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Triaxial HTS AC power cables, that incorporates three HTS phases wound around a single core within a single cable is considering the optimal solution for low and medium voltages. Such a design permits to save expensive HTS conductor and to increase the power density transmitted. It is difficult to increase the transmission capacity of a triaxial cable by increasing its operating voltage and therefore, insulation thickness between phases. The other way is to increase the operation current by using a multilayer structure in each phase of the triaxial cable. This task demands a complicated optimization analysis and a precise manufacturing technology. Following our previous studies, we developed and tested the triaxial cable prototype with two layers per phase made of ReBCO wire. The optimal parameters of the cable (twist pitch, diameters of phases, etc.) have been determined using the numerical simulation methods developed. DC and AC tests of the cable has been performed. In the result, the feasibility to provide a uniform current distribution in a triaxial cable, each phase of which consists of two layers, has been demonstrated. With the outer diameter of ∼24 mm only, this is the most compact triaxial HTS power cable manufactured and tested up to date with currents up to 4 kA per phase. The anticipated power transfer with three phases of this cable can be as large as ∼34 MW.

In the framework of the project where space in a cryostat is strictly limited, the high temperature superconductor (HTS) segments of the current leads have to be placed horizontally and in a scattered magnetic field. We developed the current leads optimization model for such conditions. In this paper, we present the results of optimization for 2000 A HTS current leads, comprised of 2G HTS tapes and able to work in the magnetic field up to 2 T. The optimal parameters of the current leads segments have been determined using a numerical simulation. In order to approach the maximum efficiency, it is necessary to have a very good heat exchange between the current leads and evaporating helium gas. The impact of the external magnetic field on the heat leak from current leads into liquid helium is studied.

Triaxial HTS power cables are the optimal solution for low and medium voltages. This design permits to save expensive HTS conductor and to increase the power density transmitted. They could be used at medium voltages ~10-20 kV for distribution grids. At relatively low voltages (

This article describes results of the development, construction, and test of the first three-phase distribution power transformer prototype in Russia with windings made from second-generation HTSC wire and iron core from amorphous electrical steel. The design of its elements is described and improvement of HTSC transformer performance characteristics compared to conventional power transformers with the same power rating is shown.

Progressive advance in technology of second-generation (2G) high-temperature superconducting (HTS) superconductors leads to the continuous rise of their critical-current density. Critical current of 200 A at 4-mm tapes and ~600 A at 12-mm tapes is already state of the art. One problem is that critical currents of HTS tapes have sounding anisotropy. Moreover, it was found that in external magnetic field, the Lorentz force direction is affecting the critical currents of 2G HTS tapes. Two hypotheses were suggested as a reason for this phenomenon. This is the result of the properties of crystal lattice in a 2G HTS tape or of spatial inhomogeneity of critical currents across the width of HTS tapes. In this paper, the verification of hypotheses was done by the cutting of wide 2G HTS tapes into narrow tapes and measuring of critical-current anisotropy in narrowed tapes. We present the experimental results and their discussion. We concluded that the spatial inhomogeneity of critical-current density in the cross section of a tape is the reason for the Lorentz force direction influence on the anisotropy of 2G HTS tape critical currents.

In the framework of the second stage of the Russian R&D program for the development of hybrid energy transfer lines (HETLs), the new 30-m $\hbox{MgB}_{2}$ superconducting cable with high voltage insulation has been developed and tested. The superconducting cable was inserted into a newly developed flexible 30-m hydrogen cryogenic line that has three sections with different types of thermal insulation in each section. High-voltage current leads were also developed. The superconducting cable, cryostat, and current leads have been tested in October 2013. Cable critical current was $\sim$ 3500 A at $\sim$ 21 K. Cable and current leads passed a high voltage test with 50-kV dc at liquid hydrogen temperature. The tests were performed at temperatures from 20 to 26 K, hydrogen flow from 70 to 450 g/s, and pressure from 0.25 to 0.5 MPa. It was found that the active evaporating cryostatting system as a thermal insulation practically eliminated heat transfer from room temperature to liquid hydrogen. The flexible 30-m HETL developed is able to deliver $\sim$ up to 60 MW of chemical power and $\sim$ 75 MW of electrical power, i.e., $\sim$ 135 MW in total. $\hbox{MgB}_{2}$ cable design and test results of hybrid energy transfer lines are presented and discussed.

