NbTi Superconductors Research Articles

Nb-Ti superconductors of a high current-carrying capacity

The effect of multiple prolonged heat treatment on the structure and critical current density in Nb-50 wt.% Ti composites was studied in the case of monofilamentary superconductors. The treatment temperature range, in which high Jc values can be obtained, is determined. The influence of intermediate treatments on the increase in the quantity of precipitated alpha -phase is studied. The exponent specifying the kinetics of particle growth from the supersaturated solid solution is estimated. The interrelation between the critical current density and the quantity of precipitated phase is established. The maximum critical current density is 4*105 A cm-1 (B=5 T).

Enhanced flux creep in Nb-Ti superconductors after an increase in temperature

The magnetic fields of Superconducting Super Collider (SSC) dipole magnets change with time when the magnets are operated at constant current. The decay of the field is thought to be a consequence of flux creep in the Nb-Ti filaments in the superconducting cables. However, measured magnetic relaxation of small samples of SSC cable as a function of time is unlike the large decays that are observed in the fields of the actual magnets. We have made relaxation measurements on sample SSC conductors at 3.5 and 4.0 K after field cycling. The decay at both temperatures was 2.8% in 50 min. However, the relaxation measured after a temperature increase from 3.5 to 4.0 K was 4.8% in 50 min. A likely reason for the greater magnetization decay is that, after an increase in temperature, the Nb-Ti is in a supercritical state, with shielding currents flowing at a density greater than the new critical current density. This causes enhanced flux creep. We suggest that a small temperature rise during the operation of SSC magnets may contribute to the unexpectedly large magnetic field decay.

An Analysis of Normal Front Propagation in Superconducting Coils under Quench Condition.

A computer code was developed to simulate normal front propagation and the associated transient characteristic behavior of superconducting coils under quench condition. The analysis method considers the normal propagation in both longitudinal and transverse directions by introducing a logical coordinate system. In a logical coordinate system, superconducting coils are assumed to consist of long wires which are divided into finite lengths of elements. Thus, three-dimensional problems can be transformed to one-dimensional problems. In order to examine the capability of the computer program, the quench propagation in solenoid coil systems with epoxy bonded NbTi superconductors has been analyzed. The results showed good correlation of the computed quench time with the experimental data, and the applicability of the code was demonstrated.

Superconducting coaxial turn transformer

The concept of a superconducting coaxial turn transformer is presented. The turns enclose each other like hoses and the circularly wound warm iron core encloses the coils outside the cryostat. This concept promises some remarkable advantages over other geometries. The potentials of this concept as well as of existing superconductor technology are demonstrated by design studies on various actively short circuit protected and inherently short safe 400 MV A, 400/21 kV, 50 Hz, three phase coaxial turn generator transformer banks. Based on an experimental NbTi superconductor it is shown that actively switch-protected transformers which quench at certain overcurrents exhibit losses and weights which are only fractions of those of corresponding conventional transformers. These types could also serve as current limiters with fast recovery times. It is further demonstrated that even inherently short safe varieties with losses and weights less than those of normal conducting transformers are possible.

Critical currents and flux pinning in Ag stabilized high Tc superconductor wires

Y 1Ba 2Cu 3O 7− x and Bi 2Sr 2Ca 1Cu 2O 8+ x Ag sheathed high T c superconductor wires have been prepared using the powder in tube technique. Characterization of wire specimens was performed mainly by measuring critical current densities at 77 K and 4.2 K, as well as by a.c. susceptibility measurements. Whereas the critical current density in Y 1Ba 2Cu 3O 7− x /Ag wires is limited by weak links, there are virtually no weak links present in Bi 2Sr 2Ca 1Cu 2O 8+ x /Ag wires. Above ≈20 K, critical current density in this material seems to be limited by flux creep. At lower temperatures flux creep becomes insignificant. At 4.2 K critical current densities of up to 5.7 × 10 4 A cm −2 in zero magnetic field and of up to 1.5 × 10 4 A cm −2 in a field of 26 T could be achieved, rendering possible the use of this material for very high field application. The results will be compared with those obtained on conventional metallic Nb-Ti superconductor wires. Flux pinning in these materials will be discussed, based on a model of direct summation of elementary pinning forces.

