Patent Description:
Cryogenic cooling systems are commonly used to perform experiments at low temperatures below <NUM> Kelvin. Systems are generally customised for a specific experiment by installing experimental apparatus in a particular arrangement. Installation of experimental apparatus can be difficult and time consuming, commonly requiring the use of cranes or elevated platforms to access the system. Furthermore, testing is generally required after the installation of equipment to ensure it is functioning satisfactorily, which can take a significant amount of time. The more time spent installing and troubleshooting, the less time spent collecting experimental data.

Cryogenic cooling systems can reach millikelvin temperatures when in use, typically by including a number of platforms which are held at intermediate temperatures between room temperature and millikelvin temperatures. In this way, the cooling can be staged, such that the final platform of the system can provide continuous cooling to millikelvin temperatures. Installed experimental apparatus and other components of the system can provide a path from room temperature to the final platform. In order to prevent unintentional heating through these components, each platform provides a thermal sink to remove additional heat.

It is possible for experimental services to be assembled on a module outside a system and installed pre-assembled. This is typically faster than direct installation of experimental services. However, it is important for the module to be well thermalised, so that millikelvin temperatures can be obtained. In the prior art, thermalisation is achieved using clamps and/or complex and extensive adjustment processes.

It is the case that a minor offset will result in poor thermalisation within the system.

Low temperature physics experiments are becoming increasingly complex, and the experimental services required to perform the experiments is consequently increasing. Quantum Information Processing (QIP) experiments, for example, use radio-frequency (RF) wiring to address devices with large numbers of qubits. As the number of qubits scales up, the amount of RF wiring required correspondingly increases. Cryogenic cooling systems are expected to accommodate the growing amounts of experimental services. One way the growing demands can be accommodated is by providing modular upgrades for a core system. However, manufacturing tolerances can accumulate to result in mismatched joints and poorly thermalised platforms within the cryogenic cooling system, thus requiring extensive minor adjustments to improve performance.

<CIT> discloses an arrangement of a cryogen free cooling apparatus in which the first stage and the second stage of a mechanical cooler are thermally coupled to a first shield and a second shield, respectively, of the cryostat.

It is desirable to have a more convenient way of installing experimental services in a cryogenic cooling system.

A first aspect of the invention, which is defined in claim <NUM>, provides a cryogenic cooling system comprising when in use: a primary insert comprising: a plurality of primary plates, each primary plate having a primary contact surface; and one or more primary connecting members arranged so as to connect the plurality of primary plates; a demountable secondary insert comprising: a plurality of secondary plates, each secondary plate having a secondary contact surface; and one or more secondary connecting members arranged so as to connect the plurality of secondary plates such that the secondary insert is self-supporting; and one or more adjustment members; wherein the one or more adjustment members are configured such that, when in use, when the secondary insert is mounted to the primary insert, the adjustment members cause the primary and secondary contact surfaces of the respective primary and secondary plates to be brought into conductive thermal contact.

Advantageously, the system comprises adjustment members which cause the primary and secondary contact surfaces of the respective primary and secondary plates to be brought into conductive thermal contact with each other. This removes the need for numerous minor adjustments to overcome a misalignment between two portions of a cryogenic cooling system such that they are in effective thermal communication. When demounted, the secondary insert can also be moved with respect to the primary insert as a self-supporting body, which further simplifies the mounting and demounting process. For example, each plate of the secondary insert can be aligned with respect to the corresponding plate of the primary insert in a single step.

The one or more adjustment members may form part of the primary insert. In this case, the adjustment members may form part of the plurality of primary plates, or form part of the one or more primary connecting members, or form part of both the plates and the connecting members. Similarly, the one or more adjustment members may form part of the secondary insert. In this case, the adjustment members may form part of the plurality of secondary plates, or form part of the one or more secondary connecting members, or form part of both the plates and the connecting members. It is also possible for the adjustment members to form part of the primary insert and the secondary insert. Alternatively the adjustment members may take the form of fasteners that are configured to couple corresponding plates of the primary and secondary inserts. The choice of location of the adjustment members may depend on a specific implementation. For example, if the secondary insert is designed to accommodate rigid experimental apparatus, the position and type of adjustment member will be chosen accordingly.

The primary and secondary plates typically extend in a generally planar manner and are connected in use along mutually adjacent peripheral surfaces of the plates, and these may be stepped. Preferably, the conductive thermal contact between a primary plate and the corresponding secondary plate is provided by area contact between conformal planar regions of the respective primary and secondary contact surfaces. Each of the primary and secondary plates may comprise a flange. When a primary plate is brought into contact with the corresponding secondary plate, a lower surface of the flange of the primary plate matches an upper surface of the flange of the secondary plate to form a continuous structure. Typically, the primary plates and the secondary plates are formed from a high conductivity material and thus a joint in which the plates are intimately connected over a large area will provide a good thermal connection across the joint.

The adjustment members typically cause the primary and secondary contact surfaces of the respective primary and secondary plates to be brought into conductive thermal contact by accommodating a misalignment between each of the plurality of secondary plates of the demountable secondary insert and the corresponding primary plate of the primary insert. There may be a misalignment between primary and secondary plates as a result of manufacturing tolerances. Any misalignment between plates, if left unadjusted, will reduce the thermal conductance between plates.

Although the cryogenic cooling system comprises both a primary insert and a secondary insert, the secondary insert (or, alternatively, the primary insert) is demountable and thus is removable from the system. When the secondary insert is in a demounted state, the secondary plates are typically spatially positioned with respect to one another in a secondary configuration. The secondary insert is self-supporting in its demounted state, and the spacing between adjacent plates within the secondary insert may be determined by the secondary connecting members. Similarly, when the secondary insert is in a demounted state, the primary plates are typically spatially positioned with respect to one another in a primary configuration. The spacing between adjacent plates within the primary insert may be determined by the primary connecting members.

During a mounting process, the secondary insert may be mounted to the primary insert. A plate of the secondary insert is preferably configured to be brought into contact with a corresponding plate of the primary insert. However, there may be a misalignment between the above-mentioned plates. The misalignment may be the offset between the plane of a secondary plate and the plane of the corresponding primary plate in the respective primary and secondary configurations. Each pair of plates may have a different misalignment, and the misalignment may be positive or negative. Consequently, each adjustment member may provide a different level of adjustment, and typically is capable of providing a range of motion of at least <NUM> millimetres, preferably at least <NUM> millimetres in order to accommodate the misalignment.

The secondary insert is demountable from the cryogenic cooling system. The secondary insert may be fully demounted, i.e. all of the plates of the secondary insert may be separated from the primary insert and removed. Optionally, the secondary insert may only be partially demounted. If the secondary insert is partially demounted, some of the plates of the secondary insert remain attached to the primary insert, whilst the remainder of the plates of the secondary insert are removed from the primary insert. Preferably, one or more of the secondary connecting members are removable such that two or more of the plurality of secondary plates may be detached from the demountable secondary insert as a unitary, self-supporting body or assembly.

The secondary insert may comprise a first secondary plate, a second secondary plate and a third secondary plate connected using secondary connecting members, wherein the second secondary plate is positioned between the first and the third secondary plate. If the secondary connecting members connecting the second secondary plate and the third secondary plate are removed, the second secondary plate and the first secondary plate may be removed as a unitary structure. The partially demounted secondary insert (the first and second secondary plates) is preferably self-supporting in a similar way to the self-supporting nature of the fully demounted secondary insert.

