An Experimental Investigation of the Transient Response of HTS Non-insulation Coil

A single pancake coil without turn-to-turn insulation was tested in this paper to investigate the transient responses under different situations. We performed charging and discharging test, AC current test, and regional quench emulation test on the non-insulated (NI) coil. The experimental test results show a significant time delay for charging and discharging characteristics of NI coil and can be validated by a simple proposed equivalent electrical circuit. Under the AC operating current, the NI coil can bypass nearly all the AC current from the coil spiral path to the radial path such that it is not possible for NI coil to store or be affected by the AC magnet field. Additionally, while carrying AC current, the AC loss dissipation of NI coil is inversely proportional to the frequency of the AC operating current. When a regional quench occurs, the NI coil can bypass the current in the regional quench zone to avoid further temperature accumulated and protect the NI coil itself.     

Study on cross section of high temperature superconducting coil

It is in particular of importance for HTS coils to secure a larger central magnetic field and/or a large stored energy with shorter length of HTS tapes. The critical current of an HTS tape depends on both the flux density and the flux angle against tapes. From this point, the performance improvement of HTS coils is taken into account with an analytical model. The minimum volume coil derived from the Fabry Factor constant curve is taken concerning the original coil shape, which is often employed in low temperature superconducting coils. The coil critical current was analyzed in consideration of the anisotropic properties of the tape.The electric field of HTS tapes in the coil was calculated at the coil critical current and the high electric field portion were cut out. The optimal coil cross section is obtained by iterating this calculation process. As a result, the critical current and the stored energy density of the coil were improved. The stored energy density increased about 17% and the central magnetic field was almost kept constant regardless of 19% reduction of HTS tapes, as compared with the original coil with the rectangular cross section.     

Influence of self-magnetic field on a.c. quench current level of superconducting coils

The a.c. quench current level of a superconducting cable when formed into a coil is, in general, lower than that of a short sample. The current in the coil induces a self-magnetic field on the superconducting winding. It was found from our experiments that the transition from the superconducting state to the normal one in a superconducting coil originates in that part of the winding where the self-magnetic flux density is estimated to be the largest. It is concluded that degradation of the a.c. quench current level in the superconducting coil is mainly brought about by the influence of the self-magnetic field.     

Instruments for measuring the energy parameters of kinetron fluxes

Practical methods of measuring the energy parameters of kinetron fluxes using the Lorentz effect, due to the excitation of kinetron fluxes by a moving magnetic field, which initiate a current in the collector coil of a measuring system, are considered. Fairly simple specific designs of the measuring instruments are described. Translated from Izmeritel’naya Tekhnika, No. 5, pp. 62–66, May, 2009.     

Effect of Write Current Waveform on Magnetization and Head-Field Dynamics of Perpendicular Recording Heads

The response of perpendicular magnetic recording heads with a single turn and two turn coils is calculated using a full micromagnetic model, including return pole and soft underlayer. We study the effect of coil current waveforms with different rise times and overshoots. For fast coil current rise times, the head field shows very little response, until the coil current changes its polarity and it is limited by the intrinsic magnetization dynamics. Even the effect of overshoot is limited by the same mechanism. Shorter yoke length and coil turns close to the air-bearing surface improve the head-field dynamics.     

Experimental analysis of AC loss and recovery characteristics for proposal of effective thickness of spacers in pancake type

For the design of superconducting fault current limiter, the research about recovery time and AC loss is essential issue because this characteristic is closely related to stability and efficiency. In general, superconducting fault current limiter (SFCL) modules can be made into pancake or solenoid type coil. While the pancake type coil has smaller AC loss, it has longer recovery time compared with the solenoid type coil. In this paper, a new pancake type coil was proposed to decrease recovery time by improving their cryogenic condition. A new pancake type coil is made with spacers between adjacent superconducting tapes. However, as the thickness of the spacer was increased, the AC loss was increased unlike the recovery characteristic. From the experimental results, our group confirmed that the pancake type coil with the spacer has trade-off relationship between AC loss and recovery characteristics with the spacer thickness.The proposed results in this paper would be utilized to design superconducting fault current limiters.     

