N. Diaczenko

Stress management in high-field dipoles

The management of Lorentz stress and preload forces is the biggest single challenge in the effort to develop collider dipoles with ever greater field strength. Were the Lorentz forces permitted to accumulate through a coil, they would exceed the limit for strain degradation for the A15 and high-temperature superconductors which are capable of sustaining such field strength. A strategy has been devised for intercepting Lorentz stress within the coil to overcome this problem in high-field block-coil dipoles. The coil is fabricated in multiple independent shells, in which a high-strength structure and a soft-modulus spring are used to bypass stress between succeeding layers. Finite-element analysis and experimental studies have demonstrated that this strategy can limit the maximum stress anywhere in a coil so that it nowhere exceeds strain degradation limits for fields at least to 20 Tesla.

Block-coil dipole for future hadron colliders

A first model dipole is being built for a block-coil dipole for future hadron colliders. The design incorporates stress management, in which Lorentz stress is intercepted between successive sections of the coil and bypassed through a support matrix. By controlling stress, the dipole should make it possible to utilize Nb/sub 3/Sn and BSCCO superconductors without strain degradation at high field. The first model dipole is being built using NbTi cable in order to evaluate fabrication techniques and stress management performance.

Optimization of block-coil dipoles for hadron colliders

A first model dipole is being built for a 16 Tesla block-coil dipole for future hadron colliders. The design uses stress management: a support matrix that intercepts Lorentz stress between successive sections of the coil and bypasses it to prevent strain degradation of the superconductors and insulation. The block-coil methodology has also been used to design dipoles for 12 Tesla and 15 Tesla, in which the amount of superconductor is minimized by cabling copper stabilizer strands with superconductor strands. The 12 Tesla block-coil dipole requires only one-fifth as much superconductor as does a 12 Tesla cos /spl theta/ dipole that is being developed elsewhere.

Optimized block-coil dipoles for future hadron colliders

We are developing an improved technology for high-field dipoles, aimed at making a robust, affordable Nb/sub 3/Sn dipole for future hadron colliders and other accelerator applications. The technology incorporates five elements that depart from conventional dipole design. The coil is arranged in rectangular blocks, rather than the usual cos /spl theta/ geometry. The coil contains a structural support matrix that provides stress management. The superconducting cables in the coil contain an admixture of superconducting and pure copper strands, with the ratio chosen in each coil region to optimize the use of superconductor. Multipoles are controlled over a large dynamic range by current programming a trim winding. Finally, persistent-current multipoles are suppressed at low field by a close-coupled planar steel boundary. We show that these five design elements enable the design of conductor-optimized dipoles up to at least 16 Tesla. We describe a particular design for a 12 Tesla dipole that could triple the energy of the Fermilab Tevatron and support a new generation of hadron collider physics at the existing facility. Progress is reported on the construction and testing of model dipoles.

Strain-tolerant cable using Bi-2212 superconductor

The high strain sensitivity of Bi-2212 is a major obstacle for its use as a practical conductor in high-field magnet applications. Most efforts to improve the mechanical behavior of Bi-2212 focus on strengthening the silver matrix by means of alloying. We are reporting the design and preliminary testing of a strain-tolerant Bi-2212 cable that takes a different approach: Instead of reinforcing the conductor, it channels mechanical stress and strain away from it by prudent design of a Cable-in-Conduit (CIC) assembly. Six strands of Bi-2212 are cabled around a thin-walled Inconel X 750 tube and then sheathed in an outer armor that is drawn onto the 6-on-1 cable configuration.

Stress management of HTS conductor

A Bi-2212 cable-in-conduit featuring incorporated stress management is being developed. By using a compliant thin wall tube made of Inconel X 750 in the center of the CIC the strands of the cable are protected from the accumulated Lorentz stress. This significantly reduces the mechanical loading on the superconductor while it is still being held rigidly in place. A finite element analysis of a cable matrix is presented, and mechanical tests of the Inconel X 750 tubes are shown.

Test Results of a

A second phase of a high field dipole technology development has been tested. A Nb3Sn block-coil model dipole was fabricated, using magnetic mirror geometry and wind/react coil technology. The primary objective of this phase was to make a first experimental test of the stress-management strategy pioneered at Texas A&M. In this strategy a high-strength support matrix is integrated with the windings to intercept Lorentz stress from the inner winding so that it does not accumulate in the outer winding. The magnet attained a field that was consistent with short sample limit on the first quench; there was no training. The decoupling of Lorentz stress between inner and outer windings was validated. In ramp rate studies the magnet exhibited a remarkable robustness in rapid ramping operation. It reached 85% of short sample(ss) current even while ramping 2-3 T/s. This robustness is attributed to the orientation of the Rutherford cables parallel to the field in the windings, instead of the transverse orientation that characterizes common dipole designs. Test results are presented and the next development phase plans are discussed.

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

The second phase of development of a new high-field dipole technology has been completed. A model dipole employing wind/react Nb 3Sn cable and stress-managed block coil geometry was fabricated and will soon be tested at LBNL. The dipole features stress-strain management in its internal windings and metal-filled bladder preload. Pending validation of performance of these new features, the new technology should result in improved cost-effective fabrication of dipoles for 16 T and beyond. Construction experience and plans for the next phase of development are presented

Wind/React “Stress-Managed” Block Dipole

The high strain sensitivity of Bi-2212 is a major obstacle for its use as a practical conductor in high-field magnet applications. Most efforts to improve the mechanical behavior of Bi-2212 focus on strengthening the silver matrix by means of alloying. We are reporting the design and preliminary testing of a strain-tolerant Bi-2212 cable that takes a different approach: Instead of reinforcing the conductor, it channels mechanical stress and strain away from it by prudent design of a Cable-in-Conduit (CIC) assembly. Six strands of Bi-2212 are cabled around a thin-walled Inconel X 750 tube and then sheathed in an outer armor that is drawn onto the 6-on-1 cable configuration.

Construction of a Mirror-Configuration Stress-Managed

The current status of the Texas A&M superconducting magnet R&D program is reported. The program is implementing a design philosophy in which Lorentz stress is managed within the coils of a block-coil geometry, isostatic preload is delivered using an arrangement of pressurized Woods metal filled bladders, insulation utilizes fine-filament Silane-sized S-glass, and low-field multipoles are constrained by a flux plate integrated with the coil package. Construction progress on TAMU3 is reported and plans for a full-aperture dipole TAMU5 are discussed.