Quantitative description of a very high critical current density Nb-Ti superconductor during its final optimization strain. II. Flux pinning mechanisms

Flux pinning by α-Ti precipitates has been studied in a carefully made Nb 48 wt. % Ti composite having very high critical current density and a macroscopically rather uniform precipitate array. The pinning was studied as a function of field (0–15 T) and temperature (2.3 K—Tc) for a ratio of the precipitate thickness (tppt) and the coherence length (ξ) which was varied between 10 and 0.1 by a large drawing strain. Surprisingly, the maximum bulk pinning force (Fp) occurred for tppt/2ξ≂0.1 and a precipitate spacing about one-eighth the fluxon spacing, suggesting that the flux pinning occurs at clusters of precipitates. In contrast to earlier studies, strict temperature scaling of Fp was not observed; the peak of the pinning force shifted to lower reduced fields as the temperature increased. The effect was largest for the finest scale microstructure. This nonscaling is interpreted in terms of two pinning mechanisms having different field and temperature dependencies. At low reduced fields, and at temperatures close to Tc, the core interaction (δHc) dominates. At higher reduced fields and low reduced temperatures, pinning due to variations in κ also contributes to Fp. Clusters of very fine α-Ti ribbon precipitates give rise to these variations in Hc and κ. A comparison of the specific pinning force with the core interaction model was made close to Tc, where the δHc pinning mechanism dominates. Good agreement was obtained for all precipitate thicknesses, indicating that the pinning can be quantitatively described by the core interaction and the direct summation models. No significant proximity effect suppression of the pinning force at fine precipitate dimensions was observed.

Influence of filament spirality on current-voltage characteristics of twisted multifilamentary NbTi superconductors

It is shown experimentally, that the current-voltage characteristics of a twisted NbTi multifilamentary superconductor placed in a perpendicular magnetic field differ from those of untwisted superconductors. It was explained by the current transfer across the normal metal matrix due to the inhomogeneous electric field distribution along each filament. Electric field inhomogeneities along each filament arise as a result of varying the angle between the external magnetic field and filament axis. Several model conductors were prepared to investigate the role of the normal metal matrix resistivity and the geometrical arrangement of filaments. A two-dimensional theoretical model explaining the measured characteristics is also presented.

Advanced superconducting MHD magnet design for a retrofit power plant

A magnet has been designed for an MHD (magnetohydrodynamic) topping cycle retrofit of a conventional power plant. The channel power output will be approximately 35 MWe. The magnet which will have 4.5-T peak on-axis field, will be constructed of an unusual NbTi superconductor wound into four subunits per dipole half. These will consist of three 45 degrees saddle coils with circular or ellipsoidal end turns and a single planar coil with a modified racetrack shape that will serve principally as a field-shaping coil. This planar coil also allows a substantial reduction in the ratio of peak to central field strength. Among the unique features of this design will be the use of flexible bands in tension as the primary element of the transverse force containment structure. The conductor will be of the cable-in-conduit type with a cable having a low copper-to-superconductor ratio and a thick-walled aluminum conduit sheath. The sheath will support the axial loads on the saddles and will also provide thermal mass for protection against overheating in the event of an energy dump. The analysis and design of this magnet system and its projected advantages in both performance and economics are discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Comparison of Nb3Sn and NbTi Superconductor Magnet ITER Devices

The TETRA tokamak systems code is used to compare designs for the International Thermonuclear Experimental Reactor (ITER) that use Nb3Sn and NbTi superconductor magnets. Similar minimum-cost device...

Current ripple effect on superconductive d.c. critical current measurements

The effect of current ripple or noise on d.c. critical current measurements was systematically studied. Measurements were made on multifilamentary NbTi superconductor. A low-noise, battery-powered current supply was required in this study in order to make the pure d.c. critical current measurements. Also, an electronic circuit that stimulates a superconductor's general current voltage characteristic was developed and used as an analysis tool. In order to make critical current measurements in which current ripple was present, the battery supply was modified to allow the introduction of controlled amounts of a.c. ripple. In general, ripple in a current supply becomes more significant in current supplies rated above 500 A because effective filtering is difficult. The effect of current ripple is a reduction in the measured d.c. critical current; however, ripple of sufficient amplitude can result in arbitrary measurement results. The results of this work are general and quantitatively applicable to the evaluation of critical current data and measurement systems. A theoretical model was developed to further support and explain the ripple effect. An unexpected benefit of this work was a more precise method for general critical current data acquisition. Problems common to all large conductor critical current measurements are discussed.