To form a cable-in-conduit conductor (CICC) a cable should be inserted into a jacket under tensile force that is increasing with length of insertion. This tensile force can lead to both elongation and untwisting of a cable with increase of a twist pitch. The untwisting can also be induced by stress accumulated in the material during the cabling process. Understanding the untwisting process is very important, because the twist pitch of a cable inside a CICC strongly affects properties such as ac loss. In this paper, we present the results of an experiment on RF toroidal field (TF) cable sample tensile tests using the ≪Hitachi Cable≫ (Japan) tensile machine, and also the results of an experiment performed at the jacketing line in the Russian Scientific R&D Cable Institute (VNIIKP) after TF cable reinsertion. In the latter experiment, twist pitches were measured along the full cable length. A model to calculate untwisting and stretching processes has been developed. The results of the calculations are compared with the experimental data.

The residual resistance ratio (RRR) is an important parameter affecting the stability of superconductors and the quench protection properties of magnets. During the manufacture of cable-in-conduit-conductors based on Nb3Sn strands, RRR may change noticeably. We studied the RRR of strands in as-received condition and at different stages of the TF conductor manufacturing (cleaning and Cr-plating of strands, cabling and final compaction). The RRR of strands extracted from a TF conductor sample after electromagnetic and thermal cycling at the SULTAN test facility has been studied also. For this purpose after testing at SULTAN the strand samples were extracted from different locations in the cross-section of the TF conductor and from different positions along the axis. The results about the RRR variation during the conductor manufacturing process and after SULTAN tests are presented and discussed.

The residual resistance ratio (RRR) is an important parameter affecting the stability of superconductors and the quench protection properties of magnets. During the manufacture and testing of cable-in-conduit-conductors, RRR may change noticeably. Recently the RRR changes of Nb 3 Sn strands for conductors of the ITER toroidal coils have been studied during the manufacturing process, heat treatment and SULTAN testing. Here we present the similar study of RRR of Nb-Ti strands used for conductors of the ITER poloidal coils. RRRs of the strands were measured in as-received condition and at different stages of the PF conductor manufacturing. RRR of strands extracted from ITER PF conductor sample after electromagnetic and thermal cycling in the SULTAN test facility was also measured. For this purpose the strand samples were extracted from different locations in the cross-section of the PF conductor and from different positions along the axis after testing in the SULTAN. The results about the RRR change during the manufacturing process and after the SULTAN tests and also the details of experiments are presented and discussed.

The Russian R&D Program for superconducting power devices is underway, supported both by government and electric power companies. In this program R&D on HTS power cables is considered as most advanced and close to commercialization. In the framework of the Program, several heavily instrumented 5 m cables have been tested followed by the 30 m - 3 phase experimental power cable development and testing in 2008-2009. The latest achievement is the development and testing of the first long length 3x200 m power cable with rating 1.5/2kA - 20 kV. In parallel with the cable development an innovative cryogenic system has been also developed for the cable cooling. The system is using neon as the working substance and radial turbo-machines in the refrigerator. Cooling power is up to ~8 kW at 65K, the inter-maintenance time ~30 000 hours. The cryogenic pump with superconducting motor can be used to provide subcooled liquid nitrogen flow of ~0.1 to 1.5 kg/sec at 0.1 to 2.5 MPa pressure. After extensive tests at a special test facility, the HTS power cable and its cryogenic system should be installed at some substation in the Moscow utility grid. In this review, we present some details of the Russian HTS power application program, the 200m cable and its cryogenic system design. Tests results are also presented.

Results of developing and testing an experimental hybrid power transmission line with liquid hydrogen and superconducting power (SCP) cable based on magnesium diboride (MgB2) are presented. Critical currents of the MgB2 based prototype SCP cable have been determined for the first time at the forced flow of liquid hydrogen in a temperature interval of 20–26 K. Various regimes of SCP cable cryostatting with both subcooled saturated liquid hydrogen have been tested in a broad range of supply rates (7–220 g/s) and pressures (0.15–0.4 MPa).

The behavior HTS materials and devices at overload currents is determined in a very high level by cooling, particularly, by the transient heat transfer from HTS device to liquid nitrogen. Recently we observed sharp peaks in the voltage during rectangular current pulse. In the present paper we explain these peaks by analysis of transient regimes in liquid nitrogen. The sharp peak in the voltage is occurring after temperature exceeds certain level before nucleate boiling starts. Basing on measurements of the voltage and temperature of HTS tapes during current overload and numerical analysis of the process we estimated overheat temperatures of nitrogen in transient regimes. Influence of covers of HTS tapes on overheat temperature of nitrogen is discussed. Experimental data and theoretical calculations' are compared.

