There is significant interest, both in scientific research and in certain commercial sectors, in being able to provide ultra-low temperature environments. Such environments can be thought of as having a temperature of 1 milliKelvin or less. This temperature range is beyond the reach of conventional dilution refrigerators. It is however possible to achieve such temperatures by using the effect of “nuclear demagnetisation”. Essentially, using this technique, a cooled member is located within a relatively strong magnetic field so as to align the nuclear spins, whilst at the same time also being cooled to a temperature of a few milliKelvin using a dilution refrigerator for example. The material is then thermally decoupled from the dilution refrigerator and the material is allowed to demagnetise. The increased misalignment of the nuclear spins causes an increase in entropy of the material which in turn lowers its temperature further. By placing such a material undergoing demagnetisation in close thermal contact with a sample allows that sample to attain a similar temperature to the material itself. With the use of this nuclear demagnetisation technique temperatures of tens of microKelvin are achievable.
In many circumstances it is desirable for a sample to be held within a magnetic field (for example for performing NMR analysis) whilst being cooled to ultra-low temperatures. This requires the provision of two magnets, a first experimental magnet within the bore of which the sample is positioned, together with a second demagnetisation magnet located nearby and positioned so as to be available to provide the cooling power necessary for the sample to be cooled to the desired ultra-low temperature. Each of the experimental and demagnetisation magnets is formed from a superconducting material which is maintained at a temperature of around 4 Kelvin using liquid helium. This is advantageous since it ensures that only a relatively small temperature difference exists between the magnet coils themselves and either of, the sample to be cooled, or the material which is to undergo nuclear demagnetisation.
Since such systems include two relatively powerful magnets in close proximity to one another, one of which must be able to undergo a full magnetic ramp up or ramp down whilst the other remains at operational field, it is critical to their design that these magnets do not unduly influence each other. For this reason, each magnet is typically provided with an associated shielding magnet (such as a coil arrangement). The role of the shielding magnet in each case is primarily to prevent the magnetic field of one magnet influencing the other magnet. Thus, the shielding magnets in each case are designed to provide a cancelled or near zero-field region positioned between the experimental and demagnetisation magnets. Due to the relative changes in magnetic field between each magnet and it's associated shielding magnet being simultaneous, it follows that it is also necessary to ensure that each shielding magnet provides this cancelled field for its respective magnet, rather than the shielding magnets working together to produce a zero-field only by superposition. Known shielding magnets for such systems typically have a number of features, these being as follows:                a) The magnet coils have a narrow dimension in the axial direction of their cross section which may even be less than the radial extent of their cross section (this cross section relating to the coil windings on a single side of the coil axis);        b) Multiple, spatially dispersed coils are typically provided for each shielding magnet;        c) The coils are typically positioned at an axial location between that of the magnet they are shielding and the cancellation region; and,        d) The coils are provided and immersed in the same liquid cryogen reservoir of the magnet which they are shielding.        
In summary therefore, the shielding coils of known demagnetisation systems have a very specific and well developed design driven by the somewhat unique nature of demagnetisation systems. It is in this context that the present invention has been devised.