF broadband reflecting material for such applications as radar reflectors, broadband antennas with prescribed or variable gain pattern, optically transparent RF shields, EMI shields and RF filters. Such absorptive material is useful in antenna-communication links (low elevation angles), RF microwave laboratory insulation, EMI absorptive shields. Such reflective material is useful for satellite antennas, communication links, cable television, shielding electronic computers, computer games, microwave ovens commercial RF microwave laboratory shielding and in high RF energy discharge technology in industrial research and development.

Dual shielded relay reed pack

Abstract: A reed relay is provided with two shields. The shields are fabricated from copper foil and insulated from each other by polyester tape or film. One shield is connected to one lead of a switching element and the other shield is connected to another lead of a switching element. The shields are wrapped around the switching element in an overlapping manner. The dual shield configuration reduces leakage and interference to improve performance, particularly in low current applications.

EMI Shields made from intercalated graphite composites

Abstract: Electromagnetic interference (EMI) shielding typically makes up about twenty percent of the mass of a spacecraft power system. Graphite fiber/polymer composites have significantly lower densities and higher strengths than aluminum, the present material of choice for EMI shields, but they lack the electrical conductivity that enables acceptable shielding effectiveness. Bromine intercalated pitch-based graphite/epoxy composites have conductivities fifty times higher than conventional structural graphite fibers. Calculations are presented which indicate that EMI shields made from such composites can have sufficient shielding at less than 20% of the mass of conventional aluminum shields. EMI shields provide many functions other than EMI shielding including physical protection, thermal management, and shielding from ionizing radiation. Intercalated graphite composites perform well in these areas also. Mechanically, they have much higher specific strength and modulus than aluminum. They also have shorter half thicknesses for x-rays and gamma radiation than aluminum. Thermally, they distribute infra-red radiation by absorbing and re-radiating it rather than concentrating it by reflection as aluminum does. The prospects for intercalated graphite fiber/polymer composites for EMI shielding are encouraging.

High-T/sub c/ superconducting image surface magnetometers

Abstract: We present an overview of the concepts of image surface magnetometry, and analyze various shapes of superconducting shields in the presence of uniform background noise fields and current sources that simulate biomagnetic sources. We show that, for certain geometries and sizes, these shields can selectively shield background noise fields as opposed to signals of interest. We also show that clever use of these shields can, in some cases, allow an undisturbed measurement of the signals of interest while attenuating noise fields by as much as four orders of magnitude. >

Magneto-resistive head including a selectively placed low-reluctance path between shields

Abstract: A magnetic head including a shielded MR element which incorporates a low reluctance path between the shields to reduce interference picked up by inductive reading of the shields. The low reluctance path is provided outside a conductive loop of material which includes the MR element and its associated leads. The low reluctance path may include a magnetic short between the two shields.

Squid storage container and squid cooling method

Abstract: PURPOSE:To make it possible to perform magnetic field measurement precisely without creating a magnetic flux trap CONSTITUTION:Inside a SQUID storage container 1 for storing SQUID and liquid nitrogen 3 in an adiabatic state, a plurality of cylindrical magnetic shields 121 to 12n comprising a superconducting material of a high critical temperature are supported and shifting to the superconducting state is realized by cooling the magnetic shields 121 to 12n with liquid nitrogen 3 under the state of shielding the external magnetic field And magnetic shield spaces S1 to Sn are generated inside the magnetic shields 121 to 12n, then shifting to the superconducting state is realized by moving the SQUIDs 21 to 2n to the magnetic shield spaces 81 to Sn and cooling by the liquid nitrogen 3

Antenna device for nuclear spin resonance device

Abstract: PURPOSE: To provide an antenna device for a nuclear spin resonator which can be realized by perfect uniformity flat as much as possible and sensitivity CONSTITUTION: A partial antenna 4 or 7 composed of shields 4 and 5 in which a main magnetic field is formed between two magnetic poles 1 and 2 and are closed by and earthed to the magnetic poles and turn in the magnetic pole direction and planar conductor structures 6 and 7 arranged at intervals to the shields substantially in parallel to the respective shields, is arranged, and a high frequency current is supplied to one side of the conductor structure bodies, and an electric current is made to respectively flow to the conductor structure bodies arranged corresponding to the two magnetic poles

Electromagnetic interference on computer monitors caused by currents in riser cables

Abstract: Riser cables in building were simulated by a laboratory model as well as a finite-element analysis software. The resulting stray magnetic field with different cable spacing was assessed and compared. Its effect on microcomputer monitors was evaluated. It was discovered that magnetic field strength of 15 milligauss could produce noticeable distortions on the monitor. The type of video display unit, and its orientation with respect to the magnetic field has significant effects on its susceptibility to interference. Based an theoretical analysis and computer simulation results: electromagnetic shields for the cable riser and computer monitor were fabricated. Depending on operating conditions. Cable shields can achieve shielding effectiveness up to 35 db while monitor shield with practical design using high-permeability material can have shielding effectiveness of around 22 db.

