M. Kosuge

Nb3Al Multifilamentary Wires Continuously Fabricated by Rapid-Quenching

The Nb tube process has recently been developed at NRIM(Japan) for fabricating Nb3Al multifilamentary superconductors,1–3 which are characterized by the heat treatment at low temperature ( 1700°C), many attempts were made to form Nb3A1 by heat treating Nb/AI composite at high temperatures.4–6 However, rapid grain growth decreased the density of pinning center (grain boundary) in Nb3A1 and thereby degraded Jc in particular at low magnetic fields. To overcome this problem, Nb3A1 conductors fabricated through a rapid-quenching process have been studied. In this process the A15 Nb3A1 phase with fine grain structure can be precipitated from the supersaturated Nb-Al bcc phase by aging at ~800°C. However, no attempt was made to fabricate a practical-structure conductor such as a long multifilamentary wire through continuous rapid-quenching and subsequent annealing (post annealing) process.

Generation of 23.4 T using two Bi-2212 insert coils

Development of a 1 GHz superconducting NMR magnet is in progress at the Tsukuba Magnet Laboratory (TML) of the National Research Institute for Metals (NRIM). This magnet will contain a BSCCO inner coil, which should generate a central field of 23.5 T in a backup field of 21.1 T. In order to accomplish this targeted field, we fabricated two Bi-2212 double-pancake coils (Coil A and Coil B). They were installed in the high-field superconducting magnet system at the TML/NRIM. Their performance was measured in a backup field of 18 T. Coil A was made of 20 double-pancakes wound with Ag sheathed Bi-2212 tape conductors. Ag-Mg tape was co-wound for mechanical support. Its winding was 147 mm in outer diameter and 220 mm in height. It generated a central field of 21.4 T in a clear bore of 61 mm. Coil B was located inside Coil A. Its 6 double-pancakes were wound with Bi-2212 tape conductors reinforced with Ag-Mg-Ni alloy sheath. The outer diameter and height of the winding were 48 mm and 63 mm, respectively. Coil B generated the highest field of 23.4 T in a backup field of 21.4 T. This study confirmed that the present performance of the Bi-2212 coils had already satisfied the required conditions for the inner coil of the 1 GHz NMR magnet from the viewpoint of high-field generation.

Enhanced current capacity of jelly-roll processed and transformed Nb/sub 3/Al multifilamentary conductors

In order to enhance the current carrying capacity, we have developed an improved fabrication process where the wire diameter can be increased from 0.5 to 1.25 mm and the Nb-matrix ratio decreased from 1.5 to 0.52, without degrading the critical current density, J/sub c/, of Nb/sub 3/Al phase. The critical current for a monolithic conductor at 21 T and 4.2 K has now been enhanced to 166 A which used to be 15 A. The compacted-strand-cables were fabricated to investigate feasibility for large-scale application uses. We have found that stranding and flat-rolling the as-quenched Nb/Nb(Al)/sub ss/ composite cause no degradation in J/sub c/. Attempts were made to stabilize the resulting high current conductors.

Effects of additive elements on continuous ultra-fine Nb/sub 3/Al MF superconductor

Nb/sub 3/Al multifilamentary (MF) wire can be fabricated by the composite process using various Al-based alloy cores and pure Nb matrix. Additive elements of Mg, Ag, Cu and Zn harden the Al core preferentially and then improve the workability of the Nb/Al composite, permitting it to be cold-drawn into a wire with ultrafine Al-based alloy filaments (filament number: 1.8 million, diameter: about 0.1 mu m). Reacted wires at 700-900 degrees C show T/sub c/ of 15-16 K, mu /sub 0/H/sub c2/ (4.2 K) of 21-22 T and J/sub c/ (4.2 K, 10 T) of 1-1.5*10/sup 9/ A/m/sup 2/. A two-stage reaction consisting of a reaction above 950 degrees C and a subsequent reaction around 700 degrees C is carried out to improve critical values; T/sub c/ and mu /sub 0/H/sub c2/ are increased to 17.4 K and 25.4 T, respectively. Excellent superconducting properties involving good strain tolerance ( epsilon /sub irrev/=1.3%) indicate that this Nb/sub 3/Al is very promising for practical high-field superconducting cables. >

Incorporation of stabilizer to rapid-quenched and transformed Nb/sub 3/Al multifilamentary superconductors

A Nb/sub 3/Al superconducting conductor based on Jelly-Rolled (JR) Nb/Al composites is fabricated by the rapid-heating and quenching of a wire of such composites with subsequent transformation-annealing to form Nb/sub 3/Al phase. Since the conductor is heated up to 1900/spl deg/C, far above the melting point of Cu, during fabrication, Cu stabilizer cannot directly be included as a basic composite constituent. In the present study, attempts have been made to incorporate the stabilizer to the Nb/sub 3/Al conductor. In a mechanical-cladding method, the as-quenched composite is wrapped longitudinally with a Cu tape, and groove-rolled for mechanical bonding. For internal stabilization, Jelly-Rolled Nb/Al filaments are isolated from the stabilizers by a Nb-filaments barrier and the formation of unwanted Nb-Al-Cu (Ag) compounds are prevented. Ag-stabilizer, little reactive with Nb, can dramatically decrease the Nb-barrier thickness required and allows the increase in the volume fraction of the stabilizer instead.