Proposal of rectifier type superconducting fault current limiter with non-inductive reactor (SFCL)

A rectifier type superconducting fault current limiter (SFCL) with non-inductive reactor has been proposed. The concept behind this SFCL is the appearance of high impedance during non-superconducting state of the coil. In a hybrid bridge circuit, two superconducting coils connected in anti-parallel: a trigger coil and a limiting coil. Both the coils are magnetically coupled with each other and have same number of turns. There is almost zero flux inside the core and therefore the total inductance is small during normal operation. At fault time when the trigger coil current reaches to a certain level, the trigger coil changes from superconducting state to normal state. This super-to-normal transition of the trigger coil changes the current ratio of the coils and therefore the flux inside the reactor is no longer zero. So, the equivalent impedance of both the coils increased thus limits the fault current. We have carried out computer simulation using EMTDC and observed the results. A preliminary experiment has already been performed using copper wired reactor with simulated super-to-normal transition resistance and magnetic switches. Both the simulation and preliminary experiment shows good results. The advantage of using hybrid bridge circuit is that the SFCL can also be used as circuit breaker. Two separate bridge circuit can be used for both trigger coil and the limiter coil. In such a case, the trigger coil can be shutdown immediately after the fault to reduce heat and thus reduce the recovery time. Again, at the end of fault when the SFCL needs to re-enter to the grid, turning off the trigger circuit in the two-bridge configuration the inrush current can be reduced. This is because the current only flows through the limiting coil. Another advantage of this type of SFCL is that no voltage sag will appear during load increasing time as long as the load current stays below the trigger current level.     

Thermal-Electromagnetic Analysis for 14 kA Fast Discharge of a Model Coil

A model coil for 40-T hybrid magnet superconducting outsert magnet has been constructed and tested at the High Magnetic Field Laboratory, Chinese Academy of Sciences. The model coil was wound with Nb₃Sn cable-in-conduit conductor (CICC) cabled in a 316LN jacket cooled with supercritical helium. The model coil alone can produce about 4 T maximum magnetic field with an operating current of 14 kA. The model coil, in combination with 7.57-T NbTi background coil, can produce 11.5 T central field at 14 kA. During the test campaigns, a fast discharge was triggered by a dump resistor of 3.6 mΩ to evaluate the thermal-electromagnetic behavior of the model coil. In order to avoid a quench of the background coil, no current was exerted on the background coil through a power supply during the fast discharge of the model coil. The test results show that the central magnetic field is not scaled proportionally to the current decay of the model coil. The circuit model gives excellent results compared with the measured ones for the central magnetic field evolution as a function of time in this paper. For the thermal-hydraulic behavior during the fast discharge, the maximum temperature at the inlet simulated by the 1-D Gandalf code gives excellent agreement results compared with the measured ones with the conductor coupling time constant of 63 ms.     

Eddy-Current Displacement Sensing Using Switching Drive Where Baseband Sensor Output Is Readily Available

In eddy-current displacement sensing, existing techniques mostly involve a high-frequency sinusoidal carrier wave and deal with the demodulation of this signal. In this paper, we give a new design idea. We use switching circuits, in which the sensing coil is included as an inductor and driven by a switching voltage, and some baseband sensor output is readily available. Thus, no explicit demodulation is needed, and the electronics are simplified. We give three designs of such switching circuits. The first is based on coil inductance, the second is also based on coil inductance but is different from the first in many ways, and the third is based on eddy-current power loss. These circuits are tested on hardware electronics. The results seem to indicate that the presented techniques would permit performances comparable to those achieved by existing techniques.     

ORPUS-1 - A pulsed superconducting solenoid

A recent series of reference designs for Tokamak Experimental Power Reactors (EPR's) has indicated that superconducting poloidal field (PF) coils will be necessary for successful operation of these devices. It would also be desirable to use superconducting PF coils in earlier tokamak fusion devices if such coils could be developed quickly enough. In this paper, the PF coil performance requirements are briefly reviewed and some implications for the coil design are developed. A small coil (stored energy 14 kJ) has been built using construction techniques similar to those which could be employed for PF coils. The coil has been charged at rates up to 2 T/sec. Both maximum field and charging rate were limited by available power supplies. Loss measurements were carried out during pulsed operation and data for hysteretic and eddy current loss are presented. The loss measurement system used allows considerable insight into the effects of conductor motion and training.     

Analysis of current distributions in a multi-laminated HTS tape conductor for solenoid coils

