Youzhu Zhang

Nb3Sn Conductor Development for Fusion and Particle Accelerator Applications

Future large‐scale applications of superconductors, such as ITER or possible LHC upgrades, will likely use Nb3Sn conductor. Although the strand requirements for fusion and particle accelerators do have some overlap, there are significant differences that may require a variety of manufacturing approaches. Oxford Instruments, Superconducting Technology (OI‐ST) produces high performance Nb3Sn wire via several different “internal Sn” routes. Recently, research at OI‐ST has produced strand made by the restacked rod process that has achieved 12 T, 4.2 K non‐Cu critical current density (Jc) values of 3000 A/mm2. This high Jc level was achieved by a combination of optimized Cu, Nb, and Sn ratios and improved heat treatment schedules. Similar conductors developed for high field use have shown engineering current density (JE) values well over 150 A/mm2 at 23.5 T, 1.8 K. OI‐ST is also involved with research for the High Energy Physics National Conductor Program. Results on distributed barrier internal Sn composites ...

Advances in Nb/sub 3/Sn strand for fusion and particle accelerator applications

Nb/sub 3/Sn conductor made by the internal tin route is the material of choice for the highest field superconducting magnets. These include systems ranging from solenoids used in 900MHz NMR and 20 T laboratory magnets, to large-scale applications such as ITER and possible LHC upgrades. We present our latest results on internal tin strands having critical current density (J/sub c/) values of 3000 A/mm/sup 2/ (4.2 K, 12 T), as it relates to such magnet systems. One obstacle to wider use of internal tin strand is the relatively small billet size, typically limited to 50 kg or less. As part of the R&D for the U.S. High Energy Physics National Conductor Program, we have developed a method of scaling up the distributed barrier internal tin process to billet sizes several times larger. In the past year we have successfully produced a high J/sub c/ distributed barrier strand made entirely by hot extrusion. Results are also presented on a new method of supplying Ti dopant for the Nb/sub 3/Sn that does not rely on alloying the Sn cores, matrix Cu, or the Nb filaments directly. Material made with this new doping method reached a J/sub c/ (4.2 K, 12 T) value of 2500 A/mm/sup 2/. Finally, the state of development of composites having lower AC losses is described. Such conductors are being developed for possible future fusion applications including ITER.

Latest Improvements of Current Carrying Capability of Niobium Tin and Its Magnet Applications

Practical high field superconducting magnets are exclusively built with Nb3Sn multi-filamentary composites. Over the last few years there have been significant improvements in the current carrying capability of Nb3Sn strand, and these improvements offer the possibility to build more efficient and higher field strength magnets. The Nb3Sn composite requirements are somewhat different depending on the application, such as magnetically confined fusion, high energy physics accelerators, and solenoids for NMR or laboratory magnets, and thus require different designs. We will present the current status of Nb3Sn strand development at Oxford Instruments, Superconducting Technology (OST) for these applications, along with magnet results

Development of Internal Tin

Performance improvements are needed for large-scale applications of Nb₃Sn, such as ITER or LHC upgrades. The highest critical current density (J_c) values are achieved in distributed-barrier strand made by the Restacked Rod Process, which can reach 12 T, 4.2 K J_c values of 3000 A/mm², with high residual resistivity ratio (RRR) values. For purposes of accelerator magnet stability, it is desirable to combine high J_c with a small effective filament diameter ( D_eff ) . Initial experiments show reducing D_eff from 80 mum to 40 mum leads to a 10% reduction in J_c. For fusion applications, a single-barrier design with well-spaced filaments is used to achieve the low hysteresis losses that are required. The status of our fusion strand development program is presented, including results for strand made using Nb-47 wt%Ti rods to supply Ti dopant. Such strands can reach 12 T, 4.2 K J_c> 1000 A/mm², with losses < 1000 mJ/cm³.

{\hbox{Nb}}_{3}{\hbox{Sn}}

The critical current density (J_c) of Nb₃Sn strand has been significantly improved over the last several years. For most magnet applications, high J_c internal tin has displaced bronze process strand. The highest J_c values are obtained from distributed barrier strands. We have continued development of strands made with Nb-47 wt%Ti rods to supply the dopant, and have achieved J_c values of 3000 A/mm² (12 T, 4.2 K). Such wires have very good higher field performance as well, reaching 1700 A/mm² at 15 T. To reduce the effective filament diameter in these high J_c strands, the number of subelement rods incorporated into the final restack billet has been increased to 127 in routine production, and results are presented on experimental 217 stacks. A new re-extrusion technique for improving the monofilament shape is also described. For fusion applications such as ITER, we have developed single-barrier internal tin strands having non-Cu J_c values over 1100 A/mm² (12 T, 4.2 K) with hysteresis losses less than 700 mJ/cm³ over non-Cu volume. The J_c-strain behavior of such composites is also presented.