Current decay evaluation of closed HTS coil circuits

Persistent current mode operation of HTS coils is one of the key technologies required for very high-field NMR magnets composed of LTS and HTS coils. We prepared four closed circuits which consisted of an HTS coil made of a single conductor, a persistent current switch and their joints. Three HTS persistent current switches were fabricated for this study and they functioned well. The current decay behavior of the four circuits was measured in liquid helium by monitoring the magnetic field in the HTS coil with a Hall probe. One circuit maintained 80 % of the initial current even after 50 hours. A resistance less than 1.6/spl times/10/sup -11/ /spl Omega/ was obtained in another circuit. The small resistivity of HTS coils, caused by their small index number values, was thought to accelerate the current decay.

Pinning mechanism in a continuous ultrafine Nb3Al multifilamentary superconductor

For Nb3Al multifilamentary superconductors fabricated by a newly developed composite process using continuous ultrafine Al‐based alloy cores and pure Nb matrix, Jc properties have been investigated in detail in regard to the size and morphology of the Al core. A significant enhancement in pinning force caused by reducing the Al core size and an anisotropy in Jc observed in a rolled tape have indicated that the Nb3Al‐matrix (or core) interface acts as a dominant pinning center of fluxoids.

Development of internally stabilized RHQT Nb3Al superconductors

Recent progress in the internally stabilized rapid-heating, quenching and transformation (RHQT) processed Nb3Al superconductors is described and their various features are compared to those of the externally stabilized Cu clad conductor. Based on the favourable feature that Ag is almost non-reactive to Nb during joule heating, we have succeeded in thinning a diffusion barrier of Nb to less than 5 µm in the finished cross section by adopting an Ag rod jacketed with Nb as a stabilizer module when stacking the subelements. The Ag/non-Ag ratio could be increased to as much as 0.345 by additionally replacing the Nb-centre rod of a jelly roll (JR) with Ag. The superconducting properties of the critical current density, the residual resistance ratio, the magnetoresistance and the n-index in the voltage–current characteristic were studied for both as-quenched and subsequently deformed conductors, finally transformed at 800 °C for 10 h. A loading test of a small coil wound with a 30 m length of internally stabilized RHQT JR conductor has shown the uniformity of the internally stabilized conductor used.

Microstructure and n-value of RHQT processed Nb/sub 3/Al superconductors for NMR uses

We have examined the critical current I/sub c/ and n-values of RHQT processed Nb/sub 3/Al conductors at 4.2 K in a wide range of fields from 14.5 to 25 T in relation to the microstructures obtained. For a point and a tail of Cu-clad rectangular conductor used for a winding, the n-values were determined by fitting the voltage over a taps spacing of 110 or 220 mm to a relationship of V = CI/sup n/, in a range from 10/sup -5/ to 10/sup -4/ V/m. Such n-values were almost the same with each other, indicating the uniformity of a 370 m length of RHQT Nb/sub 3/Al conductor as the I/sub c/ consistency does. The n-value increased monotonously with decreasing a field from 25 T to 18 T, but got saturated to be around 35 at 4.2 K in fields less than 18 T. The n-value at 21 T and 4.2 K, corresponding to 23.5 T and 1.8 K, was about 25, and confirmed to be large enough for NMR uses.

Generation of magnetic fields over 21 T in a 61 mm clear bore using low copper ratio (Nb,Ti)/sub 3/Sn conductors

A 21 T superconducting magnet system has been developed and improved at the National Research Institute for Metals in Japan. As the steady progress of high-T/sub c/ superconducting coils required a larger bore for testing their performance, a new innermost coil has been developed. We increased the available bore from 50 mm to 61 mm. The new coil employed three kinds of rectangular (Nb,Ti)/sub 3/Sn conductors. External copper stabilized these conductors, with a Cu:SC ratio of only 0.25. The new coil operated in a 160 mm diameter clear bore with a background field of 18 T in saturated superfluid helium, generating a central field of 21.5 T without any training behavior. A double walled dewar, the outer diameter of which was 60 mm, was also developed. With this dewar, the new system provides a temperature-variable bore of 50 mm.