Pinning force in thin superconductors with small number of flux line rows

It is shown that the volume pinning force (and hence the critical current density) of thin superconducting slabs decreases strongly after the transition of the one-dimensional flux line lattice into the two-dimensional structure. The results are applied to superfine filamentary super-conductors (diameterd0·1 μm) and they are in accordance with experiments on NbTi superfine filaments, as well as on weakly reacted Nb3Sn multifilamentary superconductors. The maximum of the volume pinning force appears therefore at the transition fieldB12 from the one-dimensional to the two-dimensional flux line lattice. This field is much smaller than the fieldBm at whichFp is maximum for macroscopic superconductors. So, e.g. for filament diameterd=60 nm of NbTi superconductor,B12≈1·2 T, whereas theBm value for macroscopic NbTi is about 5 T.

Losses in transformer-like coils wound from a very fine filament NbTi superconductor

Special cylindrical coils with oppositely wound layers have been experimentally investigated under different conditions: as fed by a.c. transport current with and without d.c. background field and at normal LHe boiling, as well as at lowered temperatures. Unusual loss-field dependence proportional to B 2 has been observed at low (<0.1 T) a.c. fields. The results obtained are analysed in two ways: as bulk hysteretic losses occurring exclusively in the superconductor or as magnetic hysteresis losses in the CuNi matrix. None of the presented models has been fully confirmed by experiments. Further investigations, therefore, are needed in the field of loss theory in fine filament superconductors.

Progress on the superconducting magnets for the Large Hadron Collider

After a review of the proposed CERN Large Hadron Collider (LHC) project, the present design of the high-field superconducting magnets is described. One half of each standard cell for the ring of the magnet system under development consists of four ( approximately 10-m long) dipole magnets, a focusing quadrupole magnet, a tuning quadrupole, a combined sextupole/dipole corrector magnet, and a beam observation station. Only the main features of the dipole design are given. Both presently possible routes to high-field/high-current density magnets, using NbTi superconductors at superfluid helium temperatures or Nb/sub 3/Sn at 4.5 K, are summarized. Other major systems such as the vacuum chamber, proton synchrotron, and cavities are also noted. The work then presents the main aspects and the first results of the research and development program that was undertaken in close collaboration with various European industries and laboratories. Emphasis here is on the development of suitable NbTi and Nb/sub 3/Sn wires and cables, and the construction of magnet models (1-m long) and of a 9-m long twin-aperture prototype dipole.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Recent developments in NbTi superconductors at Vacuumschmelze

Superconductors based on NbTi have been recently the most widely used conductors for superconducting magnet. The fabrication technology of monofilamentary and multifilamentary conductors in a Cu matrix has reached a very high standard mainly in conjunction with the applications in analytical measuring techniques using nuclear magnetic resonance (NMR) and in medicine using magnetic-resonance imaging (MRI). The specific requirements of NbTi for the fields of NMR/MRI, particle accelerator technology, nuclear fusion, and power generators are briefly discussed. The state of the art of conductor technology is described by presenting the parameters and the performance of conductors produced or developed for recently completed or ongoing projects. Areas for further research and development are identified. >

Critical current transition study on multifilamentary NbTi superconductors having a Cu, a CuNi, or a mixed matrix

Measurement of voltage-current characteristics in multifilamentary wires, showing the transition from the superconducting to normal state, provides information about the quality of the wires and their production techniques. Two methods of describing this transition are discussed. In general, the so-called n-power law turns out to be adequate in the critical current region. The dependence of n on the magnetic field yields information about the inner structure of the wire, especially whether the limitation of the current is caused by intrinsic or extrinsic effects. Measurements were carried out at 4.2 K on several wires in order to investigate the above mentioned aspects with respect to the critical current transition. The current range in which the n-power law was valid was tested for two wires with highly resistive matrices. Then the dependence of n on the applied field was investigated. A wire with very thin filaments and one with thicker filaments were used. When submicrovolt measurements were carried out, it was found that in several ranges of current other n-values can be distinguished. This is thought to be caused by a current diffusion process. In an alternate way of assessing the resistive transitions, the authors used a critical current distribution function found by calculating the second derivative of the voltage with respect to the current. >

