GB2547720A - Thermal shield - Google Patents

Abstract

A cryostat 1 for use with a Scanning Probe Microscope (SPM) comprises a cryogen chamber 10 for containing a cryogen with a superconducting magnet 38 positioned therein. A radiation shield system 22 at least partially surrounds the cryogen chamber configured to insulate the cryogen chamber from external thermal energy thereby providing a shielded volume. The radiation shield system comprises a thermal mass 20 and a pre-cooling system 42 configured to be capable of cooling the radiation shield system to an initial temperature. During operation of the cryostat, the thermal mass is adapted to maintain the radiation shield system at a temperature of less than an upper temperature limit of 250 Kelvin (K) for a period of at least 72 hours when the initial temperature is 77 Kelvin when the temperature of the cryogen chamber is less than or equal to the initial temperature.

Description

Thermal shield

Field

The present invention relates to a cryostat for use with a Scanning Probe Microscope (SPM), in particular to cryostats used in providing a low temperature environment for SPM operation for an extended period.

Background

Cryostats are commonly used when an object or volume is required to be kept at a low temperature, such as below 100 Kelvin (K), for a period of time. For example, cryostats are used to keep superconducting magnets below their critical temperature, and are also used to keep a volume in which an experiment is to be conducted cold when this is required for the experiment.

Cryostats are therefore useful in scanning probe microscopy to keep a sample at a low temperature, thereby reducing the thermal energy of the sample by causing a reduction in atomic vibrations. This allows noise in data to be reduced and higher resolution images of the sample to be produced.

Cryostats are often required to have an operational temperature close to the boiling point of liquid helium (about 4.2 Kelvin at atmospheric pressure). There are a number of configurations that can be used to achieve this. However, usually such cryostats adhere to one of two general configurations.

The first general configuration is a cryostat that uses a single cryogen, such as liquid helium, held in a chamber, which is thermally shielded. This provides cooling of a target by use of the latent heat of vaporization of the helium, whereby the liquid helium remains at its boiling/condensation point, with the thermal energy received being used to progressively “boil off” the liquid helium. This process is prolonged by providing sufficient insulation to reduce the rate at which the helium boils.

This configuration has disadvantages, as the helium generally boils off quickly, meaning that a significant amount of helium, which is expensive, is required because the chamber needs frequent/constant refilling. Without replacing the helium, or providing an additional source of cooling power, there is often not sufficient time to conduct experiments or to keep a superconducting magnet operational. This also leads to increased thermal cycling and extended periods of “down time” when the apparatus is not operational, each of which is undesirable.

Accordingly, a second general configuration is used as an alternative. This uses two cryogens (usually helium and nitrogen). The helium is held in an inner chamber (analogous to that of the first configuration), which is surrounded by a further chamber holding nitrogen which acts as a radiation shield. Both cryogens are provided as liquids at appropriate respective temperatures.

By using this configuration, the helium takes longer to boil off during operation, as the nitrogen surrounding the helium absorbs heat energy from outside the cryostat and boils off in preference to the helium. This effect thermally insulates the chamber holding the helium. This allows experiments to be conducted over a longer period, as the helium boil-off rate is lower. This also means that to provide cooling for a set amount of time, the amount of helium required is reduced due to the reduced boil-off rate.

However, there is also a disadvantage to using the second general configuration. When cryogens boil, vibrations are caused by the gas bubbles that are created. These vibrations are similar to those produced by water boiling in a domestic kettle that cause the kettle to shake.

Liquid nitrogen is approximately four times more dense at its boiling point than liquid helium is at its boiling point. Accordingly, the liquid nitrogen causes significantly stronger vibrations than liquid helium. In a number of experiments, and in particular in scanning probe microscopy, keeping the vibrations, and therefore noise, to a minimum is a key factor. Therefore, the vibrations caused by the liquid nitrogen boiling in a cryostat using the second general configuration are undesirable.

Thus, there is a desire to reduce the vibrations generated in the cryostat during its operation whilst also prolonging the period of time in which the cryostat is able to maintain an acceptable temperature for conducting the desired operations.

Statement of Invention

According to a first aspect, there is provided a cryostat for use with a Scanning Probe Microscope (SPM), the cryostat comprising: a cryogen chamber for containing a cryogen, with a superconducting magnet positioned therein; a radiation shield system at least partially surrounding the cryogen chamber configured to insulate the cryogen chamber from external thermal energy thereby providing a shielded volume, the radiation shield system comprising a thermal mass; and a pre-cooling system configured to be capable of cooling the radiation shield system to an initial temperature, wherein during operation of the cryostat, in the absence of cooling applied from outside of the shielded volume, the thermal mass is adapted to maintain the radiation shield system at a temperature of less than an upper temperature limit of 250 Kelvin (K) for a period of at least 72 hours when the initial temperature is 77 Kelvin when the temperature of the cryogen chamber is less than or equal to the initial temperature.

