Electricity: conductors and insulators – Conduits – cables or conductors – Superconductors
Reexamination Certificate
2002-01-24
2003-03-18
Paladini, Albert W. (Department: 2827)
Electricity: conductors and insulators
Conduits, cables or conductors
Superconductors
C029S599000, C505S230000, C505S430000
Reexamination Certificate
active
06534718
ABSTRACT:
BACKGROUND TO THE INVENTION
1. Field of the Invention
The present invention resides in the field of superconducting coils and more particularly relates to the reinforcement of adiabatic superconducting coil magnets using composite superconducting wires that are clad with high-strength materials.
2. History of the Prior Art
Superconductivity is the phenomenon where, when a particular material, such as a wire, is subjected to successively colder temperatures, it undergoes a state transition where all electrical resistance disappears, i.e. the material can conduct electricity without generating any heat. At this stage the material is said to have become a superconductor. This phenomenon only occurs for specific metals/alloys and compounds. The state of superconductivity for a given superconductor is a function of its temperature, background magnetic field, and transport electric current.
Superconducting magnets almost exclusively consist of windings of superconducting wire, also known as coils. In very large magnets the “wire” may take the form of cabled superconducting wires. For some superconducting magnets the cable of wires is contained, together with helium for cooling, in a metal conduit. All wires used in building such magnets include insulation that surrounds the wire to be wound.
Turns and layers of the winding of a magnet are often bonded to each other by a bonding agent, such as epoxy. Such windings are said to be impregnated.
In high field magnets used for nuclear magnetic resonance (NMR) and for magnetic resonance imaging (MRI) the windings are usually solenoids. Often NMR and MRI magnets are comprised of a number of solenoid coils that are electrically connected. The magnetic fields produced by the system of coils combine to produce an intended magnetic field intensity over an intended volume inside the system of coils of the magnet. The field intensity of a solenoid, as well as most other magnet configurations, scales with the number of turns of the wire that makes up the windings of the coil and the electric current (amperes) that passes through the wire. Often it is said that the field intensity scales with “ampere-turns.” For reasons of: 1) minimizing space, 2) using less superconducting wire, and more importantly 3) reducing the stored energy of the coil, most coils used in NMR and MRI magnets are designed and built such that a given coil is as compact as possible.
When current passes through the wire of a coil, a magnetic field is produced. When current passes through a system of coils, a combined magnetic field is produced. The magnetic field (B) which is produced by a coil or a system of coils around a given turn inside the winding volume (pack) of a coil interacts with the current in the wire and produces a force on the wire, referred to as the Lorentz Force. Because the orientation of the magnetic field varies according to the location of the turn within the coil, or system of coils, the direction of the force varies also. The Lorentz Forces are balanced by the wire and other materials that make up the coil, resulting in mechanical stresses in the wire. In a given solenoid coil the Lorentz Forces on the turns combine to produce two components of stress in the wire. A tensile hoop stress is developed in the wire of magnitude B
x
I
x
R where B is the local field strength, I is the current and R is the radius of curvature. A transverse stress is also incident because radial components of the field in regions of a winding produce an axial force. Of these components of stress the hoop stress is generally the more significant, but both must be considered in the structural design of a high field magnet. When a solenoid coil is charged by current passing through it, there is a balance between its electromagnetic energy and mechanical elastic energy. Therefore a charged coil, or a system of coils, stores energy.
Design parameters and conventional approaches to solenoid design are discussed, for example, in:
Superconducting Magnets,
M. N. Wilson, Oxford University Press, New York, N.Y. (1983) and
Case Studies in Superconducting Magnets
, Y. Iwasa, Plenum Press, New York, N.Y. (1994).
The stresses on the wires inside MRI and NMR coils may reach hundreds of MPa. Therefore a main part of the design optimization of coils is calculation of stresses and bearing of the stresses by the coil.
To achieve the highest possible fields inside the bore of the NMR and MRI magnets, and most effectively support against Lorentz Forces, the wires used in these magnets are usually a solid monolithic wire, and their winding packs are devoid of any space for helium which is sometimes used as a heat sink inside the winding pack of some superconducting magnets. The coils of NMR and MRI magnets are comprised of solid matter. These are often referred to as being adiabatic coils as any release of local energy within the winding pack is absorbed by the winding pack itself first, and then it is transferred to the cooling medium that is external to the winding pack.
For reasons that relate to stability of superconductors, superconducting wires used in most magnet application are multifilamentary (MF) composites. [see for example
Stability of Superconductor
, by Lawrence Dresner, Plenum Press, New York, N.Y., 1995]. The superconducting filaments in MF wires in use in most superconducting magnets today are made from niobium-titanium (Nb—Ti) alloy. The Tc and Bc
2
for Nb—Ti are about 10 Kelvin (K) and about 10 (T), respectively. Nb—Ti alloy is ductile and basically insensitive to strain; and its use in fabricating MF wires, and subsequently in a magnet, is straightforward and comparatively less expensive than using other materials. MF Nb—Ti composite wires are often comprised of Nb—Ti filaments inside a copper or copper alloy matrix.
Superconducting magnets for operation at fields higher than about 10 T rely principally on the use of type A15 superconductors. Among the A15 superconductors the Nb
3
Sn based wires are most practical for large-scale production, basically due to the fact that fabrication of Nb
3
Sn is economical and less complicated. Almost all operating A15 magnets to date have used Nb
3
Sn conductors. The Tc and Bc
2
for Nb
3
Sn are about 18 Kelvin (K) and about 23 (T), respectively. Other A15 superconductors, such as Nb
3
Al, that have better superconducting properties than Nb
3
Sn are under development.
Nb
3
Sn, like other A15 phases, is an intermetallic compound and is inherently brittle. Therefore it does not lend itself to normal conductor fabrication methods where a given material undergoes significant plastic deformation. For most applications in magnet technology, Nb
3
Sn superconductors are produced by a two-step process in which a multifilamentary composite wire, that contains Nb and Sn in separate regions, is formed into wire (Nb
3
Sn precursor wire) and then, during a subsequent reaction heat treatment at, for example 650 C.-750 C., the Nb
3
Sn is formed by solid state reaction. MF Nb
3
Sn composite wires are often comprised of Nb
3
Sn filaments inside a copper and bronze alloy matrix. Because Nb
3
Sn is brittle, windings of Nb
3
Sn wire are often produced by winding un-reacted wires and then heat-treating the winding as a whole. This approach is referred to as the wind-then-react method. If the winding diameter is large enough the winding can be performed using a reacted Nb
3
Sn wire. This approach is referred to as react-then-wind method. Because MF Nb
3
Sn wires have to be heat-treated at temperatures of 650 C.-750 C., the non-superconducting matrix in the wires which is most often copper or bronze is in a fully annealed state and is therefore mechanically weak and cannot contribute much to the bearing of Lorentz Forces. Therefore it is particularly desirable to reinforce Nb
3
Sn wires and coils for NMR and MRI magnet applications.
A main challenge of design and manufacturing coils for NMR and MRI magnets is to minimize the size of the coils by using conductors that have enough superconducting material to provide the required ampere-turns and optimizing
Pourrahimi Nadder
Pourrahimi Shahin
Nitkin William
Norris Jeremy
Paladini Albert W.
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