Influence of hydrogen on the current density of Nb3Sn multifilamentary conductors

The overall current density of Nb3Sn multifilamentary conductors heat-treated with hydrogen and argon is quite different. At flux densities above 11 T hydrogen samples carry more current than argon samples. below 11 T argon samples are superior to hydrogen samples. The transition temperature of hydrogen samples is generally found to be some tenths of a Kelvin below that one of the argon samples. X-ray investigations at room temperature indicate an increase of the lattice spacing in hydrogen samples corresponding to a growth of the unity cell up to 0.6% . The critical current of hydrogen samples undergoing tensile stress will increase in a way similar to argon samples.     

Laboratory scale solenoids with multifilamentary Nb3Sn superconductors

By the bronze technique Nb3Sn multifilamentary conductors with different numbers and densities of filaments and partly with an integrated CuTa stabilization were fabricated. For currents exceeding 1000 A at 10 T we have manufactured fully transposed and calibrated flat cables with different numbers of strands and different ratios of superconducting to CuTa stabilizing strands. From long lengths (km) of these cables we have fabricated solenoids with manifold graded windings using the wind-and-react-technique and a final vacuum impregnation after reaction. Two different variants of this technique will be described. Magnetic flux densities up to 15 T at 4.2 K in a free bore of 40 mm diameter have been reached in a 7 T NbTi background field. The solenoids attained the short sample current within some percent requiring only short training on initial excitation but no training after heating to room temperature. High excitation speeds were obtained without degradation of the quench current.     

Neutron irradiation of Nb3Sn and NbTi multifilamentary composites

NbTi and Nb3Sn multifilamentary composites have been irradiated with fast-neutrons at 60 ± 5°C to fluences of 1.2×10²⁰n/cm²(E > 1 MeV). The NbTi samples show only a moderate reduction of Icas a function of neutron fluence in an applied field of 40 kG. Reductions in Icwere observed for fluences greater than 3 × 10¹⁷n/cm²and saturate at 18% for fluences greater than 3-4 × 10¹⁹n/cm². The Nb3Sn composites showed large neutron radiation induced changes in Tc, Icand Hc2. Reductions in Tcwere observed for fluences greater than 7 × 10¹⁷n/cm². No measurable changes in Ic(40 kG) were observed below 10¹⁸n/cm². Between 2 and 3×10¹⁸n/cm², however, there is an apparent threshold where a very rapid reduction in Ic(40 kG) is initiated. At the threshold the decrease in Tcis 13%. Between the threshold and 1.1 × 10¹⁹n/cm², I2(40 kG) has been reduced to 4% of the unirradiated value. These changes in superconducting properties in NbTi and Nb3Sn are analyzed in terms of the radiation induced defects. The impact of the response to irradiation of both materials on their applications in fusion reactor magnets is discussed.     

The temperature and magnetic field dependence of superconducting critical current densities of multifilamentary Nb3Sn and NbTi composite wires

Commercial NbTi and Nb3Sn multifilamentary superconducting wire is becoming increasing important for use in research and commercial magnet systems. In both materials the temperature dependence of Jcplays a major role in the determination of magnet system operating parameters and design stability margins. We report here critical current density measurements as a function of temperature from 4.2 to 19 K and of applied magnetic field upto 8 T for multifilamentary Nb3Sn wire and for 2 alloys of NbTi superconducting wire. From this data [partial J_{c}(H_{a})/partialT] and[partialH_{c2}/partialT]T=T_{c}can be obtained and stability criteria and other superconducting parameters of the wires may be extracted.     

Josephson effects in Nb3Sn microbridges

We have studied Josephson effects in long narrow Nb3Sn microbridges at temperatures up to 17 K. These microbridges are formed by photolithographic techniques and subsequently subjected to controlled electrical discharges to modify the intrinsic Tcof the bridge region. The bridges exhibit 10 GHz micro wave steps in their I-V characteristics whose amplitudes are in excellent agreement with the Resistively Shunted Junction (RSJ) model. I-V characteristics (with and without microwaves) can be fit assuming an effective temperature approximately 15 K above the bath temperature. We have also investigated in detail structures in the I-V characteristics in the absence of microwaves. We show experimentally that phase-slip centers are induced at weak superconducting positions along the bridge when the S-N boundary of an expanding hot spot reaches within a thermal healing distance. The critical current of the phase-slip center thus formed exhibits a temperature dependence (1-T/Tc)^1/2instead of the usual mean field result (1-T/Tc)^3/2.     