The demountable nature of the secondary insert advantageously allows the secondary insert to be modified away from the cryogenic cooling system. However, in cases in which it is not necessary to remove the entire secondary insert, it may be beneficial to leave a portion of the secondary insert attached to the primary insert. For example, as low-temperature experiments are typically performed in a vacuum, one of the joints between the primary insert and the secondary insert may form part of a barrier between atmospheric pressure and low pressure. Therefore, additional sealing may be required such as the use of an o-ring or other vacuum seal such as to reduce the possibility of any gas leaks. It may be beneficial to leave the plates forming the above-mentioned barrier in place to avoid repeatedly reforming the seal.

An advantage of the secondary insert being demountable from the cryogenic cooling system is the ability to assemble, modify and test experimental services mounted to the secondary insert away from the cryogenic cooling system. Furthermore, the modifications may only need to be performed on two, or any number of, plates of the secondary insert. It may be easier and therefore preferable to partially demount the secondary insert, only removing the necessary plates.

Typically, experimental services are positioned within the cryogenic cooling system and are used to perform experiments at low temperatures. Preferably, one or more the plurality of secondary plates is configured to accommodate experimental apparatus. This is particularly advantageous if the experimental apparatus mounted to the secondary insert is complex and time-consuming to assemble. The experimental services can hence be assembled and tested away from the cryogenic cooling system before being mounted to the primary insert.

The cryogenic cooling system can be used for low temperature experimental procedures and cooling can be achieved using a number of refrigeration apparatus. It is particularly desired for such systems to achieve millikelvin temperatures. To this end, a dilution unit preferably forms part of the cryogenic cooling system, for example the primary insert may comprise a dilution refrigerator or components thereof. The dilution refrigerator may be thermally coupled to one or more plates of the primary insert. Alternatively the primary insert may comprise a helium-<NUM> refrigerator or a <NUM> kelvin pot. In such a way, one or more plates of the primary insert may attain millikelvin temperatures. The conductive thermal contact between the primary insert and the secondary insert ensures that the secondary insert may reach similarly low temperatures during operation.

One or more of the primary plates or the secondary plates may comprise a rigid portion and one or more deformable portions. Preferably, the deformable portions are deformable with respect to the rigid portions to accommodate the misalignment. The one or more adjustment members may hence comprise the one or more deformable portions. During the mounting of the demountable secondary insert to the primary insert, the one or more deformable portions may locally deform so as to cause the conductive thermal contact. The deformable portions of the plates may be provided at the plate edges, for example in the form of flanges. One advantage of this mode of adjustment is the ability to maintain the primary and/or secondary configurations within the respective inserts. For example, operation of the adjustment member may not change the separation between adjacent primary plates of the primary insert or adjacent secondary plates of the secondary insert. In practice this means that the primary or secondary inserts respectively can remain fixed and can therefore accommodate rigid experimental apparatus which is mounted to more than one plate. Similarly, the separation between corresponding plates of the primary and secondary inserts respectively can remain fixed. The deformation may be configured to occur locally, in pre-defined areas of a plate, such that experimental apparatus is not damaged, but conductive thermal contact is nevertheless achieved. The deformable portions may form part of the primary plates. Alternatively, the deformable portions may form part of the secondary plates. The deformable portions may optionally form part of both the primary plates and the secondary plates.

When the secondary insert is in a demounted state, the primary and secondary inserts may have respective primary and secondary configurations as described above. If the adjustment is achieved through local deformation, it is possible for the primary and secondary configurations to be maintained even when the demountable secondary insert is in a mounted state. The one or more adjustment members may alternatively cause one or both of the primary or secondary configurations to be adjustable so as to cause the conductive thermal contact. For example, the one or more adjustment members are configured to change the separation between adjacent primary plates or adjacent secondary plates. This may be achieved by configuring each of the one or more primary connecting members or secondary connecting members to deform so as to accommodate the misalignment between plates.

The one or more adjustment members may form at least part of one or more of the primary connecting members or secondary connecting members. For example, the one or more adjustment members may form respective flexible portions of the primary or secondary connecting members. During the mounting of the secondary insert to the primary insert, a misalignment could be accommodated by placing the secondary connecting members under a compressive or tensile load. In response to the load, the secondary connecting members would deform thus causing the conductive thermal contact by bringing the secondary plate into alignment with the corresponding primary plate. Similarly, a misalignment could be accommodated through the deformation of flexible primary connecting members.

Alternatively, the one or more adjustment members may be configured to allow movement of the one or more primary plates with respect to the one or more said primary connecting members, or the one or more adjustment members may be configured to allow movement of the one or more secondary plates with respect to the one or more said secondary connecting members. For example, the primary or secondary connecting members may be rotatable so as to change the separation between adjacent primary plates or adjacent secondary plates using the one or more adjustment members. This adjustment may generally be achieved where an end of the primary or secondary connecting members comprises a screw or tapped portion. In this case the adjustment member may comprise said screw or tapped portion of a connecting member in combination with a receiving member configured to engage with the screw or tapped portion so as to adjust the separation between adjacent plates of the primary or secondary insert.

Preferably, the primary and secondary connecting members are thermalised at the respective primary and secondary plates. Typically there is a heat load conducted from room temperature along the primary and/or secondary connecting members to the lower temperature stages of the system. Thermalisation at the plates advantageously intercepts this heat load, thereby forming a thermal sink that enables distal stages of the primary or secondary insert to obtain lower temperatures during operation of the system. Effective thermalisation of the primary connecting members may be achieved through the use of one or more primary shims, each said primary shim thermally coupling a said primary plate to one or more said primary connecting members and configured to allow movement of the said primary plate with respect to the said one or more primary connecting members. Similarly, effective thermalisation of the secondary connecting members may be achieved through the use of one or more secondary shims, each said secondary shim thermally coupling a said secondary plate to one or more said secondary connecting members and configured to allow movement of the said secondary plate with respect to the said one or more secondary connecting members.

Further aspects of the invention will now be described. Any features discussed in connection with one aspect are equally applicable in respect of the remaining features and each aspect shares similar advantages.

A second aspect of the invention,which is defined in claim <NUM>, provides a method of operating the system according to the first aspect, wherein the secondary insert comprises a first secondary plate, a second secondary plate and a third secondary plate, a first secondary connecting member connecting the first secondary plate to the second secondary plate, and a second secondary connecting member connecting the second secondary plate to the third secondary plate, and wherein the primary insert comprises three primary plates, each said primary plate corresponding to a respective secondary plate of the secondary insert, the method comprising: mounting the secondary insert to the primary insert such that secondary plates are thermally coupled to the corresponding primary plates using the one or more adjustment members; and partially demounting the secondary insert from the primary insert, wherein partially demounting the secondary insert comprises: removing the first secondary connecting member from the secondary insert; and removing the second secondary plate, the third secondary plate and the second secondary connecting member from the primary insert as a unitary self-supporting assembly, without removing the first secondary plate from the corresponding plate of the primary insert.

Embodiments of the invention will now be described with reference to the accompanying drawings in which:.

