Shielding Currents Research Articles

Modeling of the Reverberation Chamber Method for Determining the Shielding Properties of a Coaxial Cable

The paper considers the reverberation chamber (RC) method for the measurement of the shielding effectiveness (SE) of coaxial cables with braided shields. In particular, the voltage at the cable termination is numerically computed and compared to that measured in an RC. The RC field is represented by a finite summation of random plane waves, and a finite-difference time-domain (FDTD) code is used to calculate the outer shield current induced by the RC field. The knowledge of the shield current distribution allows the determination of the voltage at the cable termination's internal circuit after a proper numerical averaging. It is then compared to the measured voltage averaged over stirrer rotations. The method is applied to a commercially available cable model RG58, and using the nominal value for the transfer impedance of this cable type gives results in a satisfactory agreement with the measurements. Finally, the possibility of recovering the transfer impedance from the measured SE of the RC is discussed.

On Reducing the Lightning Transients in Buried Shielded Cables Using Follow-On Earth Wire

This paper investigates the importance of a follow-on buried bare earth wire for the lightning protection of buried shielded cables. The use of follow-on bare wires for lightning protection of communication towers was suggested as a recommendation in certain standards, without being complemented either by theory or experiments. When lightning transients couple to the cable shields, it induces large currents (depending on the type of coupling) causing transient overvoltages between the inner conductors and the shield. It is shown by simulations based on multiconductor transmission line theory that if the follow-on bare earth conductor is placed in parallel with the shielded cable with the bare earth wire connected to the shield at the current injection end, then the shield current, and thereby, the internal transient voltages of the cable are reduced considerably.

The Inductance Computation for the Passive Compulsator

In order to decrease the internal impedance of the passively compensated pulsed generator (or passive compulsator) and enhance its power density, air gap windings are generally selected as the armature winding of the passive compulsator, for its inductance is much lower than that of the general embedded windings in the machine slot. Due to that the peak power of an electromagnetic launcher system driven by a compulsator is simply determined by system impedance and the output voltage of the compulsator, it becomes necessary to gauge the total inductance of the air gap windings accurately with straight forward calculation, while neglecting the easily obtained resistance parameters, particularly in its design period for electromagnetic launchers. By taking into account the flux compression effects generated by the eddy current of the continuous conductive shield in the compulsator, the magnetic field of the armature coils in the air gap of generator is analyzed by Ampere's law. The analytical solution to the inductance of armature air gap coil is deduced. This one equation solution is used for re-designing a 25 MW compulsator with the final calculation result: 4.04 muH. The measurement on the compulsator (4.05 muH) verified the accuracy of the analysis equation.

Homogeneity of supercurrent flow in coated conductors

The development of coated conductors is now entering a stage where they become available in lengths of several meters. A technique capable of analysing the local superconducting properties of long samples within a relatively short time is therefore highly desirable. In this paper, the application of a scanning technique, initially developed for the characterisation of bulk materials [M. Eisterer, S. Haindl, T. Wojcik, H.W. Weber, Supercond. Sci. Technol. 16 (2003) 1282] to coated conductors is presented. The principle consists of spatially mapping the magnetic field ( B z ) perpendicular to the top sample surface, generated by local shielding currents. The supercurrents are induced by a permanent magnet placed above the scanning Hall probe. This technique was successfully applied to a sample with a length of 10 cm, providing information on the tape homogeneity. Local disturbances of the supercurrent flow and spatial variations of the critical current density are detected in this way on the scale of ⩽1 mm.

A Boundary Integral Formulation on Unstructured Dual Grids for Eddy-Current Analysis in Thin Shields

A 3-D boundary integral method (3-D BIM) capable of analyzing eddy currents in thin shields is presented. This novel approach is formulated in terms of loop currents in order to implicitly fulfill the div-free condition for quasi-magnetostatic problems. Using nonorthogonal dual grids unknowns are defined on nodes, resulting in a very efficient formulation with limited memory requirements. The proposed method is tested on an axisymmetric model showing a good agreement with the analytical solution. 3-D BIM is then applied to analyze a real case of low-frequency magnetic shielding

Effect of voltage–current characteristic smoothness on the stability margin of cable-in-conduit conductors

