GB2312328A - Superconductive magnet with a flux pump energised by electromagnetic radiation - Google Patents

Description

2312328 Charging of a Superconducting Magnet Coil The invention concerns a

device for the charging of a persistent superconducting magnet coil located inside a cryostat, by means of a flux pump.

Such devices are, for example, known from US patent 4,688,132.

In prior art cryostats, in particular those of NIVIR spectrometers, the magnet coil, which is located inside a helium tank under liquid helium, is charged and discharged by means of current leads inserted from outside. Additional electric leads, e.g. to a persistent current switch at the magnet coil are guided inside the helium tank together with the current leads The superconducting persistent current switch can be heated and thereby made resistive via these electric leads to which it is attached. In this way it is ensured that the charging current flows through the coil and not through the switch. When the desired current is reached, the switch heating is switched off, thereby the switch becomes superconducting is and the current through the current leads can be reduced to zero, so that finally in the persistent mode the entire magnet current flows through the superconducting switch.

Following final adjustments, for example to homogenise the magnetic field via leads to so called superconducting shim coils attached to the main magnet coil, the current leads together with all further electric leads to the magnet coil inside the helium tank can be extracted. Apart from the suspension, which is optimised for extremely low heat inputs into the helium tank, there is then no further connection between the magnet coil and surroundings. This has the advantage of permitting extremely low refrigerant losses and small perturbations of the thermal equilibrium state, which has a positive effect on the stability and homogeneity of the magnetic field during persistent operation.

Insertion of the current leads into the liquid helium, charging of the coil with some hundred amperes by means of an external power supply and the subsequent removal of the leads can only be performed by specially trained personnel. In particular, if an unintended quench occurs during extraction of the leads, leading to the sudden evaporation of helium, the operator is in danger because of their close proximity to the cryostat. In addition, insertion of the rigid leads may be hampered by the cryostat turrets. It may well be that for this reason alone, laboratory rooms which are otherwise suitable cannot be used.

For this reason, the above-mentioned US patent 4,688,132 suggests inter alia charging the superconducting magnet by means of a so-called flux pump. Such a flux pump requires only relatively thin leads from room temperature into the cryogenic part of the cryostat, and the leads remain inside the cryostat also after the charging procedure. However, during continuous operation they represent a permanent heat input into the cryogenic region. The general set up and function of the flux pump are described in DE-A-340531 0 as well as in the references cited there, i.e. Cryogenics 21 (1981) 195-206 and Cryogenics 21 (1981)

267-277. In particular a variety of possible embodiments and their optimisation is disclosed in these latter articles, to which the reader is specifically referred.

The use of a flux pump instead of current leads reduces the heat input into the cryostat during the charging procedure. However, flux pump still requires its own current leads and current leads for heating its superconducting switches.

is The present invention seeks to further reduce the heat input into the cryogenic part of a cryostat containing a superconducting magnet coil, in particular of the cryostat of an NIVIR spectrometer with superconducting persistent operation of the magnet coil and to avoid the complicated and time-consuming insertion of current leads during the charging procedure.

The invention provides a superconducting magnet assembly comprising a cryostat, having a cryogenic region, a superconducting magnet coil in the cryogenic region of the cryostat, and a flux pump in the cryogenic region of the cryostat, the flux pump being connected to the superconducting magnet coil for charging the magnet coil, wherein the flux pump comprises at least one secondary coil and at least one periodically switchable superconducting switch, wherein the device comprises at least one periodically switchable transmitter located outside the cryogenic region, for transmitting electromagnetic energy, at least one first receiver in the cryogenic region of the cryostat, for receiving the periodically emitted electromagnetic energy, converting the received energy into a periodic current, and transferring the periodic current to the flux pump, means for transferring pulsed energy to the superconducting switch to heat the switch and thereby periodically render it non-superconductive, such that the flux pump successively increases the current through the superconducting magnet coil.

