R.M. Scanlan

Superconducting materials for large scale applications

Since the 1960s, Nb-Ti (superconducting transition temperature T/sub c/=9 K) and Nb/sub 3/Sn (T/sub c/=18 K) have been the materials of choice for virtually all superconducting magnets. However, the prospects for the future changed dramatically in 1987 with the discovery of layered cuprate superconductors with T/sub c/ values that now extend up to about 135 K. Fabrication of useful conductors out of the cuprates has been difficult, but a first generation of silver-sheathed composite conductors based on (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10/ (T/sub c//spl sim/110 K) has already been commercialized. Recent progress on a second generation of biaxially aligned coated conductors using the less anisotropic YBa/sub 2/Cu/sub 3/O/sub 7/ structure has been rapid, suggesting that it too might enter service in the near future. The discovery of superconductivity in MgB/sub 2/ below 39 K in 2001 has brought yet another candidate material to the large-scale applications mix. Two distinct markets for superconductor wires exist-the more classical low-temperature magnet applications such as particle accelerators, nuclear magnetic resonance and magnetic resonance imaging magnets, and plasma-containment magnets for fusion power, and the newer and potentially much larger market for electric power equipment, such as motors, generators, synchronous condensers, power transmission cables, transformers, and fault-current limiters for the electric utility grid. We review key properties and recent progress in these materials and assess their prospects for further development and application.

HTS conductors for magnets

We have successfully improved the J/sub c/ value of Bi-2212 round wire to 500 kA/cm/sup 2/ at 4.2 K in self-field. The J/sub c/ value strongly depended on the filament size, aspect ratio, and geometry. Adjusting the filament size made it possible to obtain wires with J/sub c/ values higher than 450 kA/cm/sup 2/ in a range of wire diameters from 0.8 to 1.3 mm. The J/sub c/ value remained constant up to an applied tensile strength of 150 MPa. A 1 + 6 stranded cable fabricated using 1.02 mm diameter wires carried an I/sub c/ value of 4.5 kA at 4.2 K in self-field. A 16-strand Rutherford cable could also be fabricated using this wire.

An approach for faster high field magnet technology development

J 2LC02 SC-MAG#773 LBNL-49918 An Approach for Faster High Field Magnet Technology Development R. R. Hafalia, S. Caspi, L. C hiesa, M . Coccoli , D.R. Die tde rich, S. A. Gourlay, A.F. Lietzke, l .W. ONeill, G. Sabbi, and R.M. Scanlan Abstract- The Superconducting Magnet Program at LBNL has developed a magnet design supporting our new Subscale Magnet Program, that facilitates rapid testing of small superconducting racetrack coils in the field range of 10·12 Tesla. Several coils have been made from a variety of NbJSnlCu cables, insulated, wound, reacted, potted and assembled into a small reusable yoke and shell loading structure. Bladder and key technology have provided a rapid, efficient means for adjusting coil pre-stress during both initial assembly, and between thermal cycles. This affords the opportunity to test moderately long cable samples under magnet conditions on a time scale considerably closer to that for traditional short-sample cable tests. We have built and tested four coils with the initial aim of determining the feasibility of reducing overall conductor costs with mixed-strand cables. Details of cost reduction improvements, coil construction, magnet structure, and assembly procedures are reported, along with the relative performance of the mixed-strand coil. II. THE CONDUcrOR The Subscale Magnet Program utilizes superconducting Nb3Sn strands produced by Oxford and lac. T he cables were wou nd with 20 strands ofO.7mm diameter. The nominal cable cross-section used by the Subscale Magnet Program is 7.9mm x 1.3mm. The cable is sheathed in a continuous, woven, fiberglass sleeve. Coil modules are wound in 2 layers in flat racetrack coil configuration around an iron pole-island - each layer having 22 turns each. (Fig. 1). Each double-layer coi l module requires 22m of cable (5 kg). The two baseline coil modules (SCOI & SC02) were used in the inaugural test of the new Subsea Ie Magnet Test Faci lity. They were both wound with Nb3Sn cable - with the same strand as was used in LBLs 14.7 Tesla RD-3B large-scale magnet. The winding and assembly, prior to reacti on, took about 2-3 days. Index Terms- Common Coil Magnet, mixed-strand cable, Nb)Sn, racetrack coil. I. I NTRODUcrlON awrence Berkeley National Laboratory has implemented a Subscale Magnet Program. The new Subscale Magnet Test Facility was fabricated to supporJ the program. The facility includes a 38 lmm-lD x - 1524mm-deep vertical cryostat, wi th supporJ structure and magnet supporJ header. T he program tests the performance of various types of superconducting cables in a medium-to-high field environment as well as investigates magnet structural design modifications, L in small scale, before implementing them into our large-scale program. The magnet structural components were mass- Fig. I . 2 layers wound around iron pole. prod uced and standardized for rapid coil mod ule assembly. Three of the four coil modules initially built (two baseline coils, one mi x.ed strand coil and a comparison coil), have been tested. Presently, two more coil modul es with mi xed Nb3SnlCu strand coil variations have been reacted and are presently being assembled. Also, a coil mod ule with a new woven ceramic insulator is being reacted. Manuscript received August 6, 2002. Thi s was supported by the Director, Office of Energy Research, O ffi ce of High Energy and N uclear Physics, Hi gh Energy Physics D ivision , U. S. Department of Energy. under Contract No. The next coil module (SC03) was wound with a mixed- strand cable consisting of 2 1 strands - 14 Nb3Sn strands cabled with 7 strands of pure copper. Due to the difference in elastic modulus between the SC and the Cu strands, decabling was evident throughout the whole length of the cable. Coil winding tension was cut in half, from 178N to 89N, to minimize decabling. Even with the lower tension, popped strands were observed. DE-AC03--76SFO0098. R.R. Hafalia is with the Lawrence Berkeley National Lab. Berkeley, CA 94720 USA (telephone: 5 10-486-5712, (e-rnail: rrhafalia @Ibl.gov). Coi l module SC04 was wound with 20 Nb3Sn strands and was intended as a comparison to the SC03 mixed strand modul e.