Experimental and numerical investigation of the AC losses induced by the transient magnet field (TMF) in cable-in-conduit conductors (CICC) revealed some interesting results. First of all, the critical current in the strands could be exceeded as a result of simultaneous increase of the strand temperature due to AC losses and the addition of the induced shielding currents to the transport current. Secondly, after the sample reaches its critical state the energy stored in the loops formed by the superconducting strands and normal “matrix” quickly dissipates in the normal matrix of the strands and in the normal “matrix” of the bundle. This results in a step increase of the conductor temperature. It was logical to assume that during the process preceding the occurrence of the first flux-jump, the maximum temperature and shielding current could depend on the smoothness of the voltage–current characteristic (VCC) of the superconducting strands. In other words, the smoothness of the VCC of the superconducting strands should influence the stability margin of a conductor. A numerical investigation of the VCC smoothness effect on stability margin was carried out for a wide range of the Stekly parameter, α, and varying the dimensionless transport current i = I t/ I c (0 ⩽ α < 50; 0 ⩽ i < 1). As a result of these calculations, it was found that the energy necessary for the first flux-jump to occur, grows as the VCC smoothness increases, when the Stekly parameter lies in the range 0 ⩽ α < 1, and the stability margin of a conductor decreases as the VCC smoothness increases, when the Stekly parameter α ⩾ 1.

Simulation of the current dynamics in a superconductor induced by a small permanentmagnet: application to the magnetoscan technique

The magnetoscan is a rather new technique for investigating local inhomogeneities of large bulkhigh-Tc superconductors. Local (shielding) currents, which are induced by a small permanentmagnet slightly above the sample surface, generate a magnetic field that is recorded by aHall probe. We report on a theoretical analysis of such a three-dimensional system usingnumerical simulations to calculate the current dynamics during a magnetoscan.The magnetoscan signal is analysed for different experimental setups and different criticalcurrent densities of the superconductor. We show that the shape of small defects(∼1 mm in size) is almost exactly displayed, even if the radius of the permanent magnet is muchlarger than the defect size. The critical current density of larger inhomogeneities can bedirectly determined at the sample surface, if all details of the experimental setup areknown. Different aspects of how to improve the experimental resolution are discussed.

Magnetization collapse in polycrystalline YBCO under transport current cycles

We report measurements of the hysteretic magnetization of YBCO under superimposedtransport current cycles, together with numerical simulations of magnetization andcurrent density profiles in the corresponding parallel configuration. Field cooled(FC) and zero-field-cooled (ZFC) experiments were carried out on polycrystallineYBa2Cu3O7−x cylinders, with both the applied magnetic field and transport current in the axial direction,and the current cycled several times, around and above the dissipative threshold. As inpreviously reported multicomponent field configuration experiments, the magnetization isseen to collapse to a more stable state both in FC and ZFC, because of the interplaybetween the shielding and transport currents. The results of our numerical simulationsare in good qualitative agreement with the measurements, and the competitionbetween shielding and transport due to vortex-pinning interactions and equilibriummagnetization effects are shown to play an important role in the range of our experiments.

Development of a single-phase 30 m HTS power cable

HTS power transmission cables appear to be the replacement and retrofitting of underground cables in urban areas and HTS power transmission cable offers a number of technical and economic merits compared to the normal conductor cable system. A 30 m long, single-phase 22.9 kV class HTS power transmission cable system has been developed by Korea Electrotechnology Research Institute (KERI), LS Cable Ltd., and Korea Institute of Machinery and Materials (KIMM), which is one of the 21st century frontier project in Korea since 2001. The HTS power cable has been developed, cooled down and tested to obtain realistic thermal and electrical data on HTS power cable system. The evaluation results clarified such good performance of HTS cable that DC critical current of the HTS cable was 3.6 kA and AC loss was 0.98 W/m at 1260 Arms and shield current was 1000 Arms. These results proved the basic properties for 22.9 kV HTS power cable. As a next step, we have been developing a 30 m, three-phase 22.9 kV, 50 MV A HTS power cable system and long term evaluation is in progress now.

Shielding current effects on diffraction patterns in the critical current of superconducting quantum interference devices

By a simple schematization of the current distribution in a two-finite-size-junction interferometer, we study the shielding current effects on the diffraction envelope of the interference figure given by the critical current as a function of the applied magnetic field. By this scheme we are able to reproduce the known results in the limit of negligible shielding currents for both conventional interferometers and for those containing one π-junction. For non-negligible values of the circulating current contribution to the superconducting phase in the junctions, we find that the superimposed quantum interference and diffraction patterns suffer a contraction with respect to the horizontal field axis.