In accordance with the invention, there is provided outside the cryogenic region of the cryostat a switchable transmitter of pulsed electromagnetic energy. At least one first receiver for the emitted pulsed energy is provided in the region of the flux pump and its superconducting switches at the cryogenic temperature level, converting the received energy to a pulsed current and transferring this current to the flux pump. At least a second receiver for pulsed electromagnetic energy is preferably also provided, arranged so as to transfer received energy directly or indirectly to at least one superconducting switch, so that it becomes temporarily non-superconducting, for example using a heater for the switch, in such a way that the flux pump successively increases the current through the superconducting magnet coil.

The flux pump and the superconducting switch can be activated from outside without time consuming preparations, and due to the transmission without physical contact, permanent is heat input into the cryogenic region of the magnet coil cryostat is avoided outside the charging periods.

In an embodiment of the invention, the switchable transmitter is an optical or infrared light source, in particular a laser, or the end of a light guide for guiding light from such a light source, respectively, and the first receiver is at least one photoelectric cell to convert the received radiation energy into electric current, which is transferred to the primary winding(s) of the flux pump in the form of current pulses.

Light from the same or additional light sources can also be used to switch the superconducting switches periodically. The heaters of the superconducting switches may thereby be actuated without physical contact by periodically guiding the electromagnetic radiation onto the heaters. This can be effected in a way analogous to the methods described in GB-A-2295492,, which is hereby fully incorporated by reference.

This has the advantage that the energy can be guided to the cryogenic surroundings of the flux pump and to the superconducting switch by optical means known per se.

In an embodiment of the invention, the heaters of the superconducting switches comprise a surface which absorb in the frequency range of the radiation emitted by the transmitter, and onto which the electromagnetic radiation is directed or focused, respectively. The heaters are connected to the corresponding superconducting switch in a thermally conductive way.

This has the advantage that a large proportion of the radiation energy is directly used to heat the switch. In a preferred embodiment, the heater surface facing the transmitter comprises a recess, into which the radiation is focused and which acts as a radiation trap. 10 In an alternative embodiment, the heater is a photoelectric cell electrically connected to a heater resistor to heat the superconducting switch. This has the advantage that, as in the prior art, heating of the switch is effected electrically, is so that no new design of the switch configuration is necessary. In an improvement of the above-mentioned embodiments, the receiver itself andlor both heaters or light guides connected to them protrude(s) above the magnet coil to such an extent that the optical path which has to be bridged starting from the transmitter only runs 20 in gas or inside a light guide, respectively, but not in a cryogenic liquid, (usually helium), even for the highest filling level of the cryogenic liquid surrounding the coil. This has the advantage that less scattering losses and absorption occur.

In an alternative embodiment, the transmitter and receiver are electric coils, which are inductively coupled, but without physical contact. The transmitter coil is electrically connected to an ac generator via a switch and the receiver coil is connected to a primary winding of the flux pump. 30 The transmitter coil may be replaced by a mechanically movable permanent magnet, whose movement effects a change of the magnetic flux through the receiver coil. The mechanical movement may be a translation or a rotation. The movable permanent magnet may be located in the upper part of the helium tank or alternatively outside the cryostat. Provided that the movement is sufficiently slow, eddy currents in the cryostat structures are generated only to a minor extent. If necessary, these may be minimised by measures such as the choice of suitable materials, geometric arrangement and slotting. This applies also to inductive coupling via a transmitter coil.

In this way, the electromagnetic energy may be transmitted inductively but without physical contact, in the form of an electromagnetic ac field instead of light. This is also an alternative for the coupling of electromagnetic energy to the flux pump as well as for for heating the superconducting switches. Generally, all combinations are possible, i.e. energy for the flux pump may be coupled in via one or more photoelectric cells or by receiving and 10 rectifying electromagnetic radiation of any suitable wavelength or it may be coupled in inductively. The same holds for heating the superconducting switches, where, if appropriately adapted, all variants of GB-A-2295492 can be applied either alone or in combination. 15 Coupling from the receiver coil into the flux pump may be direct or may be via several transformer stages. In the most simple embodiment, direct coupling is effected inductively into the secondary coil(s) of the flux pump. In this case, the primary coil of the flux pump may be omitted. 20 With inductive transmission, in general coupling into the receiver coil of additional AC fields, which might be generated during operation of the spectrometer, should be avoided which could otherwise create unwanted heat or even cause an unwanted quench. This may be achieved by careful selection of a resonance frequency, or by compensation coils, which compensate interference fields but not that of the transmitter coil. 25 Further preferred features of the invention are illustrated in the following description and in the drawings. The above mentioned features and those to be further described below in accordance with the invention can be utilised individually or together in arbitrary combination, within the scope of the appended claims. The embodiments shown and 30 described are not to be considered as exhaustive enumeration, rather have exemplary character. A number of preferred embadiments of the invention will now be described and explained in more detail with reference to the accompanying drawings, in which:-:

