Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Magnet structure or material
Reexamination Certificate
2001-07-09
2003-01-14
Barrera, Ramon M. (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Magnet structure or material
C335S216000, C335S299000, C324S319000, C324S320000
Reexamination Certificate
active
06507259
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention concerns a magnet coil arrangement with an actively shielded superconducting primary circuit, consisting of first and second magnet windings and being wound e.g. in winding chambers in a supporting body, for generating a strong magnetic field in a working volume with a small magnetic stray field and a short-circuited secondary circuit which is coupled to the primary circuit and contains third and fourth magnet windings.
Actively shielded magnet coil systems comprise in general first and second magnet windings, which are symmetrical with respect to rotation, of superconducting wire for generating a strong magnetic field in a working volume or for reducing the strength of the magnetic stray field and a means for protecting the magnet windings from overheating, high electrical voltages and uncontrolled mechanical forces in case of a quench (transition of the magnet winding from the superconducting state into the normally conducting state).
Magnet coils of this type are known from the document F. J. Davies, R. T. Elliott, D. G. Hawksworth, IEEE Trans.Magn. 27/2, 1677-1680, 1991.
Magnet systems of this type are used e.g. for generating strong magnetic fields e.g. in magnetic resonance apparatus or Fourier transform mass spectrometers.
The first magnet windings which produce fields comprise a smaller diameter than the second magnet windings which minimize the stray field. The current direction in the second magnet windings is opposite to the current direction of the first magnet windings. The magnet windings are e.g. disposed on a coil body in winding chambers and contain a plurality of windings each.
The number N
2
of windings of the second magnet windings are smaller than the number N
1
of windings of the first magnet windings. The ratio between winding numbers N
2
/N
1
is selected approximately such that the magnetic dipole moment of the first and second magnet windings are in total considerably smaller than the magnetic dipole moment of the first magnet windings.
The windings are electrically connected in series and usually form a closed electric circuit of superconducting wire. During normal operation, the electric current through all windings is identical.
The superconducting wire has a negligible small electric resistance during normal operation and can transport high electric currents practically without any electric loss. The electric current density in the superconducting wire has a magnitude of several 100 A/mm
2
and is therefore a multiple of that which would be possible with normally conducting wires, e.g. of copper.
Superconducting wires are superconducting only at very low temperatures below a critical temperature. This critical temperature is approximately 8 K for the alloy NbTi which is often used for conventional superconducting wires. The magnet coils are therefore operated in a bath with liquid helium at a boiling temperature of 4.2 K.
The magnet windings are exposed to considerable inner forces during normal operation of a superconducting magnet, caused by Lorentz forces. Tensile stresses may occur in a region far above 100 MPa in the superconducting wires.
A further physical feature of superconducting magnet coils consists in that, at their typical operating temperature of 4.2 K , the heat capacity of all materials, i.e. also of the superconducting material, becomes negligible compared to the values at room temperature. This can cause that very small local mechanical relaxations within a magnet winding which can occur easily with the large mechanical tensions, lead to considerable local temperature increases on the order of several K. The magnet winding can therefore assume locally a temperature above the critical temperature and becomes normally conducting.
Upon normal conduction, the operating current of the magnet coil does not flow without losses any more at this location, but produces considerable heat whereupon this normally conducting region assumes yet higher temperatures and expands. This process, called “quench”, usually causes complete discharge of the superconducting magnet thereby converting the complete energy stored in the magnetic field in the normally conducting regions of the magnet windings into heat.
This energy is often in the range of several MJ. As a rule, the stored magnetic energy can correspond approximately to the mechanical potential energy independent of the magnitude of the magnet coil which one obtains by lifting the magnet coil by several 100 m against gravity.
If the normally conducting regions of the magnet windings remain limited to a small volume portion of the magnet windings during a quench, the entire magnetic energy is converted into heat only in this small volume portion wherein these regions can overheat thereby destroying the magnet coil.
A further problem consists in that during a quench within the magnet windings, larger electric voltages occur which can cause electric breakdown thereby also destroying the magnet coil.
Electric protective circuits are known for avoiding overheating and excessive electrical voltages. A plurality of such protective circuits is described in the document M. N. Wilson, Superconducting Magnets, Clarendon Press Oxford, 1983, Chap. 9. In a method described therein, the magnet windings are subdivided into partial windings and ohmic protective resistors are connected in parallel to all partial windings.
During normal operation, no current flows through the ohmic resistors when the superconducting magnet is charged. In case of a quench in a partial winding, the current in this partial winding is reduced and flows via the ohmic resistance which is connected in parallel. Inductive coupling increases at first simultaneously the current in the other still superconducting partial windings.
This type of coil protection has two main advantages: on the one hand, the “first” partial winding from which the quench starts experiences less thermal load through rapid reduction of the current and on the other hand, the quench caused by an increase of the current in other partial windings can cause a transition from the superconducting to the normally conducting state can be distributed to all partial windings. The rather uniform distribution of the entire magnet coil thereby avoids inadmissible local temperature increases.
The reduction of the current in the first partial winding and the current increases in the other partial windings are larger and more rapid, the smaller the resistance values of the protective resistors compared to the ohmic resistances of the regions of the partial windings which have become normally conducting. Often diodes are used instead of protective resistors which act like low-ohmic resistances with electric voltages above some volts.
A disadvantage of such protective resistors with resistances and diodes consists in that completely different currents flow through the partial windings during a quench. Such a protective circuit is therefore little suitable for an actively shielded superconducting magnet since the good stray field shielding is achieved only when identical currents flow through all partial windings. This condition is not met during a quench.
A magnet with protection of this kind produces a considerable magnetic stray field during a quench which can cause considerable damages to things and persons, such as e.g. deletion of information on magnetic data carriers close to the magnet or damages through magnetic objects which are catapulted towards the magnet. A further disadvantage consists in that in this type of coil protection, the current in the still superconducting partial windings exceeds partially the operating current thereby possibly mechanically stressing these partial windings too much.
The protective resistor described in the document F. J. Davies, R. T. Elliott, D. G. Hawksworth, IEEE Trans. Magn. 27/2, 1677-1680, 1991 is supposed to prevent these disadvantages during a quench. In this protective resistor, an ohmic resistance is also formally connected in parallel to a partial winding. It has a high ohmic value co
Neuberth Gerald
Westphal Michael
Barrera Ramon M.
Bruker BioSpin GmbH
Hackler Walter A.
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