<FIG> provides a sectional view of the interior of a cryogenic cooling system according to a first embodiment. The system comprises a plurality of thermal stages <NUM>-<NUM> and an outer stage <NUM>. The thermal stages <NUM>-<NUM> and outer stage <NUM> are connected by primary and secondary rods <NUM>, <NUM>, thus forming a tiered assembly in which the stages are aligned and spatially dispersed along a central axis extending parallel to the rods. The primary rods <NUM> are not shown in <FIG> for clarity. The primary and secondary rods <NUM>, <NUM> are formed from a low thermal conductivity material such as stainless steel. When in use, the thermal stages <NUM>-<NUM> are contained within a cryostat <NUM>, which is typically evacuated to improve the thermal performance by the removal of convective and conductive heat paths through any gas within the cryostat <NUM>. The cryostat <NUM> is mounted to the outer stage <NUM>, and the outer surface <NUM> of the outer stage <NUM> is exposed to room temperature and pressure and is formed from a low conductivity material.

The cryogenic cooling system comprises cooling apparatus. The cooling apparatus cools the cryogenic cooling system from room temperature to an operational base temperature. The cryogenic cooling system in the first embodiment is substantially cryogen-free (also referred to in the art as "dry") in that it is not principally cooled by contact with a reservoir of cryogenic fluid. However, despite being substantially cryogen-free, some cryogenic fluid is typically present within the cryostat when in use, including in the liquid phase, as will become clear. In this embodiment, the cooling is achieved by use of a mechanical refrigerator and a dilution unit. The mechanical refrigerator may be a pulse-tube refrigerator (PTR), a Stirling refrigerator, or a Gifford-McMahon (GM) refrigerator.

In this embodiment, the mechanical refrigerator is a PTR <NUM>, and is thermally coupled to the first thermal stage <NUM> and the second thermal stage <NUM>. Each thermal stage <NUM>-<NUM> is formed from a high conductivity material such as copper and has a different operational base temperature. The first thermal stage <NUM> is thermally coupled to a first PTR stage <NUM> and attains an operational base temperature of about <NUM> to <NUM> kelvin. The second thermal stage <NUM> is thermally coupled to a second PTR stage <NUM> and attains an operational base temperature of about <NUM> to <NUM> kelvin. In this embodiment, the second PTR stage <NUM> forms the lowest temperature stage of the PTR <NUM>.

The third thermal stage <NUM>, fourth thermal stage <NUM> and fifth thermal stage <NUM> are thermally coupled to a dilution unit <NUM>. The cooling of the third, fourth and fifth thermal stages <NUM>, <NUM>, <NUM> is achieved through operation of the dilution unit <NUM>, in which an operational fluid is circulated around a cooling circuit <NUM>. The operational fluid is typically a mixture of helium-<NUM> and helium-<NUM>. The operational fluid is pumped around the cooling circuit <NUM> which comprises a condensing line <NUM> and a still pumping line <NUM> using a compressor pump <NUM> and a turbomolecular pump <NUM>. The operational fluid can be stored in a storage vessel <NUM> and supplied to the cooling circuit <NUM> using a supply line <NUM>. The third thermal stage <NUM> is thermally coupled to a still <NUM> which forms part of the dilution unit <NUM>. The operational base temperature of the third thermal stage <NUM> is typically <NUM> to <NUM> kelvin. The fifth thermal stage <NUM> is thermally coupled to a mixing chamber <NUM> of the dilution unit <NUM>. The operational base temperature of the fifth thermal stage <NUM> is typically <NUM> to <NUM> millikelvin. The fourth thermal stage <NUM> forms an intermediary stage between the third and fifth thermal stages <NUM>, <NUM> and has an operational base temperature of about <NUM> to <NUM> millikelvin.

In use, a number of heat radiation shields <NUM>-<NUM> are attached to the thermal stages <NUM>-<NUM>, wherein each shield encloses each of the remaining lower base-temperature components. The first heat radiation shield <NUM>, second heat radiation shield <NUM> and third heat radiation shield <NUM> are attached to the first thermal stage <NUM>, second thermal stage <NUM>, and third thermal stage <NUM> respectively. This reduces any unwanted thermal communication between the thermal stages <NUM>-<NUM> and allows the stages to attain different operational base temperatures.

The cryogenic cooling system of <FIG> can be controlled using a control system <NUM>. The control system <NUM> is typically a suitable computer system, although it is possible to have manual control of the system. The operation of each part of the system can be controlled using the control system <NUM>, including the operation of the PTR <NUM>, the dilution unit <NUM>, pumps <NUM>, <NUM> and associated valves; the monitoring of temperature and pressure sensors; and the operation of other ancillary equipment to perform desired procedures.

A cryogenic cooling system as described can be used to perform experiments at low temperatures, generally below <NUM> kelvin. Although not shown in <FIG>, experimental services can be mounted within the cryostat <NUM>. The choice of experimental services and their particular arrangement within the cryostat <NUM> is customisable. One such example of experimental services will be discussed with reference to <FIG>. Typically a particular arrangement of experimental services is installed, tested, and remains fixed for a period of time. Modification of the arrangement within the system to perform a different type of experiment is typically very time consuming, requiring numerous adjustments and troubleshooting procedures before the experiment can be run. Embodiments of the invention provide a primary insert <NUM> and a secondary insert <NUM>, wherein the secondary insert <NUM> is demountable from the primary insert <NUM>. Experimental services can hence be mounted to the primary insert <NUM> or to the secondary insert <NUM>, which is easy to remove and reinstall, or to both the primary and secondary inserts <NUM>, <NUM>. The primary insert <NUM> comprises a plurality of primary plates and the secondary insert <NUM> comprises a plurality of secondary plates <NUM>-<NUM>, wherein each primary plate is configured to fit to a corresponding secondary plate in order to form a respective thermal stage <NUM>-<NUM> of the system, as will be further discussed.

An advantage of mounting the experimental services to the secondary insert <NUM> arises from the ability to remove the secondary insert <NUM> from the cryogenic cooling system. Assembly and preliminary tests can be performed 'on the bench', outside the cryogenic cooling system in which the experiment will be performed. In this way, modifying or updating the experimental services to run a different experiment can be performed relatively quickly and easily. Low-temperature experiments using a cryogenic cooling system such as a dilution refrigerator typically take days, weeks or months to perform. Modifications to the experimental services within the system lead to experimental down time, i.e. time during which the cryogenic cooling system is not at operational base temperature, as the modifications typically need to be performed at room temperature. The ability to manipulate experimental services on the demounted secondary insert <NUM> on the bench (remote from the system itself) reduces the experimental down time. For example, multiple secondary inserts may be provided for use with a given cryogenic cooling system. Adjustments may be made to experimental services on a first secondary insert under atmospheric conditions whilst a cryogenic environment is maintained in the system for performing experiments on a second secondary insert.