Generally speaking, the cryostat provides a means of reducing the thermal flux received by the cryogen chamber by utilising high thermal impedances in the heat flux pathways leading to this chamber, together with providing a mechanism to absorb the thermal energy in another component (radiation shield system) so as to heat that system in preference to the cryogen chamber. Advantageously, this allows the cryostat to operate at a suitable temperature for a prolonged period of time by reducing the rate of cryogen boil-off due to the reduced rate of temperature increase of a cryogen chamber. Furthermore, there is no requirement to use (liquid) nitrogen when the cryostat is in use. This therefore reduces the vibrations generated in the cryostat, as there is no longer any nitrogen to boil when the cryostat is operated thereby entirely removing the vibrations that the use of (liquid) nitrogen produces.

The cryostat is therefore required to have the thermal capability stated. This represents the required thermal performance if the cryostat is used in practice in an external environment at standard temperature and pressure conditions and liquid nitrogen is used to cool the thermal mass and radiation shield to an initial temperature of 77K. Nevertheless, the cryostat may be able to achieve the specified thermal performance in an external environment at non-standard temperature and pressure conditions. It is of course convenient to use an initial temperature of 77K in practice since this represents the boiling point of liquid nitrogen at atmospheric pressure, which may be readily achieved. However, it is possible for the initial temperature used to be a temperature other than 77 Kelvin, as any pre-cooling of the highest temperature radiation shield would lead to lower helium boil off. This allows an energy store of cooling capacity to be developed that is not continuously cryogen driven.

In practice, 77K is the lowest initial temperature that is used, although it is possible for lower initial temperatures to be used. Higher initial temperatures may also be used, such as about 100K, about 120K, about 150K or about 200K. These initial temperatures can be achieved using gaseous nitrogen or a standard refrigerant, depending on the desired initial temperature.

Furthermore, the cryostat may be adapted to maintain the radiation shield system at a temperature between the initial temperature and the upper temperature limit for a period in excess of 72 hours. This may be achieved by the use of one or more of improved thermal shielding, a larger volume of cryogen or a larger thermal mass. Such periods in excess of 72 hours may be about 96 hours, about 168 hours or about 336 hours. This allows the cryogen chamber to maintain an operational temperature (and allows experiments to be conducted) over longer periods.

We note that the term “Scanning Probe Microscope” is intended to encompass all types of such microscopes, which may be operated at low temperatures within cryostats. In particular, these include atomic force microscopes (AFM) and scanning tunnelling microscopes (STM).

The radiation shield system may be cooled by any suitable cooling mechanism, such as by using a cryocooler (e.g. a pulse-tube refrigerator or a GM-refrigerator), a cold finger, or a Peltier cooler. These provide a cooling mechanism that produces a low heat load (for example, less than 10% of the heat load to which the cryostat as a whole is exposed during operation) or that can be decoupled after pre-cooling of the thermal mass and is not in use during operation of the cryostat. Typically however, the pre-cooling system comprises a cryogen conduit thermally coupled to the radiation shield system. This allows the radiation shield system to be cooled, using a cryogen (gas or liquid), to a known temperature quickly, and without large amounts of equipment and/or more complex means being required within the cryostat.

The cryogen conduit may be coupled to any part of the radiation shield system, although typically, the cryogen conduit is thermally coupled to the thermal mass. This allows direct cooling of the thermal mass.

The cryogen conduit is typically arranged to be evacuatable. This allows the cryogen to be used to cool the radiation shield system prior to operation of the cryostat, but ensures that no cryogen remains during the operation of the cryostat, so there is no chance of the cryogen boiling in the cryogen conduit causing vibrations in the cryostat.

The cryogen conduit may be adapted to carry any suitable cryogen. Typically, the cryogen conduit is adapted to carry nitrogen in gaseous, or preferably liquid form. This allows the radiation shield system to take the place of a standard nitrogen shield in a cryostat (eliminating the need for a nitrogen shield), thereby removing any vibrations caused by the boil-off of liquid nitrogen during operation of the cryostat.

The radiation shield system may comprise only the thermal mass, or may comprise one or more additional features. Typically, the radiation shield system further comprises a radiation shield to which the thermal mass is thermally coupled. This allows the thermal mass to be an efficient absorber of thermal energy whilst still allowing the cryogen chamber to be suitably insulated by a radiation shield.