The test facility developed in Russian Scientific R&D Cable Institute permits to perform extensive tests of heavily instrumented HTS cable models with length up to 5 m. Several HTS cables with different design have been developed and tested at this facility. The test programs included, besides usual critical current measurements, current distribution measurements among layers, joint resistance test, etc. The facility's equipment permits digital measurements of voltage and current with high accuracy and, therefore, digital ac loss analysis in model cables. In this paper we present the details of the test facility and results of tests and ac loss measurements in few 5 m model cables. ac losses in model cables made of 1 G and 2 G wires and other cable parameters are discussed and compared.

Superconductinge power cables offer the advantages of lower loss, lighter weight, and smaller dimensions, in comparison with conventional cables. It is important to developed most compact cable with a small diameter to increase the transmission power density of a cable. Precise analysis should be used to provide optimal cable layout with uniform current distribution among layers. We have developed an electric circuit model of a cable, and two FEM models using a finite-element code ANSYS. The numerical simulations can provide detailed information about the current distribution inside the cable and the optimal parameters from the point of view of current distribution of the cable have been determined. On the base of the models, the compact 2G HTS cable model with three layers in the core and two layers in the shield has been produced and tested. The outer diameter of the cable was as small as ~20 mm that is less than previously 1G and 2G HTS power cables tested. The experiment with the model cable demonstrated that even rather small mistake in the diameter of a layer leads to considerable change of current distribution among layers. This phenomenon was analyzed with the simulation models developed. It was shown that change of a layer diameter as small as the thickness of one tape (~0.1 mm) led to the current distribution change observed. Calculated results qualitatively coincide with the experiments. In addition, ac losses at different frequencies have been measured to compare them with ac loss in previously tested cables with larger diameter.

In the framework of the Russian R&D Program for superconducting power devices, an experimental 30 m HTS power cable has been developed. Three 30 m phases with nominal current of ~2 kA and 20 kV operating voltage were delivered as the result of the project. Variations were used in basic HTS materials, cryostats and current leads for the cable design in this research project. All phases were made of 1G HTS Bi-based tape. The details of the design of the cable are discussed. Before the full length cable test a 5 m witness sample has been cut, heavily instrumented and tested. Results of the witness sample tests and analysis of fault current behavior of the cable are presented.

In the framework of the Russian R&D Program for superconducting power devices, the 30 m HTS power cable has been developed made of 1 G Bi-2223 tapes. Before the full length cable production, as the first prototype the short 5 m cable model was produced and tested. After 30 m cable production the 5 m witness sample has been cut from the long piece and tested as well. To verify the calculations and designing principles both 5 m cables were heavily instrumented by potential taps and sensors to measure current distribution among layers, voltage-current characteristics and other parameters. AC losses in these short cable pieces have been analyzed by use of digital measurements of current and voltages along the cable. The witness sample has been provided by thermocouples to evaluate AC losses by calorimetric method. The details of AC losses measurements are discussed. Their analysis and comparison with calculations by standard theoretical models are presented.

The results of the technology development and first tests of the samples of the novel CICC are presented and discussed. The miniature CICC consists of thirty 0.5 mm NbTi strands inside a conduit with an outer diameter of 6.4 mm and has a projected operating current not less than 6 kA. The conductor design uses the well known and proven CICC technology. This new design looks very attractive, with several possible advantages, such as increased conductor stability margin and better mechanical rigidity of the conductor. Before N-CICC development the preliminary predictive analysis was done to justify the new conductor concept

The numerical experiment to model heating development in long HTS object has been performed by use of the specially developed computer code. Cooling by liquid nitrogen was taken into account and parameters close to the real ones for HTS tapes were used in calculations. It was shown that depending on parameters' combination, particularly on current density, stable and unstable regimes can happen. At unstable regime the fast temperature runaway is combined with the strong heat localization. The safety current density levels were determined for two model tapes. The results are important for HTS power cables that should withstand large overload currents during some time in case of faults in a grid.