Embodiments of the invention also provide adjustment members which cause the primary insert <NUM> and the secondary insert <NUM> to be brought into conductive thermal contact. Good thermal contact is important to achieve when performing low temperature measurements. In the presence of a heat flux, for example as generated by operation of a cooling source, a temperature gradient will naturally arise between the primary insert <NUM> and the secondary insert <NUM>. The difference in temperature between these components will be proportional to the heat flux and inversely proportional to the thermal conductance. For any practical experiment there is a limit to the heat flux that can be applied to the system (as the cooling power available from either of the PTR stages <NUM>, <NUM> or the dilution refrigerator <NUM> is finite). The thermal conductance of a joint will vary depending on numerous factors including its temperature and contact pressure. The adjustment members are typically configured to limit the temperature difference between corresponding stages of the primary and secondary inserts <NUM>, <NUM>, for example to within <NUM>%, and preferably within <NUM>%, of the absolute temperature of the higher temperature stage. This is achieved by making the thermal conductance between these stages sufficiently high. For example, where the second thermal stage <NUM> is cooled to <NUM> kelvin by the second PTR stage <NUM> (at a cooling power of <NUM> watt), the adjustment member for the second thermal stage <NUM> may ensure that the temperature difference between corresponding primary and secondary plates of the second thermal stage <NUM> does not exceed <NUM> millikelvin. The thermal conductance between primary and secondary plates of the second thermal stage <NUM> is therefore approximately <NUM> W/K at <NUM> kelvin. Similarly, where the fifth thermal stage <NUM> is cooled to <NUM> millikelvin by the mixing chamber <NUM> (at a cooling power of <NUM> microwatts), the adjustment member of the fifth thermal stage <NUM> may ensure the temperature difference between corresponding primary and secondary plates of the fifth thermal stage <NUM> does not exceed <NUM> millikelvin. The thermal conductance between primary and secondary plates of the fifth thermal stage <NUM> is therefore approximately <NUM> W/K at <NUM> kelvin.

The fact that there is a difference in thermal conductance expected at the second thermal stage <NUM> and the fifth thermal stage <NUM> is due to the temperature dependence of a joint, as discussed further in "<NPL>. The thermal conductance of a given joint will decrease with temperature. However as the practical heat flux that can be applied between each primary and secondary plate in the respective primary and secondary inserts <NUM>, <NUM> also decreases with temperature, all of the mounting arrangements between the primary and secondary plates can be designed and mounted in the same way to provide acceptable performance at each thermal stage <NUM>-<NUM>.

A variety of adjustment members are envisaged and embodiments facilitating different methods of adjustment will be described.

<FIG> shows the primary and secondary inserts <NUM>, <NUM> from <FIG> in further detail. As shown, each thermal stage <NUM>-<NUM> comprises an inner primary plate <NUM>-<NUM>, an inner secondary plate <NUM>-<NUM>, and an edge piece <NUM>-<NUM>. The outer stage <NUM> comprises an outer primary plate <NUM> and an outer secondary plate <NUM>. Each of the inner and outer secondary plates <NUM>-<NUM>, <NUM> are connected to the corresponding inner and outer primary plates <NUM>-<NUM>, <NUM> along a peripheral portion of the secondary plates. Each of the edge pieces <NUM>-<NUM> are connected to the corresponding inner primary plates <NUM>-<NUM> and the corresponding inner secondary plates <NUM>-<NUM> along a peripheral portion of the respective inner primary and secondary plates. The inner and outer primary plates <NUM>-<NUM>, <NUM> are connected by primary rods <NUM>, and the inner and outer secondary plates <NUM>-<NUM>, <NUM> are connected by secondary rods <NUM>. The primary and secondary rods <NUM>, <NUM> extend between the plates in a direction normal to the plates. In this embodiment, the edge pieces <NUM>-<NUM> are not connected, but in an alternative embodiment the edge pieces <NUM>-<NUM> may be connected by edge rods extending between the edge pieces.

The inner and outer primary plates <NUM>-<NUM>, <NUM> and primary rods <NUM> form part of a primary insert <NUM>. The inner and outer secondary plates <NUM>-<NUM>, <NUM> and secondary rods <NUM> form part of a secondary insert <NUM>. The secondary insert <NUM> is demountable from the cryogenic cooling system and, in particular, the primary insert <NUM>. When the secondary insert <NUM> is in a demounted state, it forms a self-supporting assembly, which does not require any additional support structures to maintain its original configuration and can be removed from the primary insert <NUM> as a unitary body.

The designs of the secondary insert <NUM> and primary insert <NUM> are such that good thermal contact will be achieved between any secondary insert <NUM> and primary insert <NUM> when the secondary insert <NUM> is in a mounted state. It is important to ensure effective thermalisation between corresponding plates in the primary insert <NUM> and secondary insert <NUM> so that any cooling applied to one of the primary or secondary plates can be effectively applied to the other of the secondary or primary plates.

Achieving good thermal contact between any secondary insert <NUM> and primary insert <NUM> when the secondary insert <NUM> is in a mounted state is not trivial. During manufacture of a primary insert <NUM> or a secondary insert <NUM>, the relative positioning of the inner and outer primary plates <NUM>-<NUM>, <NUM> and the inner and outer secondary plates <NUM>-<NUM>, <NUM> within their respective inserts <NUM>, <NUM> may vary within certain manufacturing tolerances, even if made to the same specification. Small differences can lead to a misalignment, i.e. an offset between the plane of a secondary plate and the plane of the corresponding primary plate when the secondary insert <NUM> is brought into a mounted position. Any such misalignment, even if small, can lead to poor thermal contact. This is of particular importance at low temperatures such as the operational base temperatures of the third, fourth and fifth thermal stages <NUM>, <NUM>, <NUM>.

In order to achieve good thermal contact between corresponding plates in the primary insert <NUM> and secondary insert <NUM>, the cryogenic cooling system also comprises adjustment members (examples of which will be described in further detail below) which cause the inner primary plates <NUM>-<NUM> and inner secondary plates <NUM>-<NUM> to be brought into conductive thermal contact when the secondary insert <NUM> is in a mounted state thus accommodating a misalignment. The adjustment members may form part of the primary insert <NUM> or part of the secondary insert <NUM> or part of both.

In <FIG>, the components of the cryogenic cooling system are shown in a mounted position. <FIG> provides an exploded view of the cryogenic cooling system according to the first embodiment with the secondary insert <NUM> and edge pieces <NUM>-<NUM> removed from the primary insert <NUM> to more clearly show the component parts of the system. <FIG> shows the edge pieces <NUM>-<NUM>, the secondary insert <NUM> comprising a plurality of inner secondary plates <NUM>-<NUM> and an outer secondary plate <NUM> connected by secondary rods <NUM>, and the primary insert <NUM> comprising a plurality of inner primary plates <NUM>-<NUM> and an outer primary plate <NUM> connected by primary rods <NUM>.

In this embodiment, the cooling apparatus is attached to the primary insert <NUM>. The cooling apparatus includes a PTR <NUM>, comprising a first PTR stage <NUM> thermally coupled to the first inner primary plate <NUM> of the first thermal stage <NUM> and a second PTR stage <NUM> thermally coupled to the second inner primary plate <NUM> of the second thermal stage <NUM>. The cooling apparatus further comprises a dilution unit <NUM>, wherein a still <NUM> of the dilution unit <NUM> is thermally coupled to the primary plate <NUM> of the third thermal stage <NUM> and a mixing chamber <NUM> of the dilution unit <NUM> is thermally coupled to the primary plate <NUM> of the fifth thermal stage <NUM>. In an alternative embodiment, the cooling apparatus is attached to the secondary insert. For example, the dilution unit may alternatively be mounted to the inner secondary plates <NUM>, <NUM>, <NUM> of the third, fourth and fifth thermal stages <NUM>, <NUM>, <NUM>.