Surface impedance of superconducting Nb3Sn

Microwave cavities with a resonant frequency of 8 GHz are coated with Nb3Sn by the vapour deposition technique. The surface resistance and the change of the penetration depth were determinded by measuring the quality factor and the shift of the resonant frequency of the cavity in the temperature range from 2 K to 20K. The temperature dependence of the surface resistance can be described well by the BCS-theory in the temperature rangeT < 0.5 T_{c}, however, the value of the reduced energy gapDelta_{0}/kT_{c}has to be increased from 1.76 to 2.15. The temperature dependence of the penetration depth shows significant deviations from the predictions of the BCS-theory for temperatureT < 0.5 T_{c}. The increase of the reduced energy gap is not sufficient to fit the data but one has to treat the effects of strong electron-phonon coupling consistently. Therefore, we calculated the surface impedance for strong coupling superconductors using an Einstein model for the phonon density of states. The absolute value and the temperature dependence of the surface impedance in the whole temperature rangeT < T_{c}are discussed and a comparison with the experimental data is given.     

Nb3Sn in 1978: State of the art

Seventeen years after its discovery as the first practical high field superconductor and seven years since it was demonstrated in multifilamentary form, it seems appropriate to ask, "What is the current status of Nb3Sn"? This question is approached from a user's point of view with particular emphasis on the multifilamentary form of Nb3Sn. Data from national laboratories, universities, and conductor manufacturers have been compiled and assessed. The result is a comprehensive picture of past, present, and future conductor types and their properties. Applications of these conductors in magnets are reviewed including both those devices already completed and those which are being proposed and/or constructed at this time. Finally, a summary of ongoing research and development programs is included along with the author's assessment of their impact on future Nb3Sn magnets.     

Measurements of superconducting Nb3Sn cavities in the GHz range

Superconducting Nb3Sn Cavities have potential advantages over rf cavities with Nb surfaces To test possible applications and to improve the understanding of Nb3Sn coatings on Nb, rf cavities have been measured between 1.5 and 8K and between 0.1 and 7GHz. The temperature dependence of the surface resistance R(T) indicates weak superconducting spots with transition temperaturesTmin{c}max{ast} < 1K andTmin{c}max{ast} simeq 2.5K. The normal conducting spotsTmin{c}max{ast} lsim 1K cause the large rf residual lossesR'_{res} propto f^{2}observed up to date. The spots withTmin_{c}max_{ast} simeq 2.5K cause temperature dependences ofR'(T)between 2 and 6K, where RBCS(Nb3Sn) is still negligible. In line withR_{res} propto f^{2}, the lowest rf lossesR_{res} < 2.10^{-9}Omegaand the highest field strengthB_{crit} = 83 m^{T}(wedgeE_{peak} = 29have been observed at the lowest frequency 0.1GHz measured. Surface resistance and penetration depth measurements have shown that grain boundaries or hydrogen clusters do not cause the weak spots observed withTmin{c}max{ast} < 2.5K. The origin and the chemistry of the weak spots withTmin{c}max{ast} lsim 1K, which cause the largeR_{res} propto f^{2}and the lowB_{crit} (T) simeq const, are still not clear. They seem related to the Nb3Sn surface. The weak spots withTmin{c}max{ast} simeq 2.5K consist most likely of Nb6Sn5, which in cooling below 950°C precipitates due to the excess Sn present in Nb3Sn coatings grown in Sn vapor.     