A multi-laminated HTS tape conductor has been recently developed for large coils. If the HTS tapes are simply laminated to form the conductor, the current distribution in the laminated tape conductor of the coil is imbalanced because of the differences among inductances of tapes. Transposition of the tapes in the conductor is effective for homogeneous current distribution, but the tape may be damaged due to the lateral bending. The solenoid coil has enough space to transpose the tapes at both ends. However, a proposed theory so far requires a restriction in the number of coil layers for homogeneous current distribution in the laminated tape conductor. It is very important to analyze current distributions in the multi-laminated tape conductor for the solenoid coil with arbitrary layers. In this paper, we apply the Maxwell integral equation to the region contoured by adjacent laminated tapes to analyze the current distributions of the tapes in an infinite solenoid coil, and demonstrate that the flux across the region is conserved as long as the tapes are not saturated, and finally induce the fundamental equations as functions of coil construction parameters, such as layer radii, laminated tape spaces, and winding pitches. We use the fundamental equations for 2-layer and 4-layer coils to verify the homogeneous current distribution of the laminated tape conductor for an arbitrary layer number. Since the flux between the tapes in the inner layer of a 2-layer coil is contributed from the outer layers, the tape space in the outer layer must be larger than that in the inner layer because of the balance between the two fluxes. Moreover, we have developed an analysis method for a finite solenoid coil.     

Numerical analysis of high-frequency induction heating includingtemperature dependence of material characteristics

We have carried out a numerical analysis of the magnetic field on high-frequency induction heating. This analysis includes the dependence of various magnetic properties on temperature. The required characteristics are obtained experimentally. We compare the experimental results with the theoretical values obtained by approximations. Until now, the current density inside the exciting coil on this kind of problem has been assumed to be uniform, which is different from actual phenomena. We propose a new method which takes the inhomogeneous distribution of exciting current into account. In this analysis, the eddy current of the exciting coil is also taken into account     

Design and failure modes of automotive suspension springs

This paper is a discussion about automotive suspension coil springs, their fundamental stress distribution, materials characteristic, manufacturing and common failures. An in depth discussion on the parameters influencing the quality of coil springs is also presented.

Following the trend of the auto industry to continuously achieve weight reduction, coil springs are not exempt. A consequence of the weight reduction effort is the need to employ spring materials with significantly larger stresses compared to similar designs decades ago. Utilizing a higher strength of steel possesses both advantages and disadvantages. The advantages include the freedom to design coil springs at higher levels of stress and more complex stresses. Disadvantages of employing materials with higher levels of stress come from the stresses themselves. A coil’s failure to perform its function properly can be more catastrophic than if the coil springs are used in lower stress. As the stress level is increased, material and manufacturing quality becomes more critical. Material cleanliness that was not a major issue decades ago now becomes significant. Decarburization that was not a major issue in the past now becomes essential.

To assure that a coil spring serves its design, failure analysis of broken coil springs is valuable both for the short and long term agenda of car manufacturer and parts suppliers. This paper discusses several case studies of suspension spring failures. The failures presented range from the very basic including insufficient load carrying capacity, raw material defects such as excessive inclusion levels, and manufacturing defects such as delayed quench cracking, to failures due to complex stress usage and chemically induced failure. FEA of stress distributions around typical failure initiation sites are also presented.     

Comparison of parallel- and counter-flow circuiting in an industrial evaporator under frosting conditions

This paper describes a theoretical model of a large-scale, ammonia-fed evaporator coil used in an industrial refrigeration system and operating under low temperature air and refrigerant conditions that are typically encountered in refrigerated storage spaces. The model is used to simulate the performance of counter-flow and parallel-flow circuited evaporator coil designs under frosting conditions. The counter-flow frost model is validated using in situ data obtained from a field-installed evaporator coil. The performance of an evaporator in a parallel-flow circuit arrangement is simulated and compared to counter-flow circuiting. The effects of coil circuiting are evaluated in terms of the frost distribution across the evaporator coil and the associated reduction in cooling capacity during operation.     

Instabilities above critical current region in Bi-2223/Ag superconducting coils cooled by liquid nitrogen

A sudden drop of the coil voltage and a hysteresis of I-V curve were observed in measurement of one-layer Bi-2223/Ag coils cooled by liquid nitrogen at currents well above critical current region. Their temporal behavior indicates, that the improvement of the cooling and corresponding decrease of temperature after the jump takes place. To study this phenomenon we measured I-V curves of two Bi-2223/Ag coils made from tapes with various degree of critical current homogeneity and analogical curves of two non-superconducting coils made from thin Cu tapes having various widths. In Cu coils we really observed a sudden drop of the temperature, measured in parallel with Cu resistance drop, after reaching heat flux of about 0.4 W cm⁻² during current ramping up. In spite of non-superconducting character of the tape, the hysteresis, i.e. difference between increasing branch and decreasing branch of I-V curves, was observed too! Approximately the same value of heat flux, at current corresponding to the jump, was found also in superconducting coil on segment with least value of local critical current. We conclude that observed voltage drop of the Bi-2223/Ag does not bear upon superconducting nature of the coil and, as that for Cu coil, can be explained by dynamics of heat transfer to liquid nitrogen and its history.     