The inner and outer plates <NUM>-<NUM>, <NUM> of the primary insert <NUM> are aligned along an axis <NUM> extending normal to the inner and outer primary plates <NUM>-<NUM>, <NUM> in a primary configuration. Similarly, the inner and outer plates <NUM>-<NUM>, <NUM> of the secondary insert <NUM> are aligned and spatially dispersed along a central axis normal to the inner and outer plates <NUM>-<NUM>, <NUM> of the secondary insert <NUM> in a secondary configuration. There may be an offset between the plane of a secondary plate and the plane of the corresponding primary plate in the respective primary and secondary configurations, referred to as a misalignment. Each of the inner secondary plates <NUM>-<NUM> is configured to be brought into conductive thermal contact with its corresponding inner primary plate <NUM>-<NUM> when the secondary insert <NUM> is mounted to the primary insert <NUM>, thus accommodating any misalignment. Such conductive thermal contact is caused by adjustment members. The outer secondary plate <NUM> forms a vacuum seal with the outer primary plate <NUM>, for example through use of o-rings although any suitable sealing mechanism is possible.

The installation of the secondary insert <NUM> into the cryogenic cooling system will now be described with reference to <FIG>. Firstly, the secondary insert <NUM> is aligned with the primary insert <NUM> in two dimensions, with each inner and outer secondary plate <NUM>-<NUM>, <NUM> positioned slightly below the corresponding inner and outer primary plate <NUM>-<NUM>, <NUM>. Secondly, the secondary insert <NUM> is aligned in the third dimension, wherein the third dimension is parallel to a major axis <NUM> of the primary insert <NUM>. The alignment in the third dimension with the primary insert <NUM> is achieved by raising the secondary insert <NUM> such that each inner and outer secondary plate <NUM>-<NUM>, <NUM> is brought towards its corresponding inner and outer primary plate <NUM>-<NUM>, <NUM> so as to form conductive thermal contact between each pair of primary and secondary plates. The outer secondary plate <NUM> of the outer stage <NUM> forms a seal with the outer primary plate <NUM>. The inner secondary plates <NUM>-<NUM> can then be fixed into place. In this embodiment they are fixed using fastening members, here in the form of screws. The adjustment members (not shown) cause the inner primary plates <NUM>-<NUM> and inner secondary plates <NUM>-<NUM> to be brought into conductive thermal contact in the mounted state. Finally, the edge pieces <NUM>-<NUM> are fixed into place, using screws.

Each edge piece <NUM>-<NUM> is shaped so as to shield lower base-temperature components from excess radiation. As can be seen from <FIG>, the shape of each edge piece <NUM>-<NUM> is designed to match the shape of each inner secondary plate <NUM>-<NUM> and each inner primary plate <NUM>-<NUM> to complete each thermal stage <NUM>-<NUM>. In an alternative embodiment, the edge pieces <NUM>-<NUM> can be mounted to the inner primary plates <NUM>-<NUM> without a secondary insert <NUM> in place. In another embodiment, the edge pieces are not required. Instead, each inner secondary plate <NUM>-<NUM> may be shaped so as to complete each thermal stage <NUM>-<NUM> and act as a thermal shield to block radiation between the adjacent stages.

The secondary insert <NUM> of the cryogenic cooling system is demountable from the primary insert <NUM>. <FIG> shows the cryogenic cooling system in accordance with the first embodiment, with the secondary insert <NUM> in a demounted position and the edge pieces <NUM>-<NUM> attached to the corresponding inner primary plates <NUM>-<NUM>.

Whilst the secondary insert <NUM> is in a demounted position, modifications can be made to the secondary insert <NUM> and particularly the experimental services mounted to the secondary insert <NUM>. This is practically easier for the user to achieve in the demounted position. Modifications to the secondary insert <NUM> may include, for example, updating or testing the experimental services mounted to the secondary insert <NUM>. If desired, an upgraded secondary insert <NUM> may then be mounted to the primary insert <NUM>. Furthermore, it may be advantageous to have more than one secondary insert <NUM> in order to have one secondary insert <NUM> in operation, i.e. in a mounted state and in experimental use, and one or more secondary inserts <NUM> on the bench, i.e. in a demounted state. Whilst in a demounted state, the experimental services on the secondary insert <NUM> can be modified or upgraded more easily. The experimental services on the demounted secondary insert <NUM> can be tested at room temperature, or the secondary insert <NUM> can be mounted into a donor cryostat to test the experimental services at low temperatures. The above testing, assembly, modification and upgrades can be performed in parallel to an experiment being performed in the cryogenic cooling system.

As described above, the secondary insert <NUM> forms a tiered assembly. The spatial distribution of the inner and outer secondary plates <NUM>-<NUM>, <NUM> within the assembly defines five inter-plate spaces <NUM>-<NUM> as shown in <FIG>: a first inter-plate space <NUM> between the outer secondary plate <NUM> and the first inner secondary plate <NUM>, a second inter-plate space <NUM> between the first inner secondary plate <NUM> and the second inner secondary plate <NUM>, a third inter-plate space <NUM> between the second inner secondary plate <NUM> and the third inner secondary plate <NUM>, a fourth inter-plate space <NUM> between the third inner secondary plate <NUM> and the fourth inner secondary plate <NUM>, and a fifth inter-plate space <NUM> between the fourth inner secondary plate <NUM> and the fifth inner secondary plate <NUM>.

In <FIG>, a group of four secondary rods <NUM> extends across each respective inter-plate space <NUM>-<NUM> connecting each pair of adjacent secondary plates <NUM>-<NUM>. The arrangement of each group of secondary rods <NUM> is offset with respect to the adjacent group to allow each rod from a respective inter-plate space <NUM>-<NUM> to be adjusted or removed independently. Removal of all of the secondary rods <NUM> in one of the inter-plate spaces <NUM>-<NUM> allows the secondary insert <NUM> to be divided into two parts. Two or more plates of the secondary insert <NUM> can hence be removed from the remaining plates as a unitary structure. <FIG> illustrates a cryogenic cooling system according to the first embodiment in which the secondary insert <NUM> is partially demounted.

In <FIG> the secondary rods <NUM> in the fourth inter-plate space <NUM> have been removed. The fourth inter-plate space <NUM> is between the third inner secondary plate <NUM> and the fourth inner secondary plate <NUM>, and thus the removal of the above secondary rods <NUM> allows the fourth inner secondary plate <NUM> and the fifth inner secondary plate <NUM> to be demounted from the cryogenic cooling system whilst leaving the remaining inner and outer secondary plates <NUM>-<NUM>, <NUM> mounted. The fourth and fifth inner secondary plates <NUM>, <NUM> remain held together by connecting secondary rods <NUM>, and therefore this assembly is still self-supporting once it has been demounted from the cryogenic cooling system. In alternative embodiments, any number of the inner and outer secondary plates <NUM>-<NUM>, <NUM> can be removed.

Depending on the experimental circumstances, it may only be necessary to test or modify only a subset of the secondary plates <NUM>-<NUM> of the secondary insert <NUM>. Partial removal of the secondary insert <NUM> is therefore advantageous as it allows for more flexible preparation and testing of experimental services. In addition, re-installation of a part of the secondary insert <NUM> as opposed to the whole secondary insert <NUM> is less complex for a user to perform. The cryogenic cooling system can be operated with the inner secondary plates <NUM>-<NUM> removed. However, if the inner secondary plates <NUM>-<NUM> of any of the first to fourth thermal stages <NUM>-<NUM> are removed, then they should generally be replaced with blanks to reduce radiation transfer between the thermal stages.

