Experimental Study of Potential Heat Treatment Issues of Large Bi-2212 Coils

Abstract

The route to 30 T NMR endorsed by a recent National Academy report clearly still has many challenges to achieve high stability and homogeneous high temperature superconducting (HTS) magnet. As the only HTS conductor with round wire (RW) geometry, Bi2Sr2CaCu2O8-x (Bi-2212) RW conductor is very attractive for NMR magnet applications. At present, an NMR quality demonstration magnet with Bi-2212 RW wound insert coils is under development at the National High Magnetic Field Laboratory (NHMFL). The target of this demonstration magnet is to generate a total field of 23+ T with ppm level homogeneity. Since Bi-2212 coils require Wind-and-React (W&R) technology, our initial major concern was that large Bi-2212 coils might deform during the typical partial melt Bi-2212 heat treatment (HT) due to their large self-weight. To experimentally mimic the HT of large Bi-2212 coil, several small test coils were heat treated under deadweight loads. After 1 bar Bi-2212 full reaction, these coils were characterized in terms of coil geometry, transport critical current properties, oxygenation status and insulation performance. Coil geometry and individual wire shape was in fact not distorted, nor was transport properties degradation was induced by mechanical loading. Uniform oxygen equilibration was achieved in these coils even though they were coated with dense oxide insulation. However, although the TiO2-based insulation coating was well preserved on the wire surface, several coils developed electrical shorts.

Correlation of critical current density to quasi-biaxial texture and grain boundary cleanliness in fully dense Bi-2212 wires

“…In addition to good texture, high J c in Bi-2212 RW also requires fully densified filaments which is achieved by using over pressure heat treatment (OPHT), which promotes high physical filament connectivity [24,25]. Large scale solenoids, racetrack and canted cosine theta coils have now all been successfully reacted using OPHT [26][27][28] and we now regard OPHT as routine. However, there is still much to learn about the details of OPHT of large coils because heating and cooling rates vary throughout any given coil structure.…”

Tripled critical current in racetrack coils made of Bi-2212 Rutherford cables with overpressure processing and leakage control

“…RC3 is identical to RC2 except that its cable was brush painted with several coats of TiO 2 -polymer slurry before being insulated with the same mullite sleeve used for the cables and coil winding of RC1 and RC2. The TiO 2 slurry was prepared with a recipe [21,22] used for single strand solenoids with properties already reported [23][24][25]. Figure 1 shows cross-sections of the wires and the insulated Rutherford cable.…”

Superconducting Accelerator Magnets Based on High-Temperature Superconducting Bi-2212 Round Wires

“…More than a dozen coils have been successfully constructed, reaching a peak field of 33.8 T operating a background field of 31.2 T [17]. Much understanding has been obtained in regard to OPHT [92,93], insulation materials [94], persistent superconducting joint [95], quench protection [96], and maintaining a precise winding [97] to preserve the ppb level field homogeneity required by the NMR magnets. A new mechanically reinforced strand with an axial stress limit of greater than 300 MPa has also been developed in collaboration with the Solid Material Solutions, LLC [98].…”