Magnetic and eddy current effects in an open-loop pulsed hysteresisgraph system for magnetization of rare-earth magnets

Describes an open-loop pulsed hysteresis graph system that is capable of magnetizing ceramic and rare-earth permanent magnets such as neodymium, samarium cobalt, and Alnico in cylindrical and rectangular shapes. The prototype system relies on an air-core excitation coil with an inner diameter of 3 cm and a length of 10 cm. A pulsed power supply provides a transient current pulse of up to 9000 A into the air-core coil. The pulse duration is typically about 15-20 ms. In addition, a pair of Helmholtz coils records the applied magnetic field strength, whereas a local coil, tightly wound around the permanent magnet, records the magnetic flux density in the sample. Both fields are electronically acquired, digitized, and processed in a computer to obtain the hysteresis graph response of the sample. The processing allows compensation for the presence of the demagnetization field due to the open-loop system configuration. We present an analytical approach to take into account the effect of permeability in cylindrical samples. We propose a numerical approach that can characterize conductive materials such as Alnico, whose eddy-current influence significantly affects the open-loop recording of the B-H curve     

High-frequency operation of the coils for standard magnetic-field generation using the bulk-current technique

The sum of the currents in the individual wires of a multiturn coil (here referred to as the bulk current) possesses properties that can extend the useful operating range of the device to frequencies substantially higher that those obtainable through the traditional control of the terminal current. This paper theoretically analyzes and experimentally confirms some basic results about the high-frequency behavior of coils. The analysis is initially based on the multiconductor transmission line theory applied to an equivalent line model obtained through a simple geometrical transformation of the coil; then closed-form formulas predicting the essential deviations from ideal at high frequencies (well beyond the first resonance) are presented. The response to different types of feed (balanced or single ended) is also described. The "bunch" or "bulk" current concept is a new finding, not existing in the previous literature. The analysis of the bulk current reveals that the magnetic field remains constant and predictable up to frequencies many times higher than the resonant frequency of the coil. Experimental results confirming the theoretical predictions are presented and discussed.     

The fault current limiting characteristics of a flux-lock type high-Tc superconducting fault current limiter using series resonance

We analyzed the fault current limiting characteristics of a flux-lock type high-T_c superconducting fault current limiter (HTSC-FCL) using series resonance between capacitor for series resonance and magnetic field coil which was installed in coil 3. The capacitor for the series resonance in the flux-lock type HTSC-FCL was inserted in series with the magnetic field coil to apply enough magnetic field into HTSC element, which resulted in higher resistance of HTSC element.However, the impedance of the flux-lock type HTSC-FCL has started to decrease since the current of coil 3 exceeded one of coil 2 after a fault accident. The decrease in the impedance of the FCL causes the line current to increase and, if continues, the capacitor for the series resonance to be destructed. To avoid this operation, the flux-lock type HTSC-FCL requires an additional device such as fault current interrupter or control circuit for magnetic field.This paper investigated the parameter range where the operation as mentioned above for the designed flux-lock type HTSC-FCL using series resonance occurred from the experimental results. In the design of the flux-lock type HTSC-FCL, the some methods to avoid the continuous increase of the line current were suggested and confirmed by the experiments that the suggested methods were available to prevent the continuous increase of the line current after a fault happened.     

Frequency effects in a pancake coil

An aluminum pancake coil is designed to amplify pulsed power in the range of 100-MW to a 100-mΩ electro-thermal load. For the tradeoff between the coil energy density limit and energy discharge efficiency, the frequency dependence of its current and magnetic field distributions is calculated in the range of DC to 1 kHz. Near DC magnetic field measurements are compared to the calculated results. Energy efficiency of about 85% has been measured for the discharge of 1.6 kA from the coil to a linear resistive load in the range of 40-400 mΩ     

Use of a pulsed magnetic field to increase the position sensitivity of a photomultiplier

It is well known that photomultipliers (PMTs) are sensitive to external magnetic fields. We have used this property to vary the gain of the PMT depending on the position of the light source. Various coil configurations have been evaluated to produce local magnetic fields in the region between the photocathode and first dynode to maximize the amplitude variation in the anode signal with respect to change in the coil current. The aim was to improve the position sensitivity of a PMT used in positron emission tomography (PET). The position sensitivity was tested with an array of collimated light emitting diodes directed towards the photocathodes of a Hamamatsu R1548 PMT. The best coil position was above the photocathodes. A 60% reduction in output for light pulses from two LEDs near the coil was obtained with a current of 43 Ampere-turns while signals from two LEDs far from the coil remained the same. The rise time of the magnetic field was 2 μs.