Experimental services can be mounted to the cryogenic cooling system. <FIG> illustrates the cryogenic cooling system according to the first embodiment with experimental services mounted to the secondary insert <NUM>. Examples of experimental services may include wiring which may be RF wiring, ultra-high vacuum components, electrical devices (such as attenuators, filters, circulators or other microwave components, amplifiers, resistors, transistors, thermometers, capacitors, inductors), or any other experimental services required for a chosen experiment. The experimental services illustrated in <FIG> are coaxial wires.

As described above, secondary inserts may be wholly or partially demounted from a cryogenic cooling system and inserted into another cryogenic cooling system. There may be a misalignment between each inner secondary plate <NUM>-<NUM> and the corresponding inner primary plate <NUM>-<NUM> when the secondary insert <NUM> is brought into a mounted position, which can lead to poor thermal contact. In order to ensure good thermal contact, the cryogenic cooling system comprises adjustment members. Possible adjustment members will now be described with reference to <FIG>.

<FIG> schematically illustrates a front view of an inner secondary plate in accordance with the first embodiment. Although described below in relation to the first inner secondary plate <NUM>, the description may be applicable to any one or more of the inner secondary plates <NUM>-<NUM> of the secondary insert <NUM>. The first inner secondary plate <NUM> has a rigid central part <NUM>. There are flanges <NUM> along each side of the rigid central part <NUM>, arranged in the plane of the first secondary plate <NUM>. In this embodiment, each flange <NUM> has secondary holes <NUM> distributed evenly along the length of the flange <NUM>. The holes may be tapped or untapped. A matching series of holes is positioned on the corresponding primary plate (see <FIG>) so that the first inner secondary plate <NUM> can be mounted to the first inner primary plate <NUM> using screws or any suitable attachment mechanism.

The flanges <NUM> are separated from the rigid central part <NUM> by a linking portion <NUM>. The linking portion <NUM> is a relatively thin strip of the first inner secondary plate <NUM> extending along the length of the flange <NUM> and which forms a pivot about which the flange <NUM> can move. The first inner secondary plate <NUM> further accommodates four receiving holes <NUM> for positioning the secondary rods <NUM>, although of course the number of receiving holes <NUM> may vary depending on the number of secondary rods <NUM> used.

In the first embodiment, the flanges <NUM> are configured to deform when a load is applied so as to cause the first inner primary plate <NUM> and the first inner secondary plate <NUM> to be brought into conductive thermal contact. Localised deformation allows rigid experimental apparatus to be mounted to the secondary insert <NUM>, such as an ultra-high vacuum port. Such rigid apparatus may, once mounted, effectively determine the separation between two or more of the inner or outer secondary plates <NUM>-<NUM>, <NUM>. In this embodiment, the rigid apparatus is mounted to the rigid central part <NUM> of the first inner secondary plate <NUM>, and the flanges <NUM> provide a deformable portion thus forming the adjustment members. The localised deformation of the flanges <NUM> accommodates any misalignment between the first inner primary plate <NUM> and the first inner secondary plate <NUM>. The inclusion of a rigid central part <NUM> of an inner secondary plate advantageously allows rigid experimental apparatus to remain unaffected by any adjustment required whilst ensuring effective thermalisation between the secondary insert <NUM> and the primary insert <NUM>.

<FIG> schematically illustrate a side view of a part of a cryogenic cooling system in accordance with the first embodiment during a mounting process. <FIG> illustrates a portion of the secondary insert <NUM> in a demounted state and <FIG> illustrates the portion of the secondary insert <NUM> in a mounted state, with the adjustment member in use. <FIG> depict portions of the first inner secondary plate <NUM>, the second inner secondary plate <NUM>, the first inner primary plate <NUM> and the second inner primary plate <NUM>. However, the description applies to any adjacent inner plates in the secondary insert <NUM> and the corresponding plates in the primary insert <NUM>.

The first inner secondary plate <NUM> comprises a rigid central part <NUM>, a flange <NUM> and a linking portion <NUM>. The second secondary plate <NUM> comprises a rigid central part <NUM>', a flange <NUM>' and a linking portion <NUM>'. Primed reference numerals are used to designate similar apparatus features between the second inner secondary plate <NUM> and the first inner secondary plate <NUM>. The first and second inner secondary plates <NUM>, <NUM> both take the form shown by <FIG>. The first inner primary plate <NUM> is connected to the second inner primary plate <NUM> by a primary rod <NUM>. Typically, more than one primary rod <NUM> will be used to connect adjacent plates of the primary insert <NUM>, but only one is shown here for clarity.

<FIG> schematically illustrates a portion of the secondary insert <NUM> and the corresponding portion of the primary insert <NUM> when the secondary insert <NUM> is in a demounted state. In a demounted state, the separation between the first inner secondary plate <NUM> and the second inner secondary plate <NUM> is d<NUM>. The separation between the first inner primary plate <NUM> and the second inner primary plate <NUM> is d<NUM>, wherein d<NUM> > d<NUM>. In a different example, the misalignment may be in the opposite direction, i. e d<NUM> < d<NUM>. The relative lateral positioning in <FIG> is illustrative only, and is to show the vertical misalignment clearly. The misalignment is between the first inner secondary plate <NUM> and the first inner primary plate <NUM>. The first and second inner primary plates <NUM>, <NUM> comprise a stepped portion along the periphery through which primary holes <NUM>, <NUM>' extend. The secondary holes <NUM>, <NUM>' in the first and second inner secondary plates <NUM>, <NUM> are configured to align with the primary holes <NUM>, <NUM>' in the first and second inner primary plates <NUM>, <NUM> respectively.

In an alternative embodiment, the flanges may be positioned on the plates of the primary insert <NUM>, instead of the secondary insert <NUM>. This may be particularly advantageous if there are multiple interchangeable secondary inserts <NUM> for a cryogenic cooling system, some of which may not comprise adjustment members. In another alternative embodiment, the flanges <NUM> may be positioned on the plates of the primary insert <NUM> and the secondary insert <NUM>. This may advantageously allow a larger possible misalignment as the deformation could occur on both sides.

<FIG> schematically illustrates the portion of the secondary insert <NUM> and the corresponding portion of the primary insert <NUM> of <FIG> when the secondary insert <NUM> is in a mounted state. In <FIG> the secondary holes <NUM>, <NUM>' are aligned with the primary holes <NUM>, <NUM>'. The flange <NUM> and the linking portion <NUM> are in a deformed position, deformed to cause the first inner primary plate <NUM> and the first inner secondary plate <NUM> to be brought into conductive thermal contact. The flange <NUM> is thus in area contact with the first inner primary plate <NUM> along the stepped portion of the first inner primary plate <NUM>. The planar regions of the stepped portion of the first inner primary plate <NUM> conform to the flange <NUM> of the first inner secondary plate <NUM>.

In this embodiment, the deformation of the flanges <NUM>, <NUM>' can accommodate the misalignment between d<NUM> and d<NUM> whilst the rigid central parts <NUM>, <NUM>' of the first inner secondary plate <NUM> and the second inner secondary plate <NUM> remain in a fixed position with respect to each other. The first inner primary plate <NUM> and the second inner primary plate <NUM> also remain in a fixed position with respect to each other before and after the mounting process.

