H. Higley

Correlation between strand stability and magnet performance

Magnet programs at BNL, LBNL and FNAL have observed instabilities in high J/sub c/ Nb/sub 3/Sn strands and magnets made from these strands. This paper correlates the strand stability determined from a short sample-strand test to the observed magnet performance. It has been observed that strands that carry high currents at high fields (greater than 10 T) cannot sustain these same currents at low fields (1-3 T) when the sample current is fixed and the magnetic field is ramped. This suggests that the present generation of strand is susceptible to flux jumps (FJ). To prevent flux jumps from limiting stand performance, one must accommodate the energy released during a flux jump. To better understand FJ this work has focused on wire with a given sub-element diameter and shows that one can significantly improve stability by increasing the copper conductivity (higher residual resistivity ratio, RRR, of the Cu). This increased stability significantly improves the conductor performance and permits it to carry more current.

Rutherford cables with anisotropic transverse resistance

Putting a resistive core into the center of a Rutherford cable increases resistance between strands in the crossover direction, which greatly reduces the coupling currents, even when the resistance to adjacent turns remains small. This allows one to improve stability by soldering strands together or using porous metal, without incurring a penalty of increased coupling. We describe our manufacturing methods and an experimental measurement of coupling.

HD1: design and fabrication of a 16 Tesla Nb/sub 3/Sn dipole magnet

The Lawrence Berkeley National Laboratory (LBNL) Superconducting Magnet Group has completed the design, fabrication and test of HD1, a 16 T block-coil dipole magnet. State of the art Nb/sub 3/Sn conductor was wound in double-layer racetrack coils and supported by an iron yoke and a tensioned aluminum shell. In order to prevent conductor movement under magnetic forces up to the design field, a coil pre-stress of 150 MPa was required. To achieve this level without damaging the brittle conductor, the target stress was generated during cool-down to 4.2 K by exploiting the thermal contraction differentials between yoke and shell. Accurate control of the shell tension during assembly was obtained using pressurized bladders and interference load keys. An integrated 3D CAD model was used to optimize magnetic and mechanical design and analysis.

Strand critical current degradation in Nb/sub 3/Sn Rutherford cables

Fermilab is developing 11 Tesla superconducting accelerator magnets based on Nb/sub 3/Sn superconductor. Multifilamentary Nb/sub 3/Sn strands produced using the modified jelly roll, internal tin, and powder-in-tube technologies were used for the development and test of the prototype cable. To optimize the cable geometry with respect to the critical current, short samples of Rutherford cable with packing factors in the 85 to 95% range were fabricated and studied. In this paper, the results of measurements of critical current, n-value and RRR made on the round virgin strands and on the strands extracted from the cable samples are presented.

Fabrication and test of Nb/sub 3/Sn racetrack coils at high field

A program based on exploring the benefits of racetrack coil designs for utilization of brittle superconductors to achieve high fields is underway at LBNL. As an intermediate step in the experimental program, a set of Nb/sub 3/Sn racetrack coils, using state-of-the-art conductor, have been built and tested. The coils were configured to maximize the field, providing a means to study the effects of stress on conductor performance. In addition, several design improvements were added which will be implemented in the next step of the program; construction of a racetrack dipole with a field of 14 Tesla. An evaluation of the design modifications and test results are given.

Fabrication of a Short-Period

Lawrence Berkeley National Laboratory develops high-field Nb3Sn magnets for HEP applications. In the past few years, this experience has been extended to the design and fabrication of undulator magnets. Some undulator applications require devices that can operate in the presence of a heat load from a beam. The use of Nb3Sn permits operation of a device at both a marginally higher temperature (5-8 K) and a higher Jc, compared to NbTi devices, without requiring a larger magnetic gap. A half-undulator device consisting of 6 periods (12 coil packs) of 14.5 mm period was designed, wound, reacted, potted and tested. It reached the short sample current limit of 717 A in 4 quenches. The non-Cu Jc of the strand was over 7,600 A /mm2 and the Cu current density at quench was over 8,000 A/mm2 . Magnetic field models show that if a complete device was fabricated with the same parameters one could obtain beam fields of 1.1 T and 1.6 T for pole gaps of 8 mm and 6 mm, respectively.