The thermal mass may be located inside the radiation shield, or may be integrally formed with the radiation shield such that it is either inside the shielded volume or defines the boundary of the shielded volume. Typically however, the thermal mass is positioned outside the shielded volume. This allows the thermal mass to absorb thermal energy without it first penetrating the radiation shield, which further insulates the radiation shield and/or the shielded volume from any increase in temperature.

The thermal mass may be made of any material suitable for maintaining the radiation shield system at a temperature between the initial temperature and the upper temperature limit for the period set out above. Typically, the thermal mass is made of aluminium or an aluminium alloy. This is because aluminium has a relatively high heat capacity compared to the materials commonly used in cryostats, such as stainless steel. This allows a smaller mass of aluminium to be used to absorb the equivalent thermal energy than would be required for other materials because aluminium is a good conductor, and so allows a lower mass to be used than a stainless steel or other material thermal mass providing comparable effects whilst providing the same or improved thermal energy absorption over a stainless steel or other material thermal mass. .

The thermal mass may have a mass of at least 50 kilogrammes (kg). Larger masses are able to be used such as about 100kg, about 150kg, about 200kg, about 250kg or about 300kg. Larger thermal masses provide improved longevity of cooling, although are detrimental in terms of increased size, reduced portability and longer pre-cooling times.

The cryostat may further comprise a neck between the cryogen chamber and an exterior of the cryostat, wherein the thermal mass is thermally coupled to the neck. Having a neck allows the cryogen chamber to be accessible from outside the cryostat to allow further apparatus, such as an SPM, and samples for analysis to be placed therein. By having the neck thermally coupled to the thermal mass, thermal energy conducted along the neck from outside the cryostat is absorbed by the thermal mass, which stops it from being conducted further along the neck toward the cryogen chamber.

The neck may be entirely axially encircled by the thermal mass between the radiation shield and an exterior of the cryostat. However, typically the neck has a portion that extends from the thermal mass towards an exterior of the cryostat. As such, there is typically a physical separation between the thermal mass and the exterior of the cryostat. This separates the thermal mass from the exterior of the cryostat, which provides further insulation for the cryostat as the thermal mass absorbs thermal energy conducted along the neck, and not thermal energy at the exterior of the cryostat.

In use, the thermal mass isolates the other components of the cryostat from the heat load to which the cryostat as a whole is exposed. This is achieved by the thermal mass initially being cooler than the surrounding environment, and therefore the thermal mass absorbing thermal energy from the environment, as any local heating of a component is passed to the thermal mass by thermal conduction. Similarly, as the shielded volume is initially cooler in use than the thermal mass, the thermal mass loses thermal energy to the shielded volume. This causes an energy transfer from the environment, through the thermal mass into the shielded volume.

However, the amount of thermal energy absorbed by the thermal mass is greater than the amount of energy lost to the shielded volume by the thermal mass, as some of the thermal energy warms the thermal mass. The rate at which the thermal energy warms the thermal mass is referred to as the “thermal mass heat load”, and is roughly equivalent to the difference between the energy absorbed by the thermal mass (referred to as the “heat load”, which is the sum of all thermal energy to which the cryostat as a whole is exposed during operation) and the energy lost by the thermal mass.

Typically, the thermal mass is adapted to maintain the radiation shield system at a temperature between the upper temperature limit and the initial temperature for a period of at least 72 hours on exposure to a heat load of up to 30 Joules per second (J/s). This allows the radiation shield system to be maintained within the parameters defined when a conventional cryostat is used. When the heat load is less than 30 J/s and other variables are kept the same, such as the relative size of the thermal mass compared to the size of the other components of the cryostat, the period over which the radiation shield system can be maintained between the initial temperature and the upper temperature limit may be extended.

The magnitude of the heat load to which the cryostat, and therefore the thermal mass, is exposed is primarily determined by the size of the cryostat. For a bench-top system, the heat load will typically be about 5 J/s, whereas for a larger system, the heat load may be up to about 50 J/s.

Radiation is the primary type of energy contributing to the heat load. However, the contribution of radiation energy to the heat load decreases as the size of the cryostat decreases. This is due to the reduction in the available surface area upon which radiation can be incident. In any case, the cryostat according to the first aspect may be able to maintain the radiation shield system at a temperature between the initial temperature and the upper temperature limit for a period of at least 72 hours on exposure to a heat load of between about 5 J/s and about 50 J/s, between about 10 J/s and 40 J/s, between about 15 J/s and about 30 J/s or up to about 20 J/s.

The cryogen chamber may be adapted to contain a suitable cryogen. Typically, the cryogen chamber is adapted to contain helium, allowing the cryogen chamber to operate at temperatures at least as low as 4.2K. This enables experiments to be conducted that are susceptible to very small vibrations.