The coherence between the voltage–current characteristics (VCC) of superconducting filaments, multifilament superconducting wires and multistrand cables made from such wires is discussed. The objective of the paper is to consider how the increasing of number of superconducting elements from filaments to cables affects their VCC. The influence of different factors on VCC, like stress–strain effects or current non-uniformity is discussed also. A generalized, simplified model is proposed for the calculations of the multistrand cable VCC. The application of the obtained results for the calculations of the critical currents in LTS (steep VCC) and HTS (smooth VCC) materials is discussed. The results are important for analyzing of changes of VCC in CICC made from Nb 3 Sn strands.

The qualitative analysis of the quench development in long HTS objects has been performed based on the general physical study of so-called "blow-up" regimes. Blow-up processes with strong heat localization in HTS objects with overcurrent are possible. Heat localization appears due to general instability in a media with nonlinear parameters that is valid for HTS. The sizes of overheated area and time of the heat localization development has been evaluated for typical parameters of HTS power cables.

Normal zone evolution or quench development is a major feature for resistive type fault current limiters design. We studied quench development under fast current rise in several samples of multifilament superconducting wires with a highly resistive matrix. At very fast current rise rates simultaneous quench of the entire sample takes place. It may be described by a characteristic time of a quench. We found that at the highest current rise rate this time is the same for samples with different lengths and cooling conditions made from the same wire. Apparent normal zone "velocity" determined by this time is an ultimate velocity for a given length of a superconducting wire of certain type. We present experimental data about quench development under fast current rise and provide the theoretical estimations of quench parameters.

The theory, describing thermal quench in HTSC devices, was developed. It permits evaluation of thermal quench conditions and development dependent on materials' parameters and cooling. Thermal quench threshold current and time characteristics of the quench can be predicted. The theory has been extensively verified by experiments with different superconducting HTSC devices. Good correspondence between theory and experiments has been observed. We use this theory to analyze the thermal quench emergence conditions that are dependent on cooling, sizes of the device, material properties, etc. It was shown, that with increase of the size of the device threshold thermal quench current may become less than standard determined critical current. Time characteristics of the thermal quench increase with sizes. The analysis is supported by comparison with experiments with different HTSC coils.

Stability and quench development in a HTSC magnet have been experimentally studied with the transport current in the magnet being below or above the “thermal quench current” level. The magnet was tested at both cryocooler cooling and liquid nitrogen cooling, with and without background magnetic field (up to 4 T). The temperature and electrical voltages in different sections of the magnet were measured using 20 thermocouples and 24 potential taps embedded in the winding. In this paper, the experimental procedure and the results are described. The results are compared with those obtained earlier in the experiments with the smaller HTSC specimens and are analysed by using the scaling theory of the thermal quench.

It is believed that critical current reduction in multistrand superconducting cables at nonsteady state conditions is caused by a nonuniform current distribution among strands. This was experimentally proven for small model cables, but is still not verified for large cables. In the ENEA Frascati Laboratory, an ITER relevant, large superconducting magnet has been tested at different field ramp rates. By means of numerous local miniature field sensors (Hall probes and pickup coils) located in a few positions along the conductor, current redistribution phenomena inside the cable have been studied. Fast and slow local field changes have been studied to quantify the current nonuniformity. It has been shown that severe current nonuniformity does exist in the cable and that induced current loops are generated, which decay with very long time constants (up to 10/sup 4/-10/sup 5/ s).

The KISO-32 superconductor applied to the pool-cooled Helical Coils of the Large Helical Device (LHD) consists of 15 NbTi/Cu superconducting strands and a pure aluminum stabilizer with a CuNi clad, both embedded in a copper housing. To study quench development phenomena in such a complicated structure, we prepared a special sample coil. The coil was fabricated in Kyushu University and tested in NIFS. Tiny thermocouples and electrodes were inserted into the conductor's body in several cross-sections. Resistive heaters initiated a normal zone, and internal temperature distributions as well as electric fields during quench, i.e., during a normal zone propagation and recovery were measured directly. We observed a considerable temperature gradient between the inner and outer parts of the superconductor. The inner electric field distribution was recorded too. Oscillation phenomena were observed with an appearance and disappearance of a normal zone in some areas of the conductor. In the paper, we present the experimental details and some of the results obtained. The mechanism of the oscillation phenomena as well as the reason for the temperature gradient are considered.