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.

Test results for a high field (13 T) Nb/sub 3/Sn dipole

A Nb/sub 3/Sn dipole magnet (D20) has been designed, constructed, and tested at LBNL. Previously, we had reported test results from a hybrid design dipole which contained a similar inner Nb/sub 3/Sn and outer NbTi winding. This paper presents the final assembly characteristics and parameters which will be compared with those of the original magnet design. The actual winding size was determined and a secondary calibration of the assembly pre-load was done by pressure sensitive film. The actual azimuthal and radial D20 pre-loading was accomplished by a very controllable novel stretched wire technique. D20 reached 12.8 T (4.4 K) and 13.5 T (1.8 K) the highest dipole magnetic fields obtained to date in the world.

Core-suppressed AC loss and strand-moderated contact resistance in a Nb3Sn Rutherford cable

Calorimetric measurements of AC loss and hence interstrand contact resistance (ICR), have been performed on two types of Rutherford cable wound with unplated Nb3Sn strand. One of the cable types was furnished with a thin core of AISI 316L stainless steel and the other was left uncored. The cables were subjected to pressures of 5, 10, and 20 MPa, respectively, during reaction heat treatment (RHT), and to 100 MPa during measurement. AC loss was measured at 4.2 K in sinusoidal magnetic fields of amplitudes 200 and 400 mT at frequencies of 5 to 90 mHz both with and without the presence of DC bias fields of up to 1 T. The magnetic fields were applied both parallel and perpendicular to the face of the cable. For the cored cables ICR was relatively high (≈78 μΩ). Loss and contact resistance were relatively independent of RHT pressure within the range measured. The contact resistance of the uncored cables were also RHT-pressure independent, however, the absolute values of ICR were much lower, being about 2.7 μΩ with a 1 T background field, and very much smaller in the absence of a background field. The background-field dependence was shown to be due to the presence of an unreacted Nb barrier which enveloped the strand just below the external Cu stabilizer and thereby provided very low resistance coupling paths at small bias fields. The relative roles of strand and cable designs in the determination of interstrand losses are discussed. It is concluded that a low-loss Nb3Sn Rutherford cable has been achieved through the use of a metallic-alloy core.

High Field HTS R&D Solenoid for Muon Collider

This paper presents the goal and status of the high field High Temperature Superconductor (HTS) solenoid program funded through a series of SBIRs. The target of this R&D program is to build HTS coils that are capable of producing fields greater than 20 T when tested alone and approaching 40 T when tested in a background field magnet. The solenoid will be made with second generation (2G) high engineering current density HTS tape. To date, 17 HTS pancake coils have been built and tested in the temperature range from 20 K to 80 K. Quench protection, high stresses and minimization of degradation of conductor are some of the major challenges associated with this program.

Development of a high gradient quadrupole for the LHC interaction regions

A collaboration of Fermilab, Lawrence Berkeley National Laboratory and Brookhaven National Laboratory is engaged in the design of a high gradient quadrupole suitable for use in the LHC interaction regions. The cold iron design incorporates a two-layer, cos(2/spl theta/) coil geometry with a 70 mm aperture operating in superfluid helium. This paper summarizes the progress on a magnetic, mechanical and thermal design that meets the requirements of maximum gradient above 250 T/m, high field quality and provision for adequate cooling in a high radiation environment.

Design of HD2: a 15 tesla Nb/sub 3/Sn dipole with a 35 mm bore

The Nb/sub 3/Sn dipole HD1, recently fabricated and tested at LBNL, pushes the limits of accelerator magnet technology into the 16 T field range, and opens the way to a new generation of HEP colliders. HD1 is based on a flat racetrack coil configuration and has a 10 mm bore. These features are consistent with the HD1 goals: exploring the Nb/sub 3/Sn conductor performance limits at the maximum fields and under high stress. However, in order to further develop the block-coil geometry for future high-field accelerators, the bore size has to be increased to 30-50 mm. With respect to HD1, the main R&D challenges are: (a) design of the coil ends, to allow a magnetically efficient cross-section without obstructing the beam path; (b) design of the bore, to support the coil against the pre-load force; (c) correction of the geometric field errors. HD2 represents a first step in addressing these issues, with a central dipole field above 15 T, a 35 mm bore, and nominal field harmonics within a fraction of one unit. This paper describes the HD2 magnet design concept and its main features, as well as further steps required to develop a cost-effective block-coil design for future high-field, accelerator-quality dipoles.

Fabrication and test results for Rutherford-type cables made from BSCCO strands

Wires based on the Bi-2212 HTS superconductor are becoming available commercially, with current densities that are attractive for some applications. The authors report here on their success in using these Bi-2212 wires to fabricate multistrand, kiloamp conductors that can be used to construct dipole and quadrupole magnets for particle accelerator applications. Multistrand cables have been made from several types of Bi-2212 wire supplied by two manufacturers. These cables were made with cores of various compositions and dimensions in order to optimize the fabrication process. In addition, cables have been made from aspected strands as well as round strands. Cable critical currents are reported and compared for the various cable parameters investigated in this study.