{\rm Nb}_{3}{\rm Sn}

Most accelerator magnets for applications in the field range up to 10 T utilize NbTi superconductor and a cosine theta coil design, For fields above 10 T, it is necessary to use Nb/sub 3/Sn or other strain sensitive superconductors and other coil geometries that are more compatible with these materials. This paper describes our recent efforts to design a series of racetrack coil magnets that will provide experimental verification of an alternative magnet design philosophy, with the near-term goal of reaching a field level of approximately 14 T. The conductor and fabrication issues relevant to building high field, racetrack dipoles utilizing Nb/sub 3/Sn superconductor and a wind and react approach will also be discussed.

Superconducting Undulator

Future luminosity upgrades for the LHC at CERN will require higher field magnets than those presently being utilized. The US LHC Accelerator Research Program (LARP) is addressing this need with the design and fabrication of prototype quadrupole magnets utilizing Nb3Sn technology. A part of the R&D effort is the development of suitable cables for the magnet designs being considered by LARP. The cable parameter limits for LARP prototype and production cables are presented. At present the standard strand for LARP is the Restack Rod Processed (RRP) strand being produced by Oxford Superconducting Technologies. The present strand has 54 sub-elements and 53% Cu. An optimal cable for the first generation of LARP quadrupole magnets has a width of 10 mm, thickness of 1.26 mm, and a keystone angle of 1 degree. This work shows that if a strand is properly cabled a nearly degradation-free mechanically-stable cable can be produced.

Design and fabrication of a 14 T, Nb/sub 3/Sn superconducting racetrack dipole magnet

Historically, improvements in dipole magnet performance have been paced by improvements in the superconductor available for use in these magnets. The critical conductor performance parameters for dipole magnets include current density, piece length, effective filament size, and cost. Each of these parameters is important for efficient, cost effective dipoles, with critical current density being perhaps the most important. Several promising magnet designs for the next hadron collider or a muon collider require fields of 12 T or higher, i.e., beyond the reach of NbTi. The conductor options include Nb/sub 3/Sn, Nb/sub 3/Al, or the high temperature superconductors. Although these conductors have the potential to provide the combination of performance and cost required, none of them have been developed sufficiently at this point to satisfy all the requirements. This paper will review the status of each class of advanced conductor and discuss the remaining problems that require solutions before these new conductors can be considered as practical. In particular, the plans for a new program to develop Nb/sub 3/Sn and Nb/sub 3/Al conductors for high energy physics applications will be presented. Also, the development of a multikiloamp Bi-2212 cable for dipole magnet applications will be reported.

Cable R&D for the LHC Accelerator Research Program

report on wind-and-react (W&R) Bi2Sr2CaCu2O8+x (Bi-2212) insert coils that were fabricated using a Canted-Cosine-Theta (CCT) coil technology. In the CCT technology, a conductor is wound into canted helical channels that are machined into cylindrical coil winding mandrels, thereby creating a cosine-theta current distribution and providing stress support at the conductor level. The prevention of stress accumulations by the internal structure of winding mandrels is considered an enabling technology for high field Bi-2212 insert coils that target the 20 T magnetic field range. We report on the fabrication and reaction of coils for two proof-of-principle Bi-2212 inserts, BIN1 and BIN2, each consisting of two CCT coils. The BIN1 coils use insulated 0.8 mm diameter wires and INCONEL 600 coil winding mandrels with a stainless steel 316 shell, an overall diameter of 50.0 mm and a clear bore of 35.3 mm. The BIN2 coils use insulated 6-around-1 cables from 0.8-mm diameter wires, and aluminum-bronze mandrels with an aluminum alloy 6061 shell, an overall outer diameter of 68.6 mm, and a clear bore of 38.8 mm. The coils are designed to study the fabrication, reaction, and impregnation in Bi-2212 CCT technology, and provide a baseline for a Rutherford cable-wound coil set (BIN3). These latter coils target an insert-coil set with optimized current density in the windings, to enable the construction of an 18 T Nb3Sn-Bi-2212 hybrid dipole magnet.