It is widely acknowledged that current non-uniformity is a major source of reduction of quench currents at nonsteady state conditions in multistrand superconducting cables. It is important to study current non-uniformity in experiments. In this paper we present a review of the experimental methods used to study current non-uniformity in CICC and AC multistrand superconducting cables and results received. We discuss the influence of current non-uniformity on stability of superconducting magnets made from multistrand superconducting cables and possible ways to overcome their instability.

We studied quench propagation in double-sided samples of YBCO thin films covered by a gold top layer and deposited on sapphire wafers. The length of the YBCO structures was up to 0.5 m. A critical current density /spl sim/2-3 MA per centimeter square at 77.8 K was found over the entire YBCO film. Quench development in large-area YBCO thin film appears to be a complicated process. Depending on test conditions and external circuit parameters, normal spots may appear and disappear during the quench process. In double-sided samples quench development is strongly affected by thermal interaction between the two films through the substrate's heat conductivity. This thermal conduction should be considered during the design of fault current limiters made from such films. Mutual thermal interaction of a film through a substrate may also be used for acceleration of normal zone propagation using active or passive heaters on opposite sides of the substrate.

Quench dynamics in a YBCO HTSC film and a Bi-based small HTSC coil have been studied. While the stability margin of HTSC against a local disturbance was very large, quench current was limited by a catastrophic temperature rise originated from the nonlinear characteristic of Joule heating in HTSC. The crucial parameter for the quench becomes the nonlinear resistance in HTSC as a function of temperature and transport current. It has been shown that the dynamic characteristics of the quench in both the film and the coil can be described quantitatively by the simplified one-dimensional heat balance equation even though the time scales are different by more than six orders, i.e., several hundreds micro seconds for the film and several hundreds seconds for the tape coil.

The widely used multistrand superconducting cables demonstrate quench current degradation in AC mode. Often it happens due to premature quench of one strand. It was suggested that changing of the cable's design may improve the "single strand stability", or stability of a cable in relation to the quench of a single strand. We are checking this idea by testing several AC multistrand superconducting cables made from insulated superconducting strands with different cabling technique. We determined their quench current at different frequencies and current redistribution process in case of the quench of one single strand. The experimental results are presented and influence of the cable design on the "single strand stability" is discussed.

Using a Bi-based small high T c superconductor (HTSC) coil, we have studied its stability against a local disturbance and current-induced quench in the helium gas cooling condition. While the stability margin of HTSC coil against a local disturbance was very large, quench current was limited by a catastrophic temperature rise which originated from the nonlinear characteristic of the Joule heating. The crucial parameter for the quench becomes the nonlinear resistance in HTSC as a function of temperature and transport current. It has been shown that the dynamic characteristics of the quench can be described quantitatively by the simplified one-dimensional heat balance equation.

It is widely acknowledged that current non-uniformity is a major source of reduction of quench currents in superconducting multistrand cables at non-steady state conditions. Recently we performed several experimental studies on the current non-uniformity in CICC and AC cables. In this paper we present the summary of the experimental methods used. Indirect methods with use of local magnetic field sensors can provide general information about non-uniformity inside CICCs and other large size cables. Indirect methods may be used in real superconducting devices. Direct measurements of the current in each strand provide exact information about the current distribution, but they need special sample preparations. Because no method is perfect, the best idea is to use them as complementary to each other in the study of a certain type of a cable. The results obtained from the measurements of the current non-uniformity in CICC and AC cables by both methods are briefly discussed.

*Institute of Superconductivity and Solid State Physics of Russian Research Center ‘Kurchatov Institute’, 123182 Moscow, Russia tPlasma Fusion Center, Massachusetts Institute of Technology, Cambridge, MA 02139, USA Received 18 November 1996; revised IO February 1997 Experiments were performed to measure directly the current in each strand of a 12- strand Nb,Sn cable-in-conduit superconductor during current and/or external mag- netic field ramps. The goal of the experiment was to get straightforward evidence of current maldistribution in a cable-in-conduit conductor (CICC). A heavily instrumented sample coil from Nb,Sn TPX-TF strands was specially prepared. Severe non-uniformity of the strand currents were found during field ramp. Immediately before a quench the individual strand currents within a triplet differed by as much as an order of magni- tude. During field ramps with constant transport current, the currents in some strands were observed to drop rapidly and then recover. The data show that quench develop- ment in the CICC is a complicated phenomenon involving dynamic redistribution of current among the strands. Non-uniformity of current along the strands during quench was also observed. 0 1997 Elsevier Science Ltd.

