TITLE

"LOW TEMPERATURE HEATSINKING SYSTEM"

BACKGROUND OF THE INVENTION The need to have good heat sinking is a requirement in many low temperature systems, as heat flowing from high temperature needs to be intercepted and rejected before it can penetrate into even lower temperature regions. This is especially important in cryogen-free systems, where cooling is only available at one or two temperature points and there is no evaporating liquid cryogen to provide efficient cooling at all temperatures in between. Normally thermal connection to these special points can be achieved through the use of tight, permanently fitted, clamping bolts, but in situations where a part of the apparatus, such as an experimental insert, needs to be interchanged without warming up the entire apparatus, a different disconnectable heatsinking approach needs to be used.

For cryogen-free systems operating at temperatures above 1 Kelvin it is possible to extract heat through static helium exchange gas but in systems that run to temperatures of less than 0.1 Kelvin it is necessary to provide a high vacuum around the low temperature stage and still to be able to reject heat at 4 Kelvin. The thermal connection at this point (and at the cryocooler first stage temperature of 50-70K) therefore needs to be through some kind of mechanical contact that must have a low thermal resistance to prevent heat loads from raising the starting temperature significantly above the base temperature of the cryocooler. Even when the structure of the low temperature probe can be built from low thermal conductance materials, such as glass-fibre composite, it is still necessary to obtain a 4K heatsink thermal resistance of less than 0.1 Kelvin per mW (less than 0.01 K/mW for a system built from

stainless steel). For example, the heat leak between 77K and 4K down a single glass fibre spar can be as much as 2 mW and for SS tube it can be 10-15mW per tube

At 4K the thermal conductance between pressed metal surfaces is strongly dependent upon the contact force (See for example White and Meeson -

Experimental Techniques in Low-Temperature Physics, Oxford University Press, 4 ^th ed., (2002), p.97). In order to achieve the 0.1 K/mW contact resistance it is necessary to apply a force in excess of 450N (copper to copper contact) and greater than 800N (stainless steel to stainless steel contacts). Contacts between gold-plated surfaces can in principle achieve the same thermal contact resistance with much lower forces of 3ON, hence the reason why most heatsink spring contacts are gold plated, but gold has the disadvantage that it is very soft and easily wears. There are two conventional ways to mechanically heatsink a low temperature probe of this type - via springs that slide all the way down the inside of the vacuum can, or by having a combination of screw thread (50-70K usually) and cone recess (4K) cut on the inside of the vacuum can, into which the low temperature probe gets screwed.

The first of these techniques suffers from the fact that it is impossible to obtain the required high lateral forces needed on the springs without also producing very high frictional forces that would make it very difficult to insert the low temperature probe.

The cone and screw technique suffers from its sensitivity to differential thermal contraction, especially if the low temperature probe is made of low thermal conductivity materials, such as glass-fibre, that have large expansion coefficients - whilst the screw will remain fully mated, the cone may be pulled out of its heatsink housing.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, there is provided a heatsinking system comprising: a vessel having a contact surface on a portion of an inner surface thereof to be cooled to act as a heatsink; and a component receivable in an opening in the vessel, the component having a plurality of protrusions for engagement with the contact surface of the vessel; wherein when the component is received in the vessel such that the protrusions are located adjacent the contact surface, rotation of the component causes the protrusions to engage with and slide over the contact surface such that said engagement increases the radial contact force between the protrusions and the contact surface.

Typically, the vessel is generally cylindrical having the opening in an end thereof and the component is slid axially into the vessel such that the protrusions are located adjacent the contact surface. The rotation comprises rotation about the longitudinal axis of the vessel.

Preferably, the contact surface comprises a plurality of sections each corresponding to one of the protrusions and each section having a decreasing radius around the periphery thereof such that said rotation causing engagement of the protrusions with the contact surface is in the direction of decreasing radius of the sections of the contact surface. Preferably, each of the protrusions is generally dome shaped.

In one embodiment, each of the protrusions is separated radially from the body of the component and spring biased relative to the component such that initial rotation of the

component engages the protrusions with the contact surface and moves the protrusions against the spring bias until the protrusions engage with the body of the component, after which further rotation causes said increase in the radial contact force between the protrusions and the contact surface. The protrusions may be mounted on flexible arm members extending from the component. Alternatively, the protrusion may comprise separate members separated from the component by coil springs. Slots may be provided in the component into which are received the coil springs and the protrusions comprise dome headed pins received in the coil springs.

Preferably the contact surface comprises a heatsink ring secured to the inner surface of the vessel.

The contact surface and the protrusions may be made partly or fully of a material with lower thermal expansion than the contact surface.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a side cross sectional view of a heatsinking system in which a component is inserted into a vessel directly heatsunk to a reservoir of cryogenic liquid;

Figure 2 is a side cross sectional view of a heatsinking system in which the component is inserted into a vacuum can or loading tube indirectly heatsunk through exchange gas; Figures 3a and 3b are a top cross sectional views of a first embodiment of the present invention showing engagement between the component and the inner surface of the vessel for heatsinking;

Figures 4a and 4b are a top cross sectional views of a second embodiment of the present invention showing engagement between the component and the inner surface of the vessel;

Figures 5a and 5b are a top cross sectional views of a third embodiment of the present invention showing engagement between the component and the inner surface of the vessel;

Figure 6 is a perspective view of a fourth embodiment of the heatsinking system of the present invention.

DESCRIPTION OF THE INVENTION

Referring to the Figures, there is shown a heatsinking system 10 for heatsinking a removable component 12 in a low temperature system.