Fig. 1 shows a first embodiment of a schematic configuration of an apparatus according to the invention with optical energy transmission; Fig. 2 shows a schematic detail circuit diagram of the flux pump of the apparatus according to the invention of Fig. 1; Fig. 3 shows a second embodiment of a schematic configuration of an apparatus according to the invention with inductive energy transmission; Fig. 4 shows a schematic detail circuit diagram of the flux pump of the apparatus according to the invention of Fig. 3; Fig. 5 shows a schematic detail circuit diagram of the flux pump of an apparatus according to the invention with inductive coupling from a mechanically rotatable permanent magnet.

In detail, Fig. 1 and Fig. 2 show very schematically a first embodiment of an apparatus according to the invention to charge a superconducting magnet coil 2 being during operation in a superconducting persistent mode and being located at a cryogenic temperature level inside a cryostat 1, by means of a flux pump 3. The transmission of electric energy from outside the cryostat 1 to the flux pump 3 located at cryogenic temperature inside a helium tank 4 of the cryostat 1, is effected without physical contact by means of laser light 90. In the embodiment illustrated, the periodic heating of the superconducting switches 31, 32 of the flux pump 3 and of the superconducting persistent switch 21, are also effected optically. However, in other embodiments, heating of the switches 31, 32, 21 can be effected in a conventional manner by thin current leads or also inductively as described below in connection with a further embodiment of the invention.

Pulsed intensive light 90 (visible, IR or UV) is guided from a light source 6, which may be of various forms, but is preferably a laser, via optical switch and beam guide means 7, e.g.

via controlled, movable mirrors, onto light guides 81 - 85. These light guides 81 - 85 extend via a cryostat tower 11 into the upper gas-filled part of helium tank 4 of cryostat 1.

Opposite to those there are further light guides 91 - 95, extending from below out of the region of liquid helium. Laser light 90, being periodically guided to one or more of light guides 81 - 85 at room temperature, bridges in this way - if needs be with the aid of further focusing means, e.g. lenses 61 - 65 and 71 - 75 - a helium gas filled distance inside helium tank 4 and enters one of the further light guides 91 - 95. Two of these further light guides 91, 92 lead to the light-sensitive surfaces of two photoelectric cells 101, 102 with their maximum sensitivity matched to the laser wavelength. Photoelectric cells 101, 102 are each electrically connected to primary coil 33 of flux pump 3 via diodes 103, 104 such that illumination of one of photoelectric cells 101 causes a positive current in primary coil 33 and illumination of the other one 102 a negative current. Flux pump 3 is driven by illuminating both photoelectric cells 101, 102 alternatingly at a given rate to generate an ac current with a corresponding frequency through primary coil 33. In more simple embodiments, diodes 103, 104 and one of the photoelectric cells can be omitted.

is The remaining further fight guides 93 - 95 lead to superconducting switches 31, 32 and 21. As known in the art, in order to use the flux pump 3, switches 31 and 32 have to be switched in a push-pull way synchronously to the current polarity through primary coil 33.

They alternatingly switch one of the two secondary coils 34, 35 of flux pump 3 in opposite directions to magnet coil 2. In this embodiment, periodic opening of superconducting switches 31, 32 is effected by direct heating by means of laser light via light guides 83, 84 and further light guides 93, 94. In this way, superconducting persistent switch 21, too, may be kept open via light guides 85 and 95. Variations of the circuit diagram according to Fig.

2 are definitely possible. In particular, we refer in this context again to the two review articles cited at the beginning.