Direct measurements of the current in each strand of a chrome coated Nb/sub 3/Sn CICC were performed during current and/or external magnetic field ramps. Severe non-uniformity of the strand currents was found immediately before quench. The currents in some of the strands were also observed to change abruptly at various points during external field ramps. The observed peculiarities of the strands' current behaviors are considered in the present paper and a simple "ad hoc" model of the ramp rate limitation phenomenon is proposed based on the hypothesis that current non-uniformity is a main cause of RRL. This model is compared with the experimental results.

Spike voltages observed during ramp rate limitation tests on sub-sized Nb/sub 3/Sn cable-in-conduit superconductors are analyzed using current loop model. The effects of loop currents on the ramp limitations of multi strand superconducting cables are discussed. Current loops existing in multi strand cables generate excess local currents that quench strands and produce voltage spikes. Experimental results previously reported as abnormal ramp rate limitations are explained by loop current phenomena.

A superconducting pulse coil set was constructed for transient stability experiment of CICC (cable-in-conduit conductor). The pulse coil set was composed of an inner and an outer coil connected in series. With 5 T background field, the pulse coil produced an additional 2 T on the CICC. In order to simulate the TPX (Tokamak Physics Experiment) plasma initiation scenario, 2 T field drop from 7 T was tried with an approximate ramp rate of -25 T/s. This paper describes the design for the coil and presents the experimental results of its successful AC operation.

A 27-strand hybrid superconducting cable-in-conduit conductor (CICC) was fabricated and tested under quickly-ramped high magnetic fields. When the field increased linearly on the CICC, the voltage signal showed several intermittent spikes before it quenched. This paper describes an observation of peculiar voltage spikes during these ramp-rate limitation experiments. The voltage spikes are interpreted as quench precursors and understood as current redistribution events within the local cable inside the conduit. A quantitative correlation is obtained for the magnetic field at which the first voltage spike occurs during ramping fields. The non-uniform current distribution among the strands and the induced loop current in the cable, which is generated by ramped fields, are found to be responsible for the voltage spikes.

The quench development process was studied with two six-strand superconducting cable samples with insulated strands and different cabling design. One sample was the traditional ‘six-around-one’ cable, the other was of the ‘round-braid’ type. Quench was initiated by a heating pulse applied to a single strand. A significant difference was observed in the current redistribution among strands due to the different cabling designs. It has an important influence on the stability of multistrand cables. A new concept of the ‘single-strand stability’ was proposed and corresponding stability criteria were offered. A possible way to improve cable stability by using special cabling design is discussed.

Ramp-rate limitation experiments were done in a new facility at the MIT (Massachusetts Institute of Technology) Plasma Fusion Center. The features of this new facility include (1) a superconducting pulse coil that can superimpose high ramp-down rates, up to 25 T s−1, (2 T in 80 ms) at a background field up to 5 T, (2) new power supplies that can supply high rates of dl dt and dB dt to the sample under test and (3) a forced-flow supercritical helium system for cooling CICCs (Cable-In-Conduit Conductors). This paper discusses the results of the ramp-rate limitation experiments on a 27-strand hybrid Nb3Sn cable. The cable was tested under field ramps of up to 2.5 T s−1 with various operating currents. It did not quench with dB dt , field and average strand currents that were simultaneously above the operating range of TPX-PF (Tokamak Physics Experiment Poloidal Field) coils. Further ramp-rate limitation experiments revealed that the tested 27-strand hybrid cable has very high transient stability at ramped fields, extending out to average strand currents that are nearly triple the TPX-PF operating current.

The influence of current redistribution between strands on the quench process in a superconducting cable is studied. A theoretical model is developed which accounts for non-uniformities in strands. In agreement with experimental data, the model predicts that three different current redistribution modes may be observed in the cable, depending on the initial current; namely the current sharing mode, and slow and fast quench modes. It is shown that fast current redistribution can initiate multiple normal zone nucleation in strands. This phenomenon is responsible for an abnormally large normal zone propagation velocity. The results obtained are of importance for understanding the quench process in multistrand superconducting cables.

