Effects of Core Type, Placement, and Width, on the Estimated Interstrand Coupling Properties of QXF-Type Nb3Sn Rutherford Cables

The coupling magnetization of a Rutherford cable is inversely proportional to an effective interstrand contact resistance, Reff, a function of the crossing-strand resistance, Rc, and the adjacent strand resistance, Ra. In cored cables Reff varies continuously with W, the core width expressed as percent interstrand cover. For a series of un-heat-treated stabrite-coated NbTi LHC-inner cables with stainless-steel (SS, insulating) cores Reff(W) decreased smoothly as W decreased from 100% while for a set of research-wound SS-cored Nb3Sn cables Reff plummeted abruptly and remained low over most of the range. The difference is due to the controlling influence of Rc – 2.5 μΩ for the stabrite/NbTi and 0.26 μΩ for the Nb3Sn. The experimental behavior was replicated in the Reff(W)s calculated by the program CUDI© which (using the basic parameters of the QXF cable) went on to show in terms of decreasing W that: (i) in QXF-type Nb3Sn cables (Rc = 0.26 μΩ) Reff dropped even more suddenly when the SS core, instead of being centered, was offset to one edge of the cable, (ii) Reff decreased more gradually in cables with higher Rcs, (iii) a suitable Reff for a Nb3Sn cable can be achieved by inserting a suitably resistive core rather than an insulating (SS) one.Funding was provided by the U.S. Dept. of Energy, Office of High Energy Physics, under Grants No. DE-SC0010312 & DE-SC0011721 (OSU) and DEAC02- 05CH11231 (LBNL).The coupling magnetization of a Rutherford cable is inversely proportional to an effective interstrand contact resistance, Reff, defined as Reff = [1/Rc + 20/N3Ra]-1. In uncored cables Reff is primarily controlled by Rc. The LHC magnet’s uncored NbTi cables, wound with specially heat treated stabrite-coated strands, evidently have acceptable Rcs. It has been reported that the current ramping of LHC magnets produces field errors: (i) in dipoles of about 1 unit of b1 and less than 0.1 units of cn, consistent with Rc well above 50 μΩ, (ii) in quadrupoles of about 2 units of b1 and less than 0.2 units of cn, consistent with Rc between 100 and 150 μΩ. Evidently such Rcs have contributed to the successful operation of the LHC dipoles and quadrupoles to date and hence could be thought of as new target values when designing the Nb3Sn cables for the LHC upgrades. But with measured Rcs of typically 0.3 μΩ bare Nb3Sn cables are unsuitable; the cables need to be furnished with some kind of core to separate the crossing strands. In cables with insulating cores Reff (now a function of both Rc and Ra) increases continuously with W (% core cover), with Ra eventually taking over as the controlling ICR. In seeking an optimal core width a large assortment of research cables were wound and measured over the years. The results, assembled and compared here for the first time, show Reff(W) reaching acceptable values only when W approached ~90% beyond which it increased very steeply. These experimental values were compared to modelling results using the program CUDI© choosing as our model cable a variablewidth- core version of QXF. Further application of the program demonstrated that core positioning was important, Reff decreasing by about 2½ times as the cores shifted from the center to one edge of the cable. As a result it is predicted that irregularities in core placement could produce a large scatter in Reff. The sensitivity of Reff to core width and position in the optimal large-W range leads to the suggested inclusion of a core, not of SS (which has a stable, insulating oxide surface layer), but of a resistive composite such as Cr-plated SS or Crplated Cu

Transport and Magnetization Properties of rolled RRP Nb3Sn Strands.

Restack Rod Process (RRP) strands with 54 and 108 sub-elements were rolled from 0.7 mm diameter to 0.45 mm thickness to simulate the deformation of strands at the edges of Rutherford cables. Various diagnoses were then applied to assess performance and stability. Transport measurements were used to assess the effect of rolling on the critical current. Magnetization measurements were used to probe superconducting pathway bridging between deformed sub-elements. The copper residual resistivity ratio RRR was also measured to assess tin contamination due to thinned or ruptured diffusion barriers. While systematic changes were observed in all three measurements with increasing deformation, RRR showed the strongest changes. The implications of these measurements for cable stability, and their relationship to observations of the strand cross-section by light microscopy, are discussed

Mechanical design of a high field common coil magnet

A common coil design for high field 2-in-1 accelerator magnets has been previously presented as a "conductor-friendly" option for high field magnets applicable for a Very Large Hadron Collider. This paper presents the mechanical design for a 14 tesla 2-in-1 dipole based on the common coil design approach. The magnet will use a high current density Nb/sub 3/Sn conductor. The design addresses mechanical issues particular to the common coil geometry: horizontal support against coil edges, vertical preload on coil faces, end loading and support, and coil stresses and strains. The magnet is the second in a series of racetrack coil magnets that will provide experimental verification of the common coil design approach. (9 refs)

The upper critical field of filamentary Nb3Sn conductors

We have examined the upper critical field of a large and representative set of present multi-filamentary Nb3Sn wires and one bulk sample over a temperature range from 1.4 K up to the zero field critical temperature. Since all present wires use a solid-state diffusion reaction to form the A15 layers, inhomogeneities with respect to Sn content are inevitable, in contrast to some previously studied homogeneous samples. Our study emphasizes the effects that these inevitable inhomogeneities have on the field-temperature phase boundary. The property inhomogeneities are extracted from field-dependent resistive transitions which we find broaden with increasing inhomogeneity. The upper 90-99 % of the transitions clearly separates alloyed and binary wires but a pure, Cu-free binary bulk sample also exhibits a zero temperature critical field that is comparable to the ternary wires. The highest mu0Hc2 detected in the ternary wires are remarkably constant: The highest zero temperature upper critical fields and zero field critical temperatures fall within 29.5 +/- 0.3 T and 17.8 +/- 0.3 K respectively, independent of the wire layout. The complete field-temperature phase boundary can be described very well with the relatively simple Maki-DeGennes model using a two parameter fit, independent of composition, strain state, sample layout or applied critical state criterion.Comment: Accepted Journal of Applied Physics Few changes to shorten document, replaced eq. 7-