Innovative technologies for Faraday shield cooling

Abstract: Alternative advanced technologies are being evaluated for use in cooling the Faraday shields used for protection of ion cyclotron range of frequencies (ICR) antennae in Tokamaks. Two approaches currently under evaluation include heat pipe cooling and gas cooling. Both of these technologies offer attractive alternatives to water-cooled designs primarily because they remove the time-consuming cleanup required in the event of tube weld failures and because they increase the operating temperature range for the shield. A Monel/water heat pipe cooled Faraday shield has been successfully demonstrated. Heat pipe cooling offers the advantage of reducing the amount of water discharged into the Tokamak in the event of a tube weld failure. The device was recently tested on an antenna at Oak Ridge National Laboratory. The heat pipe design uses inclined water heat pipes with warm water condensers located outside of the plasma chamber. This approach can passively remove absorbed heat fluxes in excess of 200 W/cm/sup 2/. Helium-cooled Faraday shields are also being evaluated. This approach offers the advantage of no liquid discharge into the Tokamak in the event of a tube failure. Innovative internal cooling structures based on porous metal cooling are being used to develop a helium-cooled Faraday shield structure. This approach can dissipate the high heat fluxes typical of Faraday shield applications while minimizing the required helium blower power. Preliminary analysis shows that nominal helium flow and pressure drop can sufficiently cool a Faraday shield in typical applications. Plans are in progress to fabricate and test prototype hardware based on this approach.

3D analysis of forces in the shields of SC coils in EDS-MAGLEV transport systems

Abstract: The paper firstly describes a 3D analytical method to calculate the currents in the on-ground levitation coils of an EDS-MAGLEV train; such currents are then used as inputs for a 3D FEM code to calculate the eddy currents and losses in the shields of the on-board SC coils.

Gas-insulated transformer

Abstract: PURPOSE:To provide a gas insulated transformer equipped with thin magnetic shields which are easily mounted on an inner wall of a pressure container, by piling a plurality of sheets of band-shaped magnetic materials to compose the magnetic shields and bending the magnetic shields in conformity with a curved surface of the pressure container and fixing them on the inner walls. CONSTITUTION:Magnetic shields 10 are composed of a plurality of sheets of band- shaped magnetic materials 4 made of silicon steel bands piled on an inner wall of a pressure container 3 and they are bent in conformity with a curved surface of the container and pressed/fixed with bolts 6 at both their end parts. The magnetic shields 10 composed of a plurality of sheets of piled band shaped magnetic materials 4 exert reaction force 11 against their bending and are closely adhere to the inner wall of the pressure container 3 and are allowed to be fixed with the bolts 6 at both their end parts. Because the piled magnetic materials are not required to adhere to themselves and besides because molding in conformity with the curved surface is unnecessary, manufacturing processes are simplified and reduction in cost can be performed. Since the number of the bolts 6 is much decreased, a cross sectional area of a passing magnetic flux can be increased and the thickness of the magnetic shields 10 can be made small.

Construction method of extremely large rectangular shield tunnel

Abstract: PURPOSE:To construct a rectangular road tunnel with an extremely large sectional area by a non-opened shield tunneling method in a short period. CONSTITUTION:A rectangular tunnel with an extremely large section is divided into horizontal slab parts 1', 2', 3' and respective slab parts are constructed in advance by respective rectangular shields 10, 11, 12 to construct horizontal slabs 1, 2, 3, The tunnel is divided into vertical main body parts 4', 5', 6', 7' connecting upper and lower horizontal slabs and respective vertical main body parts are constructed by rectangular shields 13, 14, 15, 16 to construct respective vertical main bodies 4, 5, 6, 7 and make a unified structure with respective horizontal slabs 1, 2, 3. In this way, an extremely large sectional tunnels 8, 9 can be rationally constructed in a low cost and in a short period by a non- open shield tunneling method.