The ramp rate limitation phenomena were studied using local field sensors to observe the intrinsic processes within the cable. Sensitive miniature Hall sensors and small pick-up coils placed around cable-in-conduit superconductor were used to measure local magnetic fields and field derivatives associated with currents in the cable. Using this method, both fast jumps and slow changes in local magnetic fields at different conditions mere observed. First jumps occured during ramping background magnetic field and may indicate a fast current redistribution processes. Slow changing of local fields may be associated with current loops closed through the current lead joints. Such current loops may also indicate the nonuniformity of current distribution in the cable strands. The new method is a promising tool for future investigations of stability of multistrand cables. >

The normal transition of a superconducting NbTi-CuNi multifilamentary wire under fast transport current decrease (dI/dt >

The feasibility of utilization of cold switches for quench protection of very large superconducting magnets (e.g., for SMES) is considered. The scheme of quench protection of large SMES is suggested. The necessary number of sections can be easily evaluated. Destructive superconducting switches seem to be the best solution. The switch has to be properly designed to avoid arcing and to offer a possibility to change the destructive elements in a reasonable time without warming up large portions of the winding. A suggestion is also made to make use of the temperature dependence of the electrical resistance of the dump resistor. A proper choice of its mass can result in 25 percent decrease of a quench load. >

The stability of multi-strand superconducting cables or cables-in-conduit is degraded by rapid changes in current or field (ramp rate limitation or current degradation). The maximum achievable AC current is significantly less then the sum of DC critical currents of strands. Measurements of the dependence of quench current on ramp rate were made for one, two, and three strand cables. Samples had different strand types and lengths. It was shown that although the theory of thermomagnetic instability explains single strand performance very well, it does not explain the reduction of quench current observed in multi-strand cables. The influence of joint resistance between cable strands and current leads is shown to be the most probable reason for the reduction. >

The key to understanding the fast quench phenomenon in multistrand cables is the normal zone origination and propagation in superconducting wires with fast changing currents. Results of experiments simulating the situation in strands during fast quench development are presented. The maximum quench current, normal zone velocity, increase of current and time before quench are measured in NbTi-CuNi composite wire at different transport current ramp rates and at different initial currents. Normal zone velocity propagation depends on I(t) and I/spl dot/(t) only. The presence of an initial current leads to the acceleration of normal zone propagation at decreasing current. The initial current also leads to a reduction of the current increase and the time when a quench starts. We use the results received to explain the appearance of fast quench phenomenon in superconducting cables with insulated or highly resistive matrix strands. >

The quench process of a multi-strand cable was investigated using the simplest system: two twisted wires. Several properties of the quench, such as the commutation of currents, the time scale, the resistance rate, and the maximum voltage, were determined experimentally or by calculation. Particular attention was given to the role of the cable length. Several samples with lengths varying from 1.5 cm to 12 m were made from an AC superconductor with CuNi matrix. In the experiment, the decay of the currents was measured after initiating a local normal spot in one of the wires. An important conclusion is that the quench propagation and stability of a cable depend on its length and can therefore be influenced by soldering it at certain intervals. >

With reference to the application of NbTi tape in a superconducting thermal switch, the critical current of a 20- mu m-thick NbTi tape was measured in several directions of the magnetic field. The critical current was found to behave strongly anisotropically, due to the deformation of the NbTi. The tape is extrasensitive to the component of the magnetic field perpendicular to the surface. Without an external field this component of the self-field reduces the critical current far below its intrinsic value. A one-dimensional model can describe the reduction of critical current due to the self-field in a thin tape. >

For pt.I, see ibid., vol.28, pp.735-738 (1992). The evolution of quenches in multi-strand cables with high resistive matrix is analyzed. During the quench process, the growing normal resistance in certain strands will cause commutation of their current to other strands with less resistance (or still superconductive). This redistribution of current can lead to a quench of the whole cable. Furthermore, above a certain level of the initial current, an extremely fast quench process occurs with an apparent propagation velocity on the order of several kilometers per second. A model is presented that describes the above phenomena and special attention is given to the fast quench. It is noted that the fast quench could be a useful phenomenon for protection purposes or for the realization of superconducting switches with very short opening times and large dR/dt. >

The measurement of quench currents of sample coils at frequencies from 0 to 1000 Hz in an external magnetic field are presented. Coils were wound with multifilamentary Nb-Ti wires with various diameters in pure high-resistivity matrices with different resistivities. The influence of cooling on quench in the AC mode was studied. The stability of the wire increased with improved cooling and increased magnetic field. Results were analyzed from the point of view of the theory of stability, which takes into account real voltage-current characteristics of the wires.