Development of 12 T-10 Al-stabilized Nb3Sn conductor for TMC-II

The Al-stabilized Nb3Sn strand has been successfully fabricated and Jcvalue of 400 A/mm²at 12 T is obtained on condition that Nb3Sn is reacted at 625°C for 200 hr. Overall residual resistivity of this strand is lowered to 77 % of that of the Cu-stabilized Nb3Sn strand (TMC-I)^1,2at 12 T by Al-stabilizer. The 12 T-10 kA cable-in-conduit conductor, fabricated by this strands, is charged up to 20 kA at 8.7 T without appearance of normal zone. From these experimental results, this conductor satisfies almost the specification of the present target conductor (TMC-II)³.     

Low temperature irradiations of Nb3Sn with 14- MeV neutrons

High energy neutron irradiations have been performed on Nb3Sn superconductors to assess their behavior in a fusion reactor environment. Irradiations were performed at 4.2 K and property measurements were made without warming the samples. The critical current Icincreased with irradiation to a level about 50% above the unirradiated value at the highest fluences reached in our experiments. These results are compared with the results of other low temperature irradiations of Nb3Sn.     

A 17.5 Tesla superconducting concentric Nb3Sn and V3Ga magnet system

A superconducting magnet system has been designed and constructed; it now operates to a field of 17.5 Tesla. The system consists of an outer Nb3Sn solenoid with a 160 mm bore producing 13.5 T, and an inner V3Ga solenoid with a 31 mm bore producing an incremental 4 T. Electrical transients were monitored in the outer magnet during normal transition and compared with predictions. The inner magnet operates close to the critical current of the V3Ga as measured in small coil tests. The magnet system was driven normal several times at a stored energy level of approximately 1.8 Megajoules, activating protective circuitry, designed to safely dissipate the energy released.     

Recent advances in hydrostatic extrusion of multifilament Nb3Sn and NbTi superconductors

Recent technical results are presented relating to the processing of superconductor wire by hydrostatic extrusion. Included are the reference processing sequence developed for the Nb3Sn system and a discussion of processing parameters affecting the formability and quality of tantalum barriers. In addition, a comparison of manufacturing costs for producing wire by conventional and hydrostatic extrusion is made. Finally, some aspects of transferring technology to industry are discussed.     

Multifilament Nb3Sn superconductors produced by the E.C.N. technique

Multifilament Nb3Sn conductors are produced by reduction of composites containing bundles of niobium tubes filled with NbSn2-powder and surrounded by pure copper. Heat treatment at temperatures between 575 and 675°C after final reduction causes the tin from the NbSn2-powder to diffuse into the niobium tubes, which results in a final Nb3Sn-layer at the inner side of the tube. Two types of experimental wire are produced, the first type consisting of 4 bundles of 9 tubular filaments, the powder cores having diameters of about 35 micron, the outer size of the wire being 1 mm square. The second type consists of 4 bundles of 45 tubular filaments. This type is fabricated in 3 sizes: round φ 2.3 and square 1.4 and 1.0 mm, corresponding with powder core diameters of 26,18 and 13 microns respectively. Critical current densities in the Nb3Sn-layers reach values of about 4 - 6 × 10⁹A/m²at 8 Tesla and 1,6 - 2,4 × 10⁹A/m²at 12 Tesla. Maximum critical temperatures are about 18.1 K with a ΔTcof 0.3 K.     

Critical current of multifilamentary Nb3Sn conductors up to 18 t

The critical current l_c of Nb₃Sn conductors containing 1615 or 3721 filaments was measured in magnetic flux densities up to 18 T. The magnetic field was generated by a Bitter magnet. The overall current density of the conductor was $\text{5 × 10}{}^4\text{A cm}{}^{\mathrm{?2}}$ at 10 T and 5 × 10³ A cm^?2 at 18 T. Values of up to 21.6 T were obtained for the upper critical flux density B_c2. The influence of the diffusion conditions upon l_c, B_c2 and the superconducting transition temperature T_c have been related to the fraction of the total conductor cross-section taken up by Nb₃Sn. This fraction serves as a rough measure of the pressure exerted upon the Nb₃Sn within the conductor.     

