GB2339968A - A magnetizing arrangement for a high temperature superconductor - Google Patents

Description

1 2339968 MAGNETISING MAGNE This invention relates to magnetising magnets

for use with high temperature superconductors (HTSCs).

HTSCs have a critical (transition) temperature, above which they cease to be superconducting, of less than 100'K and in many cases below the 77K boiling point of nitrogen. Some of these HTSCs are magnetisable and behave like permanent magnets below their critical temperatures. One such is melt-processed single crystal yttrium barium copper oxide, Y-Ba-Cu-0 (M-P YBCO), typically having the composition (Y Ba2 CU3 07-0 Use of M-P YBCO involves a number of problems, including the need to be able to generate the fields needed to magnetise it, and the fact that if, for any reason, the material warms up above its critical temperature it will lose all its magnetisation.

Then, after re-cooling, it requires further magnetisation.

The problem of magnetising the material is that it must be cooled to below 90'K (the critical temperature of YBCO) and ideally to below 500K (as the flux it traps increases markedly as its temperature is reduced). In practice, a block of material needs to be held inside a vacuum container as primary insulation supported on insulating struts with sufficiently low conductivity to minimise heat conduction to the block, but with a necessary material strength to support the forces its enormous potential magnetisation 2 could cause to act on it in the presence of an external magnetic field. A means of keeping the material cold, typically using a good thermal conductor in contact with it which can be cooled through an appropriate low thermal resistance connection to the cold head of a refrigerator, is also needed and requires insulation. Such insulation and support tends to involve several centimetres of space (perhaps 2 to 4 centimetres) around the block of material.

Such a magnet could be used to provide, or to supplement, the main magnetic field of a magnetic resonance imaging magnet. While such a magnet would have advantages in terms of the strength of field generated in terms of its size, it would suffer the disadvantage of the problems which would be caused if its refrigerator failed and it warmed up to above its critical temperature, which could take from a few minutes to a couple of hours.

In particular, it would be desirable for the magnetising magnet needed to re-magnetise it to be transportable so that it could be brought to the relevant imaging apparatus, perhaps one of several in a hospital, in order to allow re-magnetisation to take place.

However, the bulk of the HTSC in its cryostat, allied to the potentially very high fields needed (up to 8 to 10 Tesla (1)), means that the magnetising magnet would have to be very large and have a potentially significant fields spread. This would make it very difficult to locate and could make it impracticable to be transportable for re magnetisation of devices in-situ.

3 The invention provides a magnet for magnetising a high temperature superconductor (HTSQ comprising a pair of poles for generating a magnetising flux, and means for imparting relative movement between an HTSC of area greater than the cross-sectional area of the magnetising flux, and the magnetising magnet.

The arrangement permits a magnetising magnet with a relatively small field to magnetise a relatively large HTSC.

Ways of carrying out the invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which:

Figure I is a front view, partly in section, of a first magnetising magnet; and Figure 2 is a sectional view, partly schematic, of a second magnetising magnet.

Like reference numerals have been given to like parts throughout the figures.

Referring to Figure 1, the first magnetising magnet comprises a pair of drive coils 1, 2 forming poles for generating a magnetising flux indicated schematically by the dotted region 3. Between the poles, a flat thin cryostat 4 is arranged containing an HTSC 5 mounted on a carrier plate 6.

The cryostat 4 is evacuated so as to insulate the HTSC 5, which is cooled by virtue of 4 its carrier plate 6 which is thermally conducting, and via a conductor 7 which can be solid or multi-filamentary, which is in contact at its other end with the cold finger of a refrigerator, which may be of the Sterling cycle or the Gifford-McMahon type. 'Me carrier plate 6 may be of ceramic material, and the conductor 7 may be of co pper.

Thermocouples 8 are installed to monitor at least the central and edge temperatures of the HTSC 5.

The two coils 1. 2 are connected at their rear ends by an iron yoke 9. The purpose of this is to route a useful fraction of the flux generated by the coils around the outside of the HTSC 5, even though it is likely to be significantly saturated particularly near the coils. The coils are wound in the same sense, and the main magnetising flux is shown generally in the dotted region 3. There will be some leakage flux passing through regions of the HTSC 5 away from that near the axis of the coil (a typical line of force of such leakage flux being denoted 10), but the fields that these other places is will be much less than centrally.

In accordance with the invention, in order to magnetise the whole of the HTSC 5 with the relatively small area of magnetising flux, a drive mechanism (not shown) is provided for imparting movement of the cryostat 4 relative to the pole pieces of the magnetising magnet. The movement is both in the direction of the arrow, and at right angles to the direction of the arrow (into the plane of the drawing), in order that all areas of the HTSC 5 can be energised by the magnetising flux 3.