<FIG> schematically illustrate a side view of a part of a cryogenic cooling system in accordance with a second embodiment, showing a portion of a secondary insert <NUM> in a mounted state, with an adjustment member in use. The cryogenic cooling system takes a similar form to that described in the first embodiment, although the adjustment members provided are different. Each of <FIG> show a first inner secondary plate <NUM> connected to a second inner secondary plate <NUM> by a secondary rod <NUM> and a first inner primary plate <NUM> connected to a second inner primary plate <NUM> by a primary rod <NUM>. Typically, further primary rods <NUM> and further secondary rods <NUM> are used, but for clarity only one is shown in <FIG>. With the secondary insert <NUM> shown in a mounted position, the secondary holes <NUM>, <NUM>' are aligned with the primary holes <NUM>, <NUM>'.

In the second embodiment, the secondary rods <NUM> are configured to deform when a compressive or tensile load is applied so as to adjust the separation between the adjacent inner secondary plates <NUM>, <NUM>. This movement accommodates any misalignment between the corresponding plates of the primary and secondary inserts <NUM>, <NUM>. In this embodiment, the primary rods <NUM> are rigid and therefore the separation between adjacent plates in the primary insert <NUM> is fixed. The secondary rods <NUM> are formed from stainless steel and curved to allow deformation as described. The deformation of the secondary rods <NUM> causes each of the inner secondary plates <NUM>-<NUM> to be brought into conductive thermal contact with the corresponding inner primary plates <NUM>-<NUM>.

In <FIG>, the separation between the first inner secondary plate <NUM> and the second inner secondary plate <NUM> in a demounted state, d<NUM>, is less than the separation between the first inner primary plate <NUM> and the second inner primary plate <NUM>, d<NUM>, i.e. d<NUM> < d<NUM>. The secondary rod <NUM> is in a first position <NUM>, indicated in <FIG> using dashed lines, when the secondary insert <NUM> is in a demounted state. The secondary rods <NUM> are configured to be extended in response to a tensile load to a second position <NUM>, indicated in <FIG> using a solid line, in which the first and second inner secondary plates <NUM>, <NUM> are separated further to enable good thermal contact between the first and second inner primary plates <NUM>, <NUM> respectively along the contact surfaces.

In <FIG>, the separation between the first inner secondary plate <NUM> and the second inner secondary plate <NUM> in a demounted state, d<NUM>, is greater than the separation between the first inner primary plate <NUM> and the second inner primary plate <NUM>, d<NUM>, i.e. d<NUM> > d<NUM>. The secondary rod <NUM> is in a first position <NUM>, indicated in <FIG> using dashed lines, when the secondary insert <NUM> is in a demounted state. The secondary rods <NUM> are configured to be compressed in response to a compressive load to a third position <NUM>, indicated in <FIG> using a solid line, in which the first and second inner secondary plates <NUM>, <NUM> are brought into good thermal contact with the first and second inner primary plates <NUM>, <NUM> respectively.

In the second embodiment as described above with reference to <FIG>, the secondary rods <NUM> can accommodate the misalignment between corresponding inner plates of the primary insert <NUM> and secondary insert <NUM> of the cryogenic cooling system. The secondary rods <NUM> are configured to adjust the separation between adjacent secondary plates in order to bring each plate of the secondary insert <NUM> into alignment with each plate of the primary insert <NUM>.

In an alternative embodiment, the primary rods may be configured to deform when a compressive or tensile load is applied, as described above in relation to the secondary rods <NUM>, and the secondary rods may be rigid thus fixing the position of the inner and outer secondary plates with respect to one another. This may make the secondary insert more secure in a demounted state.

<FIG> schematically illustrate a side view of a part of a cryogenic cooling system in accordance with a third embodiment. Similar to the second embodiment (<FIG>) and unlike the first embodiment (<FIG>), the third embodiment comprises an adjustment member configured to adjust the separation between adjacent plates of an insert. <FIG> illustrates a portion of a secondary insert <NUM> in a demounted state, whereas <FIG> illustrates the portion of the secondary insert <NUM> in a mounted state, with the adjustment member in use. <FIG> depict a first inner secondary plate <NUM>, a second inner secondary plate <NUM>, a first inner primary plate <NUM> and a second inner primary plate <NUM>.

In <FIG>, the first inner secondary plate <NUM> is connected to the second inner secondary plate <NUM> by a secondary rod <NUM>. An upper secondary rod <NUM>' connects the first inner secondary plate <NUM> to the outer secondary plate (not shown). A lower secondary rod <NUM>" connects the second inner secondary plate <NUM> to the third inner secondary plate (not shown). Each of the secondary rods <NUM>, <NUM>', <NUM>" comprises a shoulder <NUM>, <NUM>" provided at the proximal end of each rod <NUM>, <NUM>', <NUM>" and adapted to receive a grub screw <NUM>, <NUM>'. The first inner primary plate <NUM> is connected to the second inner primary plate <NUM> by a primary rod <NUM>. An upper primary rod <NUM>' connects the first inner primary plate <NUM> to the outer primary plate <NUM> (not shown). A lower primary rod <NUM>" connects the second inner primary plate <NUM> to the third inner primary plate <NUM> (not shown).

<FIG> schematically illustrates a portion of the secondary insert <NUM> and the corresponding portion of the primary insert <NUM> when the secondary insert <NUM> is in a demounted state. Secondary holes <NUM>, <NUM>' are configured to align with primary holes <NUM>, <NUM>' when the secondary insert <NUM> is in a mounted state, with a fastening member extending between them. The primary holes <NUM>, <NUM>' and/or the secondary holes <NUM>, <NUM>' may be threaded, or may form clearance holes, for example where the fastening member is used in conjunction with a backing nut. In <FIG>, the separation between the first inner secondary plate <NUM> and the second inner secondary plate <NUM> in a demounted state, d<NUM>, is greater than the separation between the first inner primary plate <NUM> and the second inner primary plate <NUM>, d<NUM>, i.e. d<NUM> > d<NUM>. The relative lateral positioning in <FIG> is illustrative only, and is to show the vertical misalignment clearly. The misalignment is between the second inner secondary plate <NUM> and the second inner primary plate <NUM>.

In a demounted state, the first inner secondary plate <NUM> and the second inner secondary plate <NUM> are positioned on the shoulders <NUM>, <NUM>" of the secondary rod <NUM> and lower secondary rod <NUM>" respectively. A first grub screw <NUM> is positioned between the secondary rod <NUM> and the upper secondary rod <NUM>'. An upper portion of the secondary rod <NUM> and a lower portion of the upper secondary rod <NUM>' are tapped so as to engage with the first grub screw <NUM>. A second grub screw <NUM>' is positioned between the secondary rod <NUM> and the lower secondary rod <NUM>". An upper portion of the lower secondary rod <NUM>" and a lower portion of the secondary rod <NUM> are tapped so as to accommodate the second grub screw <NUM>'. It is this combination of the tapped portion of a secondary rod and the corresponding grub screw with which it engages that forms the adjustment member in this embodiment. In an alternative embodiment, the primary rods may be fitted with an adjustment mechanism as described for the secondary rods or both the primary rods and the secondary rods may be fitted with such adjustment mechanisms.

