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[en] With the growing interest in commercial Ag-alloy sheathed Bi2Sr2CaCu2O8+x powder-in-tube conductors (Bi2212) for use in high-field magnets, it is important to understand the quench behavior and limiting criteria of a quench, including conditions that will result in a decrease in critical current. Even though the quench characteristics of low-temperature superconductors NbTi and Nb3Sn are well understood, there is still a lack of data and understanding of what conditions during quenches in high-temperature superconducting (HTS) materials cause degradation of the conductor. In this investigation, quenches are induced in short samples of Bi2212 tape conductors with local heat disturbances using a resistive heater. The voltage and temperature evolution during quenching were recorded and analyzed to determine the normal zone propagation velocity. Furthermore, the quench conditions where varied to identify the threshold quench conditions which result in conductor damage. These conditions are quantified in terms of three parameters: the maximum temperature (Tmax), the maximum rate of temperature increase (dT/dt|max) and the maximum temperature gradient (dT/dx|max). It is found that the normal zone propagation velocity for Bi2212 tape conductor is 20-30 mm s-1. The samples showed high sensitivity during quench, exhibiting large losses in critical current under certain conditions. In all cases the sections of the tape closest to the quench initiation exhibited the highest peak temperatures and loss in critical current. It was found that conductor damage is avoided under the following conditions: dT/dt|max<250 K s-1, dT/dx|max<100 K cm-1 and Tmax<250 K
[en] We have recently shown that the gas present in the only ∼ 70% dense filaments of as-drawn Bi-2212 wire agglomerates into large bubbles that fill the entire filament diameter during the melt phase of the heat treatment. Once formed, these bubbles never disappear, although they can be bridged by 2212 grains formed on cooling. In order to test the effect of these bubbles on the critical current Ic, we increased the density of the filaments after drawing using 2 GPa of cold isostatic pressure, finding that the bubble density and size were greatly reduced and that Ic could be at least doubled. We conclude that enhancement of the filament packing density is of great importance for making major Ic improvements in this very useful, round superconducting wire. (rapid communication)
[en] It is well known that the critical current density Jc of multifilamentary Bi-2212 wires tends to decline as the wire length increases, but the reasons for and the magnitude of this decline remain obscure and quantitatively unpredictable. Here we report on the Jc and mass density variation with length on ∼ 1 m long samples taken from two recent and representative wires, in which we find a strong decrease of Jc with distance from the end and a strong correlation between Jc and the local mass density. The mass density variations occur on length scales of centimeters, many times the nominal 15 μm filament diameter. The cause of the mass density variation appears to be internal gas pressure that generates bubbles which almost fill the filament diameter when the Bi-2212 melts. Control of this internal pressure seems to be vital to moderating or avoiding the length dependence of Jc.
[en] Applications of (RE = Y, Gd)BCO coated conductors for the generation of high magnetic fields are increasing sharply, this while (RE)BCO coated conductors themselves are evolving rapidly. This article describes and demonstrates recently developed and applied mathematical models that systematically and comprehensively characterize the transport critical current angular dependence of a batch of (RE)BCO coated conductor in high magnetic fields at fixed temperatures with an uncertainty of 10% or better. The model development was based on analysis of experimental data sets from various published sources and coated conductors with different microstructures. These derivations directly are applicable to the accurate prediction of the performance in high magnetic fields of coils wound with (RE)BCO coated conductors. In particular, a nonlinear fit is discussed in this article of transport critical current at T = 4.2 K versus field and angle data. This fit was used to estimate the hysteresis losses of (RE)BCO coated conductors in high magnetic fields, and to design the inserts wound with such conductors of the all-superconducting 32 T magnet being constructed at the NHMFL. A series of such fits, recently developed at several fixed temperatures, continues to be used to simulate the quench behavior of that magnet. (paper)
[en] Increasing the critical current density (Jc) of the multifilamentary round wire Ag/Bi2Sr2CaCu2Ox(2212) requires understanding its complicated microstructure, in which extensive bridges between filaments are prominent. In this first through-process quench study of 2212 round wire, we determined how its microstructure develops during a standard partial-melt process and how filament bridging occurs. We found that filaments can bond together in the melt state. As 2212 starts to grow on subsequent cooling, we observed that two types of 2212 bridges form. One type, which we call Type-A bridges, forms within filaments that bonded in the melt; Type-A bridges are single grains that span multiple bonded filaments. The other type, called Type-B bridges, form between discrete filaments through 2212 outgrowths that penetrate into the Ag matrix and intersect with other 2212 outgrowths from adjacent filaments. We believe the ability of these two types of bridges to carry inter-filament current is intrinsically different: Type-A bridges are high- Jc inter-filament paths whereas Type-B bridges contain high-angle grain boundaries and are typically weak linked. Slow cooling leads to more filament bonding, more Type-A bridges and a doubling of Jc without changing the flux pinning. We suggest that Type-A bridges create a 3D current flow that is vital to developing high Jc in multifilamentary 2212 round wire.
