Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Superconductive type
Reexamination Certificate
2002-05-17
2003-07-29
Barrera, Ramon M. (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Superconductive type
C505S211000, C505S705000, C505S924000, C505S879000, C029S599000
Reexamination Certificate
active
06600398
ABSTRACT:
This application claims Paris Convention priority of DE 101 25 429.6 filed May 25, 2001 the complete disclosure of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
The invention concerns a superconducting magnet coil for very high field with several, substantially solenoidal, multi-layer coil sections which are wound in layers onto a hollow cylindrical support body about a common central axis a, which are electrically connected in series and which can carry a current of more than 100 A during operation. The invention also concerns a method for producing such a magnet arrangement.
An arrangement of this type and the corresponding production method are known e.g. from U.S. Pat. No. 5,319,333.
High temperature superconductors (HTS) of oxidic ceramic material have been known since 1986. They are particularly characterized by very high transition temperatures of up to 120K as well as very high critical magnetic field upper limits (BC2).
Such materials have the substantial disadvantage of being very difficult to handle and require highly complicated processing steps and precise maintenance of very narrow boundary conditions in order to produce good superconducting properties.
In a processing step, thermal treatment is carried out in an oxidizing atmosphere at temperatures in the range of 800° C. To maintain the optimum superconducting properties, the oxygen content of the atmosphere must be controlled with high precision and must be continuously provided to the superconductor in the required concentrations in accordance with a desired processing procedure. To achieve optimum isotropy of the material properties, the oxygen concentration and temperature must be kept constant at all locations throughout the production process (with maximum tolerances of a few K).
These requirements can be achieved for relatively freely accessible conducting pieces and, for the generation of magnetic fields, with loosely wound “pancake” coils and, in certain cases, also for tightly wound pancake coils. The upper and lower side of the spiraled pancake coil winding are always freely accessible and permit oxygen to flow through same.
Coils appropriate for the construction of nuclear magnetic resonance (NMR) magnets having a magnetic field of high temporal stability and high homogeneity, cannot be constructed by stacking pancake coils since this would require a very large number of spacious superconducting joints which could not be disposed on a compact coil due to lack of space.
Moreover, the large number of superconducting joints increases the risk of failure of a joint which would produce drifting of the entire magnet.
A pancake arrangement can, at best, produce the required magnetic field homogeneity only with the greatest possible effort.
The preferred coil geometry for an NMR magnet is a solenoidal arrangement, wherein the winding packet should include as many layers as possible. This requirement for HTS conductors is opposed by the above mentioned reaction conditions which require that sufficient oxygen reaches the conductors during production of the winding.
To realize this, winding techniques (“wind-and-react”) are known from the field of metallic superconductors, such as e.g. Nb
3
Sn, which permit winding of e.g. helium transparent coils. With this technology, the superconductor is wound onto longitudinal rods disposed parallel to the axis a and the lateral separation between the conductors is ensured through numerous spacers. However, this production method requires neither thermal treatment nor oxygen supply. The process control is therefore substantially less critical than the above described production of HTS conductors. Pure thermal treatment is carried out which can extend over long intervals of up to a week and the windings can be denser.
The technology used for producing metallic superconductors is also inappropriate for producing HTS conducting pieces since the spacers and distancers must withstand the thermal treatment temperatures and the oxygen atmosphere without thereby reacting with the superconductor. This drastically limits the possible materials. For these reasons, a thermally treated solenoidal transparent coil of HTS material has not been designed or produced to date.
In view of the above, it is the underlying purpose of the present invention to provide an NMR-capable solenoid coil with a minimum number of superconducting joints and a topologically simple construction, which can carry a current of a few hundred amperes and which contains HTS material to permit production of magnetic fields of more than 21 Tesla, fields which cannot be achieved using metallic superconducting material alone.
SUMMARY OF THE INVENTION
In accordance with the invention, this object is achieved in a surprisingly simple and effective fashion in that the radially innermost coil section comprises superconducting wire which contains oxidic high temperature superconducting (HTS) material, wherein the layers of the radially innermost coil section are helically wound such that there is a free axial separation between the windings which is subsequently filled-up.
This permits production of the radially innermost coil section of a superconducting highest field magnetic coil of the above-described type from HTS material despite the various difficulties described above and in a technically straightforward fashion to thereby permit the entire coil to produce still higher magnetic fields than presently possible, i.e. of a magnitude in excess of 21 T, and with sufficiently high current carrying capability. The radially innermost coil section can, in particular, be designed as a compact solenoid coil with simple topology wherein only a minimum amount of superconducting joints to the radially outer coils are required for considerably reducing the danger of joint failure and to reduce the technical effort required for the superconducting joints. In this fashion, extremely homogeneous magnetic fields can be produced, e.g. for NMR applications, with field strengths which cannot be achieved using conventional metallic superconductors only, such as e.g. Nb
3
Sn.
The geometrically elegant helical winding with axial separations between the windings allows free access for the required oxygen and heat to the original HTS material from all sides during the production process without portions of the resulting superconductor sections being of differing quality. Due to the isotropy of the production conditions, the final product, the radially innermost coil section of a superconducting very high field magnet coil, has the largest possible isotropy of its properties.
For mechanical stabilization of the winding packet produced in this fashion, the hollow spaces, in particular the free axial and radial separations are filled with a moldable material. This is required since the extremely high magnetic fields would otherwise create motion of the conductors during current flow due to the Lorentz forces acting on the conductor sections which would produce considerable disturbances in the magnetic field to be produced.
The main applications of the inventive very high field magnet coils are in the field of NMR, in particular for magnet arrangements which are operated for a long time in superconducting persistent mode. Other applications are also feasible wherein magnetic fields of a particularly high field strength and high field homogeneity are to be produced e.g. for research.
One embodiment of the inventive magnet arrangement is particularly preferred wherein the pitches of the helical windings of radially subsequent layers alternate.
This maintains the full transparency of the oxygen and heat supply during the production process of the HTS coil section and in the radial direction as well.
To maintain the radial transparency during the production process while maintaining radial electrical insulation between the layers, a further embodiment of the invention provides that the layers of the radially innermost coil section are separated radially by a perforated, electrically insulating layer of high-temperature resis
Kasten Arne
Roth Gerhard
Barrera Ramon M.
Bruker Biospin GmbH
Vincent Paul
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