<FIG> schematically illustrates the portion of the secondary insert <NUM> and the corresponding portion of the primary insert <NUM> as illustrated in <FIG> when the secondary insert <NUM> is in a mounted state. The secondary holes <NUM>, <NUM>' and the primary holes <NUM>, <NUM>' are aligned, and the corresponding plates are thermally linked with a high thermal conductance. The separation between the first inner secondary plate <NUM> and the second inner secondary plate <NUM> is adjusted to align with the separation between the first inner primary plate <NUM> and the second inner primary plate <NUM>. In this embodiment, the misalignment is accommodated by separating the second inner secondary plate <NUM> from the shoulder <NUM>". In some embodiments this could be achieved by rotation of the secondary rod <NUM>. In the present embodiment the act of adjusting a fastening member extending through primary holes <NUM>' into the corresponding secondary holes <NUM>', lifts the second inner secondary plate <NUM> off the shoulder <NUM>". It should be appreciated therefore that unlike in the first and second embodiments, the adjustment members of the third embodiment facilitate movement of the second inner secondary plate <NUM> with respect to the secondary rod <NUM>, along the direction of the secondary rod <NUM>. Consequently, a thermalising shim <NUM> is positioned between the secondary rod <NUM> and the second inner secondary plate <NUM>. The thermalising shim <NUM> provides mechanical support and a thermal connection between the secondary rod <NUM> and the second inner secondary plate <NUM>, and will be further discussed in detail with reference to <FIG>.

<FIG> illustrates a cross-sectional view of a portion of a secondary insert plate according to the third embodiment as illustrated in <FIG>. Although the following describes the second inner secondary plate <NUM>, the description may apply to any of the inner secondary plates. <FIG> shows a second inner secondary plate <NUM>, a secondary rod <NUM> and a lower secondary rod <NUM>". There is a first threaded insert <NUM> arranged between the secondary rod <NUM> and the second inner secondary plate <NUM>. The first threaded insert <NUM> extends into the hollow secondary rod <NUM> at the proximal end and extends into the second inner secondary plate <NUM> at the distal end. A second threaded insert <NUM> is arranged between the lower secondary rod <NUM>" and the second inner secondary plate <NUM>. The second threaded insert <NUM> has a shoulder <NUM>" portion at its proximal end, which extends into the second inner secondary plate <NUM>. At its distal end the second threaded insert <NUM> extends into the hollow lower secondary rod <NUM>".

In this embodiment, the first threaded insert <NUM> and the second threaded insert <NUM> are threaded or tapped so as to accommodate the second grub screw <NUM>'. In alternative embodiments, the grub screw can be a set screw or any screw suitable for adjusting the separation between the secondary rod <NUM> and the lower secondary rod <NUM>". The first threaded insert <NUM> and the second threaded insert <NUM> are formed from a material with a high thermal conductivity at the operational base temperature of the relevant thermal stage, such as brass or copper. A thermalising shim <NUM> is again positioned between the secondary rod <NUM> and the second inner secondary plate <NUM>. This is also visible in <FIG>, which provides a perspective view of a portion of a demounted secondary insert <NUM> in accordance with the third embodiment. In <FIG>, experimental services are mounted to the secondary insert <NUM>. In particular, the experimental services shown are coaxial wires connected to the second inner secondary plate <NUM> and the first inner secondary plate <NUM>.

<FIG> schematically illustrates a cross-sectional view of a part of a cryogenic cooling system in accordance with the third embodiment, depicting the deformation of the thermalising shim <NUM> when the secondary insert <NUM> is in a mounted state. The inner secondary plates are moveable within the secondary insert <NUM> in the direction of the secondary rods to accommodate a misalignment. In <FIG>, the first inner secondary plate <NUM> is shown with two secondary rods <NUM> and two upper secondary rods <NUM>'. The corresponding primary plate is not shown for clarity.

A thermalising shim <NUM> connects the secondary rods <NUM>, <NUM>' to the first inner secondary plate <NUM>, providing mechanical stability to the arrangement when the first inner secondary plate <NUM> is moved along the secondary rods <NUM>, <NUM>'. In this embodiment, the thermalising shim <NUM> is formed from a material having a high thermal conductivity at the operational base temperature of the relevant thermal stage, such as brass or copper, and further provides effective thermalisation of the secondary rods <NUM>, <NUM>'. The thermalising shim <NUM> is configured to thermally couple the ends of the secondary rods <NUM>' to the inner secondary plate <NUM>. Advantageously, thermalisation of the secondary rods <NUM> and primary rods <NUM> at each thermal stage <NUM>-<NUM> reduces the time required to cool the cryogenic cooling system from room temperature to an operational base temperature. It also reduces any unwanted heat transfer between a warm end of the secondary insert and a cold end along the secondary rods <NUM>. This is achieved by increasing the thermal conductance between the secondary rods <NUM> and the secondary plates, in particular where relative movement between these components is possible.

The grub screw <NUM> has a radial protrusion around which the thermalising shim <NUM> is positioned. The outer holes in the thermalising shim <NUM> are slotted, allowing movement of the shim perpendicular to the secondary rods <NUM>, <NUM>' as indicated by the arrows. When positioned, the thermalising shim <NUM> is held in place between the first and second threaded inserts <NUM>, <NUM> by a clamping force. The thermalising shim is also fastened securely to the first inner secondary plate <NUM> using shim screws <NUM>. The thermalising shim <NUM> is flexible such that it maintains physical contact with the first inner secondary plate <NUM> and the secondary rods <NUM>, <NUM>', ensuring effective thermalisation of the secondary rods <NUM>, <NUM>' when the first inner secondary plate <NUM> is moved with respect to the secondary rods <NUM>, <NUM>'. This deformation of the thermalising shim <NUM> is visible in <FIG>.

<FIG> illustrates exemplary secondary inserts <NUM>', <NUM>", <NUM>‴ for use with primary inserts in accordance with the previous embodiments. In each case, a number of ports are shown which are axially aligned between plates. However, as shown the secondary insert can take a variety of forms. It may be advantageous to have more than one secondary insert wherein one of the secondary inserts has a different arrangement of ports. In this case, the same cryogenic cooling system could be used for more than one type of experiment, by switching one secondary insert configured with a first arrangement for another secondary insert configured with a second arrangement.

In further embodiments any combination of the adjustment members previously described may be used alone or in combination.

Claim 1:
A cryogenic cooling system comprising:
a primary insert (<NUM>) comprising:
a plurality of primary plates (<NUM>, <NUM>), each primary plate having a primary contact surface; and
one or more primary connecting members (<NUM>) arranged so as to connect the plurality of primary plates;
a demountable secondary insert (<NUM>) comprising:
a plurality of secondary plates (<NUM>, <NUM>), each secondary plate having a secondary contact surface; and
one or more secondary connecting members (<NUM>) arranged so as to connect the plurality of secondary plates (<NUM>, <NUM>) such that the secondary insert (<NUM>) is self-supporting;
a cooling apparatus attached to the secondary insert; and one or more adjustment members;
wherein the one or more adjustment members are configured such that, when the secondary insert is mounted to the primary insert, the adjustment members cause the primary and secondary contact surfaces of the respective primary and secondary plates to be brought into conductive thermal contact.