Active shielding of cylindrical saddle-shaped coils: Application to wire-wound RF coils for very low field NMR and MRI

We provide an exact expression for the magnetic field produced by cylindrical saddle-shaped coils and their ideal shield currents in the low-frequency limit. The stream function associated with the shield surface current is also determined. The results of the analysis are useful for the design of actively shielded radio-frequency (RF) coils. Examples pertinent to very low field nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) are presented and discussed.

Current Distribution Analysis in Tri-Axial HTS Cable Considering Three Phases

High Temperature Superconducting (HTS) cables have been recently studied because of low loss and compactness, compared with conventional copper cables. We, so far, analyzed current distributions in a single-phase coaxial cable with and without shield current. Three-phase cables are usually composed of the three single-phase cables. However, a tri-axial cable, composed of concentric three phases, has more advantages compared with the three coaxial cables, because of reduced HTS tapes and leakage field. We derive equations governing layer current and current density distributions in the tri-axial cable considering other phases. The current and current density distributions are described as explicit functions of cable construction parameters, such as radius, twist pitch and twist direction. We calculate the homogeneous current and current density distributions by choosing adequate cable construction parameters.

Two- and three-dimensional connectivity and current distribution in YBa2Cu3Ox-coated conductors

The microstructure of YBa2Cu3Ox sYBCOd-coated conductors depends strongly on the deposition method and thickness of the YBCO layer. This letter shows how the clear qualitative difference in grain connectivity between vacuum-deposited and solution-grown layers has direct consequences for the spatial distribution of the critical current density sJcd. Pulsed-laser-deposited YBCO conductors usually have a columnar grain structure that results in a two-dimensional current network, as demonstrated with magneto-optical imaging. Consequently, their transport Jc varies considerably on length scales from 50 mm up to 5 mm, with current suppression occurring even at defects that run parallel to the macroscopic current. In contrast, the thicker YBCO coatings in metalorganic-deposited samples have a layered structure, leading to a three-dimensional current path. Magneto-optically, this is deduced from sample-wide shielding currents, while transport experiments reveal much smaller spatial variations in Jc. These results are encouraging for the further development of nonvacuum produced YBCO-coated conductors, since such three-dimensional systems are inherently more “forgiving” of local defects.

Surface superconductivity in niobium for superconducting RF cavities

A systematic study is presented on the superconductivity (SC) parameters of the ultrapure niobium used for the fabrication of the nine-cell 1.3 GHz cavities for the linear collider project TESLA. Cylindrical Nb samples have been subjected to the same surface treatments that are applied to the TESLA cavities: buffered chemical polishing (BCP), electrolytic polishing (EP), low-temperature bakeout (LTB). The magnetization curves and the complex magnetic susceptibility have been measured over a wide range of temperatures and DC magnetic fields, and also for different frequencies of the applied AC magnetic field. The bulk superconductivity parameters such as the critical temperature T c = 9.26 K and the upper critical field B c 2 ( 0 ) = 410 mT are found to be in good agreement with previous data. Evidence for surface superconductivity at fields above B c 2 is found in all samples. The critical surface field exceeds the Ginzburg–Landau field B c 3 = 1.695 B c 2 by about 10% in BCP-treated samples and increases even further if EP or LTB are applied. From the field dependence of the susceptibility and a power-law analysis of the complex AC conductivity and resistivity the existence of two different phases of surface superconductivity can be established which resemble the Meissner and Abrikosov phases in the bulk: (1) “coherent surface superconductivity”, allowing SC shielding currents flowing around the entire cylindrical sample, for external fields B in the range B c 2 < B < B c 3 coh , and (2) “incoherent surface superconductivity” with disconnected SC domains for B c 3 coh < B < B c 3 . The “coherent” critical surface field separating the two phases is found to be B c 3 coh = 0.81 B c 3 for all samples. The exponents in the power law analysis are different for BCP and EP samples, pointing to different surface topologies.