Figures 1 and 2 show embodiments of systems in which the present invention may be used. In Figures 1 and 2, the component 12 comprises a removable low temperature experimental insert to be cooled by a cryogenic liquid 14. In each case, the component 12 is received in a vessel 16 to which contact is to be made for the purpose of heatsinking the component 12. In the embodiment of Figure 1 , the vessel

12 is directly heatsunk to the reservoir of cryogenic liquid 14. In the embodiment of Figure 2, the vessel 16 is indirectly heatsunk through exchange gas.

In the first case the heatsinking point remains cold whenever the cryostat is cold. In the second case the heatsinking point can be warmed should the vacuum can or loading tube be removed from the cryostat, as it can be through a sliding seal or load- lock. This second approach produces a system that allows the experimental insert to be more easily removed from the main part of the cryostat, which can remain cold for

extended periods of time. The heatsinking system of the present invention however can be used in either situation.

The vessel 16 is generally cylindrical and the component 12 is inserted into an opening 22 in the end of the vessel 16 in an axial manner as shown in Figures 1 and 2. The component 12 includes a portion arranged to engage around the opening 22 when the component 12 is inserted fully into the vessel 16. The portion in the embodiment shown comprises a flange 24 that engages around the opening 22 via axial insertion of the component 12 into the vessel 16.

The heatsinking system 10 of the present invention comprises a plurality of protrusions 18 on the component 12 which are arranged to make contact with a contact surface 20 on the inner surface of the vessel 16. The contact between the protrusions 18 and the contact surface 20 provide the heatsinking of the component 12. In order to provide the low thermal resistance at low temperature, it is necessary to make the contact between the protrusions 18 on the component 12 and the heatsinking point on the contact surface 20 with a sufficiently high force (100-1000N).

When the component 12 is fully inserted into the vessel 16, the protrusions 18 are located adjacent the contact surface 20. The protrusions 18 are arranged such that rotation of the component 12 causes the protrusions 18 to come into contact with the contact surface 20.

In the embodiments shown in Figures 3 to 5, the protrusions 18 are generally dome shaped and a pair of protrusions are provided radially opposite on the component 12. The inner surface of the vessel 16 is generally cylindrical. The contact surface 20 comprises two sections, each of which has a radius that decreases around the periphery thereof in an anticlockwise direction. The decreasing radius is such that

anticlockwise rotation of the component 12 causes the protrusions to come into contact with the contact surface 20.

In the embodiment of Figure 3, the protrusions are integral structures on the component. Further rotation causes the protrusions 18 to slide across the contact surface 20 with a corresponding sharp increase in the contact force between the component 12 and the contact surface 20 of the vessel 16.

Such an arrangement results in a generally radial force directly through the body of the protrusion 18 to the component 12. A sufficient magnitude of contact force can be generated to provide suitable thermal contact and the frictional forces holding the protrusions 18 in engagement with the contact surface 20 so that further application of force to maintain the connection is not required. Also, the rotational only force (i.e. no axial movement of the component) makes preservation of the seal around the opening 22 easier. Further, the sliding contact made just as the thermal contact is made can reduce thermal contact resistance by breaking up surface oxide layers and producing small amounts of contact deformation that increases the effective microscopic contact area.

Figures 4 and 5 show alternative embodiments in which the protrusions are separated radially from the body of the component 12 and spring biased with respect to the component 12. In Figure 4, the protrusions 18 are supported from the component 12 by a flexible arm. In Figure 5, the protrusion is a separate member having a coil spring 32 mounted between the protrusion and the component 12. Therefore, in each of these embodiments, initial engagement of the protrusion 18 with the contact surface 20 causes movement of the protrusion 18 toward the component 12 against the spring force. Such engagement with the contact surface 20 does not provide

sufficient contact force for the required thermal resistance. However when the protrusion 18 engages directly with the body of the component 12, further rotation causes the sharp increase in force radially through protrusion 18 directly to the component 12 as described previously. -"

It will be appreciated that the embodiment of Figure 5, in which the protrusions 18 are separate members has the advantage that the protrusions 18 can be replaced when worn. Also, the protrusions can be made fully or partly of a lower thermal expansion material than the contact surface, thereby allowing greater lateral forces to be generated by differential thermal contraction when the system is cooled to cryogenic temperatures. Further, the experimental insert is relatively free to move vertically even in the thermally locked orientation, especially, but not exclusively, if the protrusions have rounded ends and are allowed to tilt by a small angle. This allows vertical differential thermal contraction between the component and the vessel to be taken up without losing thermal contact at any other heatsinking point in the system (unlike the thread and cone heatsink approach) or needing the heatsink pins to scrape vertically between probe and vacuum can (as in a system with heatsink springs), which generates internal vibrations in the experimental insert.

Figure 6 shows an embodiment of a heatsinking system similar to the embodiment shown in Figure 5. The protrusions 18 comprise three equally spaced, spring biased, dome-headed pins 34 mounted in slots 36 in a heatsink plate 38 to be mounted in the low temperature insert. The contact surface 20 comprises a heatsink ring 40 mounted by suitable means to the inner surface of the vessel 16.

In the embodiment shown, the heatsink ring 40 includes three surface sections each having decreasing radius for each of the pins 34 to act as described previously. The shallow rake of the sections of the heatsink ring provides a mechanical inclined-plane advantage that can produce the high lateral forces needed to obtain a low thermal resistance at the ring-pin and pin-plate interfaces. A reasonable rake of the tapered cutout is 1 in 10, at a radius of 25mm, and a reasonable torque to apply to the low temperature probe is 10Nm (2 x 5kg at 10cm radius) - which yields an ideal lateral force of 4000N at each pin

Modifications and variations as would be apparent to a skilled addressee are deemed to be within the scope of the present invention