Figs. 3 and 4 show very schematically a further embodiment of a device according to the invention to charge a superconducting magnet coil 202 by means of a flux pump 203, whereby magnet coil 202 is located at cryogenic temperature inside a cryostat 201 and is during operation in a superconducting persistent mode. The transfer of electric energy from outside cryostat 201 to flux pump 203 at a cryogenic temperature inside a helium tank 204 of cryostat 201, is effected inductively without physical contact. A transmitter coil 233 inside helium tank 204 can be charged with an time-dependent current via leads 281, 281'.

Transmitter coil 233 acts as primary coil of flux pump 203. However. there is no physical contact to the secondary coils, i.e. there is no electric or thermal conductive connection.

Otherwise, flux pump 203 is constructed analogously to the embodiment of Fig. 2. It is driven by alternatingly increasing or decreasing at a given rate the current through the transmitter coil 233 by means of an external power supply 206. In this way, an ac current is flowing through primary coil 233, too. Transmitter coil 233 is inductively coupled directly to one or more secondary coils 234, 235 of flux pump 203.

Further leads 283 - 285, 283'- 285'from power supply 206 to transmitter coils 263 - 265 are provided which are inductively coupled to receiver coils 293 - 295 and serve to heat superconducting switches 231, 132, 221 without physical contact. An ac current, for a time period flowing through one of further transmitter coils 263 - 265, induces a current in the corresponding receiver coil 293 - 295 which current during this time period heats on its turn the corresponding superconducting switch 231, 232, 221 and thereby opens it. Instead of this inductive heating, however, light guides may be used as in Figs. 1 and 2, leading to superconducting switches 231, 132 and 221, so that these can be heated optically. As discussed above, during use of the flux pump 203, switches 231 and 232 have to be switched in antiphase synchronously with the polarity of the current induced through the primary coil 233. They alternately switch one of the two secondary coils 234, 235 of flux pump 203 in counter-rotating sense to the magnet coil 202. In this embodiment, the periodical opening of superconducting switches 231, 232 is also effected inductively. The superconducting persistent switch 221 of magnet coil 202 can be kept open in this way, too.

Various modifications of the circuit diagram according to Fig. 4 are possible. In this context the reader is again referred in particular to the two review articles cited above.

The periodic heating of superconducting switches 231, 232 of flux pump 203 and of the superconducting persistent switch 221 of magnet coil 202 can each be effected inductively by methods corresponding to those discolsed for heating the superconducting switch to generate a quench described GB 22 95 492 Al, but preferably it is effected optically as already described above in connection with Fig. 2 and a first embodiment of the invention.

In other embodiments heating of switches 231, 232, 221 may be effected in a conventional way via thin leads in this case, too.

In the embodiment according to Fig. 5, coupling of the energy is also effected inductively.

However, the transmitter coil is replaced by a mechanically movable permanent magnet 306, which may be located outside cryostat 301. The flux pump 303 inside the helium tank 304 has no primary coil. The magnetic flux of the permanent magnet is coupled directly into secondary coil(s) 334, 335 of flux pump 303. By mechanical movement of permanent magnet 306, for example translation andlor rotation, the flux is changed. In this embodiment, superconducting switches 331, 223 of flux pump 303 and superconducting persistent switch 321 of magnet coil 302 are switched optically analogously to the first embodiment by means of a laser 306 via light guides 383 - 385, 393 - 395 and synchronously to the movement of permanent magnet 306. Obviously, the other variants discussed above to switch the switches without physical contact addressed in the text are also possible.

In all figures, the arrangements of components in particular of the induction coils and of the permanent magnet, should by no means be interpreted as being represented to scale but rather are schematic.

It should be also be stressed that the features mentioned in connection with figures 1 to 5 in accordance with the invention can be utilised individually or collectively in arbitrary combination, within the scope of the appended claims. The embodiments shown are not to be considered as exhaustive enumeration, but they have rather exemplary character only.

Claims (20)

CLAIMS:

1 A superconducting magnet assembly comprising a cryastat, having a cryogenic region, a superconducting magnet coil in the cryogenic region of the cryostat, and a flux pump in the cryogenic region of the cryostat, the flux pump being connected to the superconducting magnet coil for charging the magnet coil, wherein the flux pump comprises at least one secondary coil and at least one periodically switchable superconducting switch, wherein the device comprises at least one periodically switchable transmitter located outside the cryogenic region, for transmitting electromagnetic energy, at least one first receiver in the cryogenic region of the cryostat, for receiving the periodically emitted electromagnetic energy, converting the received energy into a periodic current, and transferring the periodic current to the flux pump, means for transferring pulsed energy to the superconducting switch to heat the switch and thereby periodically render it non-superconductive, such that the flux pump successively increases the current through the superconducting magnet coil.