3. Characteristics of the coils

As described in section 2, the specifications for the LCT test coils were chosen to ensure that all coils were (1) compatible with the intended six-coil test array and (2) readily adaptable to meet the envisioned demands of TF coils in a tokamak fusion reactor. To encourage exploration of the various design options, the specifications intentionally allowed each design team as much freedom of choice as possible consistent with the foregoing considerations. As it turned out, the principal options were indeed represented among the six coils.All design teams accepted the judgment that the requirement of 8.0 T at the winding could be achieved by using a NbTi superconductor cooled with helium at 3.8–4.2 K. Not surprisingly, each team chose to use this material rather than the potentially more capable but definitely more difficult Nb3Sn. However, in its proposal to DOE, the WH/Airco team had stressed that either material could be used in its cable-in-conduit type conductor. To explore the problems of Nb3Sn conductor and winding, DOE therefore directed the WH/Airco team to use this material and to proceed with the necessary additional development required for commercial production.With regard to the mode of helium cooling, the teams were evenly divided, with three choosing PB cooling and three choosing forced flow. These and other design choices are evident in the coil features listed in table 3.1.

Irregularity in NbTi filament area and electric field versus current characteristics

Abstract There is a correlation between irregularity in filament area (‘sausaging’) and the shape of a superconductor's electric field ( E ) versus current ( I ) relationship. The shape of the E − I characteristic is quantified in terms of the resistive transition parameter, n , defined by E ∝ I n . Low values of n less than about 20 correlate with a wide filament diameter distribution, while n values over 50 correspond to a distribution more than 2.5 times smaller. It is proposed that the low-field (constant) value of n be used as an index of filament quality in evaluating different superconductors for practical applications. A model is also suggested to explain this effect in terms of a locally depressed filament critical current, which forces current to transfer across the normal matrix material into neighbouring filaments. The relationship between n and the statistical distribution of filament diameters may be useful as an easy method of estimating the extent of sausaging in practical multifilamentary NbTi superconductors from measurements of n .

Influence of CuNi matrix on a.c. losses and voltage-current characteristics of fine filament NbTi superconductors

The results of a.c. loss measurements at 4.2 K in two types of fine filament NbTi superconductors with a CuNi matrix are presented. The conductors were manufactured by Alsthom Atlantique (France) and Showa Electric Wire & Cable (SEWC, Japan). A.c. losses per cycle were determined from magnetization measurements on samples without transport current, subjected to the external magnetic field perpendicular to the conductor axis. Measurements were carried out at two frequencies: 0.1 and 50 Hz. The range of magnetic field amplitudes for Alsthom Atlantique and SEWC samples was 2.5 × 10 −3 − 3T and 5 × 10 −2 − 2 T, respectively. By measurements at T ≈ T c, 77 and t~300 K, we proved the existence of hysteretic loss in the CuNi matrix of these conductors. Current-voltage characteristics were also measured. Critical current densities determined from magnetization and current-voltage characteristics were compared. The observed difference is discussed.

Proximity effect depression of the critical temperature in two-phase Nb-Ti superconductors

The superconducting critical temperature of high critical current density Nb-Ti composites has been measured at various stages in the critical current optimization process. This process involves heat treatment steps which cause precipitation of normal α-Ti and make the matrix more Nb rich. Tc rises from 9.1 to 9.5 K during this stage. The final optimization stage involves extensive wire drawing during which the α-Ti precipitates are reduced to less than a coherence length in thickness. This does not result in a reduction in Jc but Tc is found to fall from 9.44 to 8.7 K during this step. The depression of Tc is found to be in fair agreement with the predicted proximity effect suppression of Tc. Wires of optimum transport critical current density are seen to have Tc of around 9 K.

Further improvements in current density by reduction of filament spacing in multifilamentary NbTi superconductors

Abstract Current densities, J c s, approaching those required in the dipole magnets of the Superconducting Super Collider have been achieved in conventional multifilamentary NbTi superconductors of the type used in the Colliding Beam Accelerator and the Tevatron magnet designs. These high J c s have resulted from a simple reduction in the spacing-to-diameter ratio, S / D , from 0.35 to 0.13, keeping other factors the same.