Modelling of the field line penetration and force transfer by the dynamic ergodic divertor of TEXTOR

For describing the penetration of the external dynamic ergodic divertor magnetic field into the plasma, a single fluid magnetohydrodynamic model has been applied which contains all three components of the magnetic and electric field, the electrical currents and the flow pattern. The set of equations is solved numerically. The analysis shows that the external magnetic field penetrates nearly unperturbed into the plasma edge; at the resonance layer, where the external coils are parallel to the internal magnetic field lines, a strong shielding current is generated. Depending on the frequency difference between the external field and the plasma, the width and the amplitude of the shielding current varies: With increasing frequency, the amplitude and the width of the resonant layer grows while with decreasing frequency both quantities are diminished. The model predicts that the shielding current is connected with a localized flow of the plasma. The force transfer function is calculated from the integration of the j × B term. Differences between the expected edge and core interactions are discussed.

Floating shield current suppression trap

AbstractShield currents or common mode currents affect coil tuning, coil‐to‐coil coupling in phased array coils, image inhomogeneity, and most importantly can cause serious patient burns. Traditionally in MRI, shield currents are reduced by cable traps; they consist of a wound coaxial cable inductor tuned to the desired resonance frequency by a capacitor between end turns of the coaxial cable ground shield. This method increases losses and effects the overall phase distance between the coil and the preamplifiers. We present a cable trap that does not connect or solder to the cable and is completely splitable, allowing easy fitting over any cable without affecting any coil parameters (tuning or phase length). Multiple cables can be placed inside the shield current trap to simultaneously attenuate the shield currents from up to eight cables, as currently designed. The shield current trap reduces shield currents at 64 MHz by ∼30 dB. © 2004 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 21B: 26–31, 2004.

‘Magnetoscan’: a modified Hall probe scanning technique for the detection of inhomogeneities in bulk high temperature superconductors

We present a novel technique for the investigation of local variations of the critical current density in large bulk superconductors. In contrast to the usual Hall probe scanning technique, the sample is not magnetized as a whole before the scan, but locally by a small permanent magnet, which is fixed near the Hall probe, during the scanning process. The resulting signal can be interpreted as a qualitative measure of the local shielding currents flowing at the surface.

Highly balanced single-layer high-temperature superconductor SQUID gradiometer freely movable within the Earth's magnetic field

We developed a gradiometer system based on a single-layer high-temperature superconductor dc superconducting quantum interference device (SQUID), which can be freely moved within the Earth's magnetic field during measurement. The problem of circumferential shielding currents in the parallel gradiometer pick-up loop is solved by the use of an appropriately designed magnetometer SQUID integrated on the gradiometer chip. The magnetometer's feedback coil of the flux-locked loop is laid out as a small Helmholtz coil pair, thus keeping the homogeneous magnetic field constant for both the magnetometer and the gradiometer. Therefore, the balance of the directly coupled gradiometer SQUID is enhanced from 100 up to 3800. The noise limited magnetic field gradient resolution of 45 pT m−1 Hz−1/2 is preserved down to frequencies of several Hz even after strong motion in the Earth's magnetic field.

Critical state models for a granular ferromagnetic superconductor

Measurements of the magnetic moment M as a function of the magnetic field B can be used to extract information about the penetration behaviour of the magnetic field in a superconducting sample. In granular superconductors the response to an applied AC field is governed by both intra- and intergranular shielding currents. We have employed low-field AC susceptibility measurements to investigate the magnetic properties of a polycrystalline RuSr 2 GdCu 2 O 8 weak ferromagnet in its superconducting state. For fields below ≃1 mT the results are consistent with the Bean critical state model. For B>1 mT the Kim model is more suitable for the description of the sample's properties.

Common mode signal rejection methods for MRI: Reduction of cable shield currents for high static magnetic field systems

AbstractThis experiment was conducted to identify common mode currents that flow on the shield of a coaxial cable and to investigate techniques that reduce their effects at high fields of ≥3 Tesla. It will be shown that there are two different methods of common mode interference for two different coil types. First, is the transceive case, where common mode cable shield currents flow as a result of unbalanced loop voltages that couple to the cable shield. In the second case, unwanted common mode signals are induced from the field of the transmission coil onto the cable shield of the receive‐only coil. Two important factors relating to the investigation of common mode signals are danger to the patient from an RF burn and the reduction of the signal‐to‐noise ratio (SNR) because of extra noise being introduced into the MRI environment. It will be shown that through the use of cable shield traps and balun matching circuits, common mode signals can be reduced significantly, increasing patient safety and SNR. © 2003 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 19B: 1–8, 2003.