2. A superconducting magnet assembly according to claim 1, characterised in that the flux pump comprises two secondary coils wound or connected in an opposite direction as well as two periodically switchable superconducting switches.

3. A superconducting magnet assembly according to Claim 1, including two said first receivers, the arrangement being such that electromagnetic energy is transmitted in an alternating fashion to the two said first receivers, and wherein the two said first receivers are electrically connected to the flux pump in such a way that they feed in an alternating fashion currents of different polarity to the flux pump.

4. A superconducting magnet assembly according to any one of the preceding claims, wherein the periodically switchable transmitter is a transmitter coil and the first receiver is a receiver coil.

5. A superconducting magnet assembly according to any one of the preceding claims, wherein the first receiver is the secondary coil(s) of the flux pump.

6. A superconducting magnet assembly according to any one of the preceding claims, wherein the periodically switchable transmitter is at least one mechanically movable magnet and the first receiver is a receiver coil.

7. A superconducting magnet assembly according to claim 6, characterised in that the mechanically movable magnet is a permanent magnet.

8. A superconducting magnet assembly according to claim 6 or claim 7, wherein the mechanically movable magnet is located outside the cryostat.

g. A superconducting magnet assembly according to Claim 1 or Claim 2, wherein the transmitter is at least one optical or infrared light source, and wherein the first receiver comprises at least one photoelectric cell which is sensitive in the spectral range of the light source.

10. A superconducting magnet assembly according to claim 9, wherein the first receiver is located such that the optical path from the transmitter to the first receiver does not run through cryogenic liquid in the cryostat, irrespective of the filling level of the cryogenic liquid.

11. A superconducting magnet assembly according to any one of the preceding claims, wherein the superconducting magnet coil comprises an additional separately heatable superconducting persistent switch.

12. A superconducting magnet assembly according to to any one of the preceding claims, wherein the means for transferring pulsed energy to the superconducting switch to heat the switch and thereby periodically render it non superconductive, includes means for receiving pulsed electromagnetic energy from the transmitter and for transferring received energy to the superconducting switch.

13. A superconducting magnet assembly according to claim 12, wherein the means for receiving pulsed electromagnetic energy from the transmitter includes a surface which is absorbent in the frequency range of the energy transmitted from the transmitter, and wherein the radiation is aimed or focused onto the said surface, and wherein the surface is in thermal contact with the superconducting switch.

14. A superconducting magnet assembly according to claim 13, wherein the said surface facing the transmitter includes a radiation trap comprising a recess into which the radiation is focused.

15. A superconducting magnet assembly according to claim 12, including a photoelectric cell electrically connected to a heater resistor to heat the superconducting switch.

16. A superconducting magnet assembly according to claim 13, wherein the said surface is a surface of the superconducting switch itself onto which the radiation is is aimed or focused.

17. A superconducting magnet assembly according to any one of the preceding claims, including a periodically switchable ac current generator heater, a primary coil, a secondary coil, and a resistor to heat the superconducting switch, wherein the secondary coil is electrically connected to the heater resistor and is inductively coupled to the primary coil, and the primary coil is connected to the periodically switchable ac current generator, and wherein the primary coil is outside the cryogenic region.

18. A method of charging a magnet coil which during operation is in a superconducting persistent mode and at a cryogenic temperature inside a cryostat, by means of a flux pump also located at the cryogenic temperature which flux pump comprises at least one secondary coil and a periodically switchable superconducting switch whereby the energy to drive the flux pump is transmitted to the flux pump from a temperature above the cryogenic temperature via electromagnetic radiation and without physical contact.

19. A superconducting magnet assembly such as hereinbefore described with reference to and as illustrated by the accompanying drawings.

20. A method of charging a magnet coil such as hereinbefore described with reference to and as illustrated by the accompanying drawings.