Effect of bronze on the compression of Nb3Sn in multifilamentary conductors

Nb₃Sn in multifilamentary conductors is subject to compressive strain as a result of the relatively small thermal contraction of the filaments as compared to bronze. The critical current l_c is consequently degraded. The critical current increases, when an external tensile stress is applied, and passes through a maximum. The ratio of the maximum critical current to the initial critical current increases with the flux density and reaches a value of two at a flux density of 16 T for technical conductors. The strain _m, at which the l_c maximum is reached, lies between 0.4% and 0.7% for the conductors investigated and depends on the material parameters. For a constant ratio of bronze to filament cross section this strain _m is reduced as the Nb₃Sn layer thickness is increased and can be determined approximately by a graphical method from the stress-strain diagram. _m is to a large extent dependent on the metallurgical properties of bronze, which vary to a considerable extent depending upon the heat treatment.     

Nb3Sn-composite conductors with Aluminum as electrical stabilizer and stainless steel as mechanical reinforcement

Three-component conductors composed of a preheat-treated Nb3Sn flat cable, aluminium, and stainless steel were soldered together in a straight alignment and then wound in a single-layer coil with a diameter of 90 mm. The performance of the composite conductor was determined mainly by the position of the steel, because the steel shifts the neutral plane considerably due to its high modulus of elasticity and can generate high compressive forces on the Nb3Sn with a consequent reduction of critical current density. The aluminum stabilizer developed its full stabilizing performance only in close contact with the superconductor, because resistive layers between them cause partial resistivity in the superconductor.     

Self-healing effect in strained multifilamentary Nb3Sn conductors

Multifilamentary Nb₃Sn conductors can be strained much more without damage than bare Nb₃Sn. The reason is a compression of the Nb₃Sn filaments which acts as a mechanical reserve. It is caused by a stronger thermal contraction of the bronze in the composite. By overloading the conductor at low temperature the compression of the Nb₃Sn disappears. Afterwards the conductor is subject to damage upon further loading. Measurements of the critical current under strain showed that at least part of the compressive strain can be regained by warming the conductor to room temperature. Analysis of the measured low temperature stress-strain diagram revealed that plastic deformation of the bronze during warming up is the cause of this self-healing.     

Formation of A15-phase in Ta/ Nb3Al0.75Ge0.25-powder - and Nb/ Nb3Al0.75Ge0.25-powder - composites and their superconducting properties

The experiments were done using a powder mixture of Nb, Al and Ge with a particle diameter of less than 50 μm which was filled into Ta- or Nb-tubes. These composites were cold-worked and heated from 400°C (10°C/min) up to 850 or 900°C and annealed 20 min on this temperature. By this heat treatment in many areas the intermetallic compound Nb(Al,Ge)3is formed within the powder mixture. After a second cold deformation the composites were annealed at 1000, 1200 or 1300°C with different annealing times. Microprobe analysis was used to investigate the phase distribution. The formation of the A15-phase was also investigated by transition temperature measurements, which were done by the inductive method. The highest transition temperature was found after 1300°C furnace annealing. Maximum critical current was achieved by short-time resistive annealing at 1200°C of 0.5 mm θ wire. The reason for the high current carrying capacity of the short-time annealed samples can be seen from the Tc-measurements which show that a A15-phase with high transition temperature is formed even after a short annealing time. Possible improvements of superconductors made from powder mixtures are discussed.     

Projected upper limits to the critical current density of superconducting Nb3Ge, Nb3Sn, and V3Ga

Applying a phenomenological theory of flux pinning developed by Kramer,¹ where the ultimate critical current density (J_c) of a superconductor is determined by plastic shearing of the flux lattice, approximate upper limits are put on J_c for Nb₃Ge, Nb₃Sn, and V₃Ga. At 4.2 K and for magnetic fields H < 100 kG, the J_c of V₃Ga is greater than that of either Nb₃Ge or Nb₃Sn and Nb₃Sn has somewhat higher values than Nb₃Ge. Above 200 kG Nb₃Ge has the highest J_c due to its having the highest upper critical field and at 14 K or above it probably has the largest J_c at all field values.