[en] We have developed TiO_2 coating on Ag-alloy sheathed Bi_2Sr_2CaCu_2O_8_−_x (Bi-2212) round-wire conductor for electrical insulation in Bi-2212 magnets. The green coating has a base layer comprised of TiO_2, polyvinyl butyral (PVB) and a small amount of polysilicate and a top layer made of polyacrylic. The coating was applied on the conductor using a continuous reel-to-reel dip coating process and showed very good adherence and flexibility that is suitable for magnet coil winding. The thickness of the coating is a function of slurry viscosity, wire withdrawal speed and wire radius. Small test coils were built with the coated Bi-2212 round-wires and were heat treated at 100 atm pressure. During the heat treatment, the PVB and polyacrylic were removed from the green coating and the polysilicate decomposed to SiO_2 that served as a sintering aid for TiO_2. After the heat treatment, the coating remained strongly adhered to the conductor and did not have a detrimental effect on the critical current (I_c) values. The breakdown voltage was about 150 V across a 7 μm thick heat treated coating on Bi-22112 round-wire conductor, corresponding to a dc dielectric strength of about 21 MV m"−"1. (paper)
[en] Self-field quench behaviours of YBa2Cu3O7-δ coated conductors with different stabilizers are studied. Samples include one with Cu on both sides (Cu-Cu), one with stainless steel on both sides (SS-SS), and one with Cu on one side and stainless steel on the other (Cu-SS). The measurements of the minimum quench energy (MQE) and normal zone propagation velocity (NZPV) are taken at various temperatures (30-75 K), and transport currents (30% Ic to 90% Ic) at a typical pressure of 10-5 Pa. Of the three samples, the Cu-Cu sample has the highest MQE while the SS-SS one has the lowest MQE at the same temperature and percentage of Ic; the NZPV in the SS-SS sample is found to be the highest while those of the Cu-Cu and Cu-SS samples are similar. The normal zone voltage and the hot-spot temperature are also compared. Both the classic adiabatic quench propagation model and the interface resistance model are used to explain the NZPV and MQE differences between the samples. The implications for conductor design and quench detection and protection are discussed.
[en] One of the biggest challenges in developing conductor on round core (CORC"®) magnet cables for use in the next generation of accelerator magnets is raising their engineering current density J _E to approach 600 A mm"−"2 at 20 T, while maintaining their flexibility. One route to increase J _E could be to add more RE-Ba_2Cu_3O_7_−_δ coated conductors to the cable, but this would increase the cable size and reduce its flexibility. The preferred route to higher J _E is a reduction in diameter of the CORC"® cable, while maintaining the number of tapes wound into the cable. The availability of very thin tapes containing substrates of 30 μm thickness enabled us to wind a 5.1 mm diameter CORC"® cable from 50 coated conductors, while maintaining a tape critical current I _c of about 97% after cabling. The cable I _c was 7030 A at 4.2 K in a background field of 17 T, corresponding to a J _E of 344 A mm"−"2, which is the highest performance of any CORC"® cable so far. The magnetic field dependence allowed us to extrapolate the cable performance to 20 T to predict an I _c of 5654 A and a J _E of 309 A mm"−"2. The results clearly show that rapid progress is being made on overcoming the J _E hurdle for use of CORC"® cables in the next generation of accelerator magnets. Further optimization of the cable layout will likely increase J _E towards 600 A mm"−"2 at 20 T in the near future, while further reduction in cable size will also make them even more flexible. (paper)
[en] Wind-and-react coils using alumino-silicate-braid-insulated Bi2Sr2CaCu2O8+x (Bi-2212) round wire consistently show reduced critical current densities (Jc) compared to short samples heat treated under nominally identical conditions. This is a significant detriment to the design of Bi-2212 insert coils for high field magnets. Here we report on the superconducting properties of a series of wind-and-react test coils and short conductor samples systematically extracted from various sections of some of these wind-and-react coils. Overall we find remarkable uniformity of the superconducting properties throughout a majority of each coil, even though the Jc of the coil is markedly below that of short samples processed at the same time. Analysis of the critical temperature (Tc) and the irreversibility field (Hirr) shows that these Jc variations are solely related to changes in connectivity within the Bi-2212 filaments, rather than to any variations of oxygen uptake within the coil winding during heat treatment. We conclude that the reduced Jc of the coils is related to a sample length dependence of Jc rather than to any lack of full oxygenation of even a 20-layer coil. Raising the Jc of the coils thus appears to be a problem that needs addressing chiefly at the strand rather than the coil level. (paper)
[en] The critical current of a short YBa2Cu3O7-δ (YBCO) coated conductor sample degrades in an unprotected quench performed in a nearly adiabatic environment at 30 K. The conductor has Cu stabilizers on both surfaces. The quench is initiated by a heater attached to the sample surface. The amplitude of the transport current is fixed as 91% of the sample's initial critical current. The duration of the current is increased to simulate an unprotected quench and to reach increasing and controlled voltage and temperature levels. A peak temperature of 490 ± 50 K and a heating rate of 1800 K s-1 are measured when the critical current degrades by ∼ 5%. The applied thermal strain on the YBCO layer from 30 to 490 K is estimated to be 0.31% and is applied at a strain rate of ∼ 1% s-1. The rate of temperature change and the time to reach a certain peak temperature, determined by the current density in the Cu stabilizer, are estimated assuming adiabatic conditions based on the short sample case. For a Cu stabilizer current density ranging from 1000 to 2000 A mm-2, achieved in commercial conductors currently available, the quench detection and protection requires a response time < 200 ms to limit the peak temperature below 200 K. A Cu stabilizer current density higher than 3000 A mm-2 may challenge the existing detection and protection techniques for the same 200 K limit. Integrating the substrate as part of the stabilizer may help reduce the stabilizer current density to gain more time for quench detection and protection while maintaining the engineering current density.