Conductor for Fusion and Particle Accelerator Applications

Oxford Instruments, Superconducting Technology (OI-ST) produces Nb/sub 3/Sn wires by two major process routes: bronze and internal Sn, including modified jelly roll (MJR). Each process has its own strengths for particular applications. We report on MJR wire designed for prototype high energy physics accelerator magnet studies which achieved short sample non-Cu critical current density at 12 T, 4.2 K exceeding 2000 A/mm/sup 2/. We also report recent results from wires fabricated by other internal Sn methods, the bronze process, and powder-in-tube processes.

Internal Tin

The past several years have seen a significant improvement in the maximum critical current density (Jc) in Nb3Sn strand. However for many applications, parameters besides high Jc values are paramount. For fusion applications such as ITER, we have developed single‐barrier internal tin strands having non‐Cu Jc values over 1000 A/mm2 (12 T, 4.2 K) with hysteresis losses less than 1000 mJ/cm3. Our most recent results are presented, with the goal of maintaining the high Jc but further reducing the losses. For high field magnet applications, higher Jc values are obtained using a distributed barrier approach. Results will be presented on a new high Jc Nb3Sn strand that is made with (Nb,Ti)3Sn instead of (Nb,Ta)3Sn. This (Nb,Ti)3Sn strand has a Jc value of 3000 A/mm2 (12 T, 4.2 K), but has improved higher field performance compared with our standard (Nb,Ta)3Sn material, reaching 1700 A/mm2 at 15 T, with further optimizations perhaps still possible. To reduce the effective filament diameter in these high Jc strand...

{\hbox {Nb}}_{3}{\hbox {Sn}}

The critical current density (J_c) of Nb₃Sn strand has been significantly improved over the last several years. For most magnet applications, high J_c internal tin has displaced bronze process strand. The highest J_c values are obtained from distributed barrier strands. We have continued development of strands made with Nb-47 wt%Ti rods to supply the dopant, and have achieved J_c values of 3000 A/mm² (12 T, 4.2 K). Such wires have very good higher field performance as well, reaching 1700 A/mm² at 15 T. To reduce the effective filament diameter in these high J_c strands, the number of subelement rods incorporated into the final restack billet has been increased to 127 in routine production, and results are presented on experimental 217 stacks. A new re-extrusion technique for improving the monofilament shape is also described. For fusion applications such as ITER, we have developed single-barrier internal tin strands having non-Cu J_c values over 1100 A/mm² (12 T, 4.2 K) with hysteresis losses less than 700 mJ/cm³ over non-Cu volume. The J_c-strain behavior of such composites is also presented.

Conductors Engineered for Fusion and Particle Accelerator Applications

Magnetization measurements have been made on several high J/sub c/ Nb/sub 3/Sn strands fabricated by different internal-Sn designs. In general these conductors have high magnetization at low fields, often exhibiting flux-jumps that are characteristic of large superconductor diameter. The effective filament size d/sub eff/ is approximately the size of the sub-element because the filament pack within each sub-element is fully coupled. Dividing the filament pack of the sub-element by adding Ta is effective for reducing d/sub eff/ and magnetization instability. But, some residual coupling across the dividers seems to remain below 6 K, perhaps due to Ta/sub 3/Sn. Implications for accelerator magnets are discussed.

Progress with Nb/sub 3/Sn conductors at oxford instruments, superconducting technology

Advances in high field magnets are driven primarily by the availability of high current density conductors. The restack rod process (RRP), internal Sn superconductors have achieved engineering current densities nearly five times that of bronze route conductors at high fields. Careful utilization of this low temperature superconductor (LTS) enables the production of magnets beyond the previous benchmark of 21 Tesla without an associated increase in magnet and cryostat volume. Steps to realize extremely compact high field magnets for a variety of applications are described. The next significant challenge is to produce magnetic fields beyond 25 Tesla solely using superconducting solenoids. High temperature superconductors (HTS) will be required and, to this end, Bi-2212/Ag matrix wires are at an advanced stage of development. The tangible objective is a new generation of compact, ultra-high field magnets.