Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Superconductive type
Patent
1992-03-09
1994-06-07
Picard, Leo P.
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Superconductive type
335299, H01F 500, H01F 100
Patent
active
053193334
DESCRIPTION:
BRIEF SUMMARY
The invention concerns a superconducting homogeneous high-field magnetic coil which is utilized for the production of a static magnetic field in combination with a nuclear magnetic resonance spectrometer.
The invention is based upon a multi-layer coil which is known in the art from the Journal of Physics E. Scientific Instruments, 1972, Pages 944 through 946. In the coil which is known in the art the current density in the middle lengthwise region is reduced through the addition of a passive wire winding in order to produce a homogeneous magnetic field.
Known in the art from patent abstracts of Japan Vol. 10 No. 233 (J-P-A. 6 165 4 11) is the reduction of the average current density in the end regions through the addition of a passively wound wires. This publication discloses, however, a one layer coil and the purpose lies in the limitation of the local increase of the magnetic fields in the end portion of the coil in order to prevent the critical field strength of the superconductor from being exceeded. There is no suggestion that this configuration should be utilized in order to reduce the pressure within the coil produced in consequence of the Lorentz forces. The problem of the field increase with extremely thin one-layer solenoid coils is treated, a problem which in principle does not occur in multi-layered coils. The accumulated pressure is, in this case, no problem. On the other hand, in multi-layer high field magnet coils which are the object of the invention, the field strength in and of itself is non-critical.
In order to produce magnetic fields of high magnetic field strength (many Tesla), among others, superconducting magnet coils are utilized which are constructed from winding layers of a superconductable wire and exhibit approximately rotationally symmetric hollow cylinder shaped winding cross sections. In order to realize the superconducting capability of the wire material, by way of example Nb.sub.3 Sn with copper stabilization, at the necessary current strengths and magnetic field obtaining in the vicinity of the wire, the magnetic coils operate within cryostats which, in the example mentioned, can be cooled by means of liquid helium to a temperature of 4.2 K. In addition to the superconducting type A15 which is suitable for the highest fields and to which Nb.sub.3 Sn belongs, most recently, other superconducting materials have become known in the art with which a utilization in a magnet construction well above 4.2 K, possibly at the temperature of liquid nitrogen, appear to be possible in the near future.
Due to the radial component of the magnetic fields produced by means of the magnet coil, axial pressures due to Lorentz forces occur in the windings. The axial Lorentz forces increase with increasing radial component of the magnetic field, that is to say, they are largest in the region of the two coil ends. The resulting axial pressures, thereby accumulate towards the coils' center, so that the maximum axial pressure occurs in the coil center. These pressures, with high field magnets, obtain a value on the order of 50 Megapascals at the center of the coil (P. Turowski et al., 9th International Conference on Magnet Technology, p. 394, 1985) and can, when charging up the magnet, in particular in places in which the winding is not homogeneous, cause movement of the conductors which, in consequence of the heat generated by said movement, can cause a transition of the wire into the normally conducting state. Since the normally conducting region, despite low resistance copper stabilization, in consequence of the electrical resistance, continues to heat up the entire magnet coil quickly becomes normally conducting.
To produce the highest magnetic fields, the high field magnet coils known in the art are also operated at 1.8 K in order to achieve a higher current carrying capability of the conductors (see by way of example P. Turowski, Th. Schneider in 2nd High Field Conference Leuven 1988).
Superconducting high field magnetic coils which are utilized for the production of very homogeneous magneti
REFERENCES:
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patent: 3177408 (1965-04-01), Mills et al.
patent: 3559130 (1971-01-01), Schraeder
patent: 4682134 (1987-07-01), Laskaris
patent: 4694269 (1987-09-01), Burnett et al.
Journal of Physics E, Scientific Instruments, 1972, pp. 944-946, Cesnak et al.: A cylindrical coil with graduated current density for very homogeneous magnetic fields.
IEEE Transactions on Magnetics, vol. 23, No. 2, Mar. 1987, New York US, pp. 565-568 M. Takeo et al.: A 17 tesla superconducting magnet with multifilamentary superconductors.
IEEE Transactions on Magnetics, vol. 24, No. 2, Mar. 1988, New York US, pp. 1409-1412, P. A. Jonas et al.: 12 tesla superconducting multifilamentary magnet.
Roth Gerhard
Tschopp Werner
Barrera Raymond
Bruker Analytische Messtechnik GmbH
Hackler Walter A.
Picard Leo P.
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