Fabrication of a Short-Period Nb3Sn Superconducting Undulator

Lawrence Berkeley National Laboratory develops high-field Nb{sub 3}Sn 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 Nb{sub 3}Sn permits operation of a device at both a marginally higher temperature (5-8K) and a higher J{sub c}, 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 717A in 4 quenches. The non-Cu Jc of the strand was over 7,600 A/mm{sup 2} and the Cu current density at quench was over 8,000 A/mm{sup 2}. 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

Critical Current of Superconducting Rutherford Cable in High Magnetic Fields with Transverse Pressure

For high energy physics applications superconducting cables are subjected to large stresses and high magnetic fields during service. It is essential to know how these cables perform in these operating conditions. A loading fixture capable of applying loads of up to 700 kN has been developed by NHMFL for LBNL. This fixture permits uniform loading of straight cables over a 122 mm length in a split-pair solenoid in fields up to 12 T at 4.2 K. The first results from this system for Rutherford cables of internal-tin and modified jelly roll strand of Nb{sub 3}Sn produced by IGC and TWC showed that little permanent degradation occurs up to 210 MPa. However, the cable made from internal-tin strand showed a 40% reduction in K{sub c} at 11T and 210 MPa while a dable made from modified jelly roll material showed only a 15% reduction in I{sub c} at 11T and 185 MPa

Simulations of the effects of tin composition gradients on the superconducting properties of Nb3Sn conductors

In powder-in-tube (PIT) Nb3Sn composites, the A15 phase forms between a central tin-rich core and a coaxial Nb tube, thus causing the tin content and superconducting properties to vary with radius across the A15 layer. Since this geometry is also ideal for magnetic characterization of the superconducting properties with the field parallel to the tube axis, a system of concentric shells with varying tin content was used to simulate the superconducting properties, the overall severity of the Sn composition gradient being defined by an index N. Using well-known scaling relationships and property trends developed in an earlier experimental study, the critical current density for each shell was calculated, and from this the magnetic moment of each shell was found. By summing these moments, experimentally measured properties such as pinning-force curves and Kramer plots could be simulated. We found that different tin profiles have only a minor effect on the shape of Kramer plots, but a pronounced effect on the irreversibility fields defined by the extrapolation of Kramer plots. In fact, these extrapolated values H_K are very close to a weighted average of the superconducting properties across the layer for all N. The difference between H_K and the upper critical field commonly seen in experiments is a direct consequence of the different ways measurements probe the simulated Sn gradients. Sn gradients were found to be significantly deleterious to the critical current density Jc, since reductions to both the elementary pinning force and the flux pinning scaling field H_K compound the reduction in Jc. The simulations show that significant gains in Jc of Nb3Sn strands might be realized by circumventing strong compositional gradients of tin.Comment: 10 pages, 8 figures, 2 tables, submitted to J. Appl. Phy

Canted-cosine-theta magnet (CCT)-A concept for high field accelerator magnets

Canted-Cosine-Theta (CCT) magnet is an accelerator magnet that superposes fields of nested and tilted solenoids that are oppositely canted. The current distribution of any canted layer generates a pure harmonic field as well as a solenoid field that can be cancelled with a similar but oppositely canted layer. The concept places windings within mandrel's ribs and spars that simultaneously intercept and guide Lorentz forces of each turn to prevent stress accumulation. With respect to other designs, the need for pre-stress in this concept is reduced by an order of magnitude making it highly compatible with the use of strain sensitive superconductors such as Nb3Sn or HTS. Intercepting large Lorentz forces is of particular interest in magnets with large bores and high field accelerator magnets like the one foreseen in the future high energy upgrade of the LHC. This paper describes the CCT concept and reports on the construction of CCT1 a "proof of principle" dipole

Insertion Magnets

Chapter 3 in High-Luminosity Large Hadron Collider (HL-LHC) : Preliminary Design Report. The Large Hadron Collider (LHC) is one of the largest scientific instruments ever built. Since opening up a new energy frontier for exploration in 2010, it has gathered a global user community of about 7,000 scientists working in fundamental particle physics and the physics of hadronic matter at extreme temperature and density. To sustain and extend its discovery potential, the LHC will need a major upgrade in the 2020s. This will increase its luminosity (rate of collisions) by a factor of five beyond the original design value and the integrated luminosity (total collisions created) by a factor ten. The LHC is already a highly complex and exquisitely optimised machine so this upgrade must be carefully conceived and will require about ten years to implement. The new configuration, known as High Luminosity LHC (HL-LHC), will rely on a number of key innovations that push accelerator technology beyond its present limits. Among these are cutting-edge 11-12 tesla superconducting magnets, compact superconducting cavities for beam rotation with ultra-precise phase control, new technology and physical processes for beam collimation and 300 metre-long high-power superconducting links with negligible energy dissipation. The present document describes the technologies and components that will be used to realise the project and is intended to serve as the basis for the detailed engineering design of HL-LHC.Comment: 19 pages, Chapter 3 in High-Luminosity Large Hadron Collider (HL-LHC) : Preliminary Design Repor