Measurements and analysis of transient electromagnetic shielding effectiveness for nested shield configurations

Abstract: Electromagnetic (EM) shielding studies on totally closed box shields, one inside the other, were carried out earlier for steady magnetic fields in the frequency range from 50 Hz to 150 kHz and for plane waves from 1 MHz to 1 GHz. The calculated and the experimental values of the shielding effectiveness for a few selected materials like copper, tin-plated iron, Mu-metal and their composites in single and nested configurations, for wall thicknesses in the range of 50 /spl mu/m to 250 /spl mu/m were compared. In order to explain the discrepancies in the calculated and experimental results, need was felt to simplify the experiments and improve the accuracy and sensitivity of the measurements. The present study is a sequel to the one above but differs essentially in two aspects, viz.; (a) the shielding effectiveness is measured for the broadband (DC to 100 MHz) transient, intense (E-field in the range of 50 to 100 kV/m) and fast (rise time less than 10 nano seconds) EM fields generated in a simulation facility, and (b) the closed nested box shields are configured as open ended structures, to overcome the experimental complexities of the former. The transient fields comprising E-field, D-Dot field and B-Dot field, were characterised by indigenously developed sensor probes. A high sensitivity of 750 mV/kV/m has been achieved for the electric field sensor. The open-ended shield structures are made from 1.5 mm. thick aluminium sheets supported at the four corners of the sheets by copper rods of 6 mm. diameter. The corresponding dimensions for the shields are 250/spl times/200/spl times/75 mm. and 200/spl times/150/spl times/50 mm respectively. The paper discusses the results of the measurements thus performed on shielding effectiveness and sufficiently elaborates the design and development of E, D-Dot and B-Dot field probes. Also, discussed are the characteristics of the transient intense EM fields generated in the simulation facility.

Design of the ICRH antenna for TPX

Abstract: A 6-MW ion cyclotron (IC) system for the Tokamak Physics Experiment (TPX) is in the preliminary design phase. In conjunction with the 3-MW Lower Hybrid system and the 8-MW neutral beam system, the IC system will provide heating and current-drive capabilities to explore advanced tokamak physics and long-pulse (1000 s) operation. The IC launcher consists of six nickel-plated current straps arranged toroidally in pairs behind three water-cooled Faraday shields. The Faraday shields can be independently and remotely detached by cutting water lines at the back of the launcher and removing bolts at the front to free each shield. The antenna can be located at the +2 cm flux line and retracted 10 cm. Faraday shields are usually copper- or nickel-plated stainless steel or inconel. Titanium is the preferred material to minimize activation without greatly decreasing electrical resistivity and therefore increasing disruption loads. The IC antenna research and development programs have provided data that confirm the feasibility of B/sub 4/C-coated nickel-plated titanium alloy in the TPX environment.

Superconducting magnet system

Abstract: PURPOSE:To provide a small superconducting magnet system with a large and uniform magnetic spatial field. CONSTITUTION:In a cryostat unit, a space between an outer wall 1a and a heat radiation shield 14 is cooled at a temperature of about 80K by a refrigerator put at an upper part of a lid 2, while a space between the heat radiation shield 14 and a heat radiation shield 13 at a temperature of about 20K. A spherical superconducting coil 9 and a reserve tank 10 are put inside the heat radiation shield 13. A space with atmospheric pressure based on a room temperature and spherical core-shaped heat radiation shields 14a and 13a are included in the spherical superconducting coil 9. The radiation shields 14a and 13a are connected to the heat radiation shields 14 and 13 at the side walls and the cryostat unit.

Open Type Magnetic Shields by High Tc Superconducting Cylinders and High-permeability Cylinders

Abstract: As conventional magnetic shielded rooms are surrounded by high-permeability materials, oppressive feeling of an investigated human is inevitable. Superconducting cylinders have high shielding effects for axial external magnetic fields. Open type magnetic shields are possible by superconductor. But its shielding effects for transverse external magnetic fields are inferior, whereas high-permeability cylinders have advantage for transverse external magnetic fields. We fabricated a 1/10 size model combining high Tc superconducting cylinders and high-permeability cylinders and studied the shielding effects. Magnetic shields combining high Tc superconducting cylinders and high-permeability cylinders make use of the characteristics of both materials and offer the possibility of the compact open type magnetic shields.

Heat protection shield for heat exchanger

Abstract: Has two heat shield sheets (2) suspended within the combustion chamber which has a cover (14) and side walls (12) next to each other. The heat shields are not in connection with the walling of the combustion chamber, so that when heating occurs they are free to expand sideways. One of the heat protection shields has five apertures (22), through which the free ends of the heat exchanger tubes and of the connecting tubes for the heat exchanger tubes are insertable. Between the individual apertures within the heat protection shields dividing sheets (24) are provided, formed as tangents to the circular apertures.