The HTSC block 5 can be magnetised in one of two ways, either by applying a field to the material which is held below its critical temperature, or to maintain a field and cool the HTSC 5 through its critical temperature. In the former case, the magnetisation achieved is only about two thirds of the applied field, whereas in the latter case the magnetisation achieved is that determined by the field.

Both modes can be accomplished by the arrangement shown in Figure 1. In both cases, the HTSC 5 in its cryostat 4 must be moved relative to the magnetising magnet.

Generally it is likely to be easier to move the cryostat, but there could be circumstances in which the cryostat would remain stationary and the magnetising magnet would be moved. The movement is required to be in two directions so that the central field between the coils covers the whole of the HTSC 5 at some time or other.

Thus, the block may be kept cold, that is, below the critical temperature of the HTSC 5, and the drive mechanism moves the block so that the central field covers all parts of it at some time or other. Alternatively, the H'I'SC block 5 starts with its temperature above its critical temperature. It is then cooled slowly, with the temperature coldest, and below its critical temperature, at the centre and moving towards the edge. The cryostat 4 is then moved rapidly and continuously, for example, in a spiral path, so that the field covers the whole of the HTSC 5. The aim is for the temperature to fall below the critical temperature at the time the magnetic flux is being applied, and the movement should be sufficiently rapid that the temperature of each incremental area is not allowed to drop more than 1'C below its critical temperature (preferably not more than 0.2C below the critical temperature) while the magnetic flux is being 6 applied. The block may also incorporate heaters so as to control the time when each area passes through the critical temperature. The path of cooling and of the corresponding magnetising flux need not be from the centre outwards but could be from one edge of the block to the opposite edge, the magnetising flux sweeping out a zig-zag path on the HTSC 5.

Referring to Figure 2, the second magnetising magnet differs from the first in that there is no iron yoke 9 connecting the drive coils 1, 2, and these drive coils are contained within a cryostat 10. The HTSC block 5, which is mounted on a carrier (not shown) does not have its own cryostat 4, and the HTSC 5 is contained in an evacuated region 11 of the cryostat 10. In the drawing, the region at the right hand end of the enclosure 11 is shown as open, but in reality it would be sealed off.

In practice, the HTSC 5 in its own evacuated cryostat would be transferred into the evacuated magnetising magnet cryostat 10 via an evacuable interconnecting region, in the manner described and claimed in our earlier patent application no. 9815540. 1.

In this embodiment, the magnetising magnet is moved, in the direction of the arrow X, and at right angles to the direction of the arrow (into the plane of the drawing), and the HTSC 5 remains stationary, but this position could be reversed if desired. A variable heat shield 12 is applied to one edge of the block and cools each part of the block in turn through its critical temperature when it is in the region of the narrow beam 3, while the relative movement takes place. The dotted outline shows the extreme 7 position of the magnet when its magnetising flux 3 has traversed the entire length of the HTSC block 5. As for the first magnetising magnet, the magnetising flux may be applied as the area of the HTSC block 5 swept is passing through its critical temperature, or the whole block 5 may be held below its critical temperature and the flux may sweep out the entire area. Again, as with the first embodiment, the movement of the magnet is required to be in two directions to cover the area of the block.

Temperature monitors are implanted in the block along with heaters.

The MC may be yttrium. barium copper oxide, Y-Ba-Cu-0 (M-P YBCO), typically having the composition Y Ba2 CU3 07-X. Alternatively, other high temperature superconductors may be used for example the material known as BSCCO (B2 Sr2 Cal Cu2 08 or B2 Sr2 Ca2 CU3 010)

Claims (9)

1. A magnet for magnetising a high temperature superconductor (HTSC) comprising a pair of poles for generating a magnetising flux, and means for imparting relative movement between an HTSC of area greater than the cross-sectional area of the magnetising flux, and the magnetising magnet.

2. A magnetising magnet as claimed in Claim 1, including a cryostat containing the HTSC in an evacuated region and moveable between the poles of the magnetising magnet.

3. A magnetising magnet as claimed in Claim 1, in which the magnetising magnet is contained in an evacuated region in a cryostat.

4. A magnetising magnet as claimed in any one of Claims 1 to 3, including means for cooling successive portions of the HTSC as they pass through the region of magnetising flux.

5. A magnetising: magnet as claimed in any one of Claims 1 to 4, in which the HTSC is mounted on a thermally conducting plate and a conductor is in thermal contact with the plate and with a refrigerator.

6. A magnetising magnet as claimed in Claim 5, in which a thermocouple is 9 associated with the HTSC for sensing its temperature.

7. The magnetising magnet as claimed in any one of Claims 1 to 6, in which the poles are formed by drive coils.

8. A magnetising magnet as claimed in Claim 7, in which the poles are connected by an iron yoke.

9. A magnetising magnet substantially as herein described with reference to the accompanying drawings.