According to a second aspect, there is provided a method of shielding a cryogen chamber in a cryostat for use with an SPM, comprising the steps: cooling, with a pre-cooling system, a radiation shield system comprising a thermal mass to an initial temperature, wherein the radiation shield system at least partially surrounds a cryogen chamber for containing a cryogen to insulate the cryogen chamber from external thermal energy thereby providing a shielded volume, the cryogen chamber having a superconducting magnet positioned therein; and operating the cryostat, wherein in the absence of cooling applied from outside of the shielded volume, the thermal mass maintains the radiation shield system at a temperature of less than an upper temperature limit of 250K over a period of at least 72 hours when the initial temperature is 77K.

The cooling step may include the step of passing a cryogen through a conduit that is in thermal contact with the radiation shield system.

Preferably, the cooling step further includes the step of evacuating the cryogen from the conduit after the radiation shield system has been cooled to a predetermined temperature.

The cryostat used in the method of the second aspect may be a cryostat according to any combination of the features of the cryostat according to the first aspect.

Brief description of figures

An example of a cryostat is described in detail below, with reference to the accompanying figures, in which:

Figure 1 shows a cross-section view of a prior art cryostat;

Figure 2 shows a cross-section view of an example cryostat;

Figure 3 shows a partial sectional view of a portion of an example cryostat;

Figure 4 shows a perspective view of a portion of an example cryostat; and Figure 5 shows a flow diagram of steps carried out using an example cryostat.

Detailed description

The cryostats according to the invention referred to herein are used with scanning probe microscopes (SPM). By using these cryostats we have significantly reduced the vibrations that cause a deterioration in the quality of data produced by an SPM, in comparison with known cryostats.

An example of a prior art cryostat 100 is shown in Figure 1. This shows a central cryogen chamber in the form of helium chamber 102 adapted to contain helium when in use, in particular liquid helium. This helium chamber is held within another chamber, which is evacuated in use to limit heat transfer into the helium chamber as much as possible, and which is referred to hereafter as the “evacuatable chamber” 104.

The helium chamber 102 is suspended by insulating supports (not shown) to reduce thermal conduction between the walls 106 of the helium chamber, and the walls of the evacuatable chamber 104. The walls of the evacuatable chamber consist of two concentric cylinders 108a, 108b, which form the sidewalls of the chamber, and end plates 108c to which each of the concentric cylinders are welded.

There is a cavity 110 between the two concentric cylinders 108a, 108b. In use, this cavity is filled with (liquid) nitrogen. This enables the cavity to act as a thermal shield to insulate the interior of the evacuatable chamber 104 and the helium chamber 102 it contains.

The walls 108 of the evacuatable chamber 104 act as a radiation shield (also known as a vapour shield when the shield is cooled by cryogen vapour as it leaves a system using such a shield) to insulate the helium chamber 102. Various other configurations of radiation shield are known in the art.

The evacuatable chamber 104 is suspended inside an outer casing 112 of the cryostat 100 using supports (not shown) to ensure there is no area contact between the walls 106 of the evacuatable chamber and the outer casing of the cryostat.

The outer casing 112 provides an outer chamber 114 between the walls of the evacuatable chamber 104 and the outer casing. The outer chamber is evacuatable in use to insulate the chambers within it. This provides further insulation for the cryostat 100.

The outer chamber 114 and the evacuatable chamber 104 are in fluid communication with each other. The fluid communication is provided by a port 118 in the wall 108 of the evacuatable chamber, shown in Figure 1, in one of the end plates 108c of the wall. To allow the outer chamber and the evacuatable chamber to be evacuated, the outer casing 112 has a valve 116 through which gas can be removed. This allows gas to be removed from the outer chamber and evacuatable chamber simultaneously due to the port between the two chambers.

The helium chamber 102 and the thermal shield 110 each have a port 120, 122 through which the relevant cryogen (i.e. helium and nitrogen respectively) is able to be passed. Liquid cryogen is able to be passed through each port 120, 122 into the helium chamber and cavity 110 respectively in order to allow them to be filled. In use, gaseous cryogen is able to vent through the ports of the cryostat to relieve pressure in the relevant chambers. This gaseous cryogen is usually collected, particularly in the case of helium.

The helium chamber 102 contains a superconducting magnet 124. The superconducting magnet is formed so that there is a bore axially through its centre.

The bore through the superconducting magnet 124 is aligned with a neck 126 that extends (usually in an upward direction) from the helium chamber 102 to the exterior of the cryostat 100 through the wall 108c of the evacuatable chamber 104, and the outer casing 112. The neck passes through the wall 106 into the helium chamber. This allows the interior of the neck to open into the helium chamber at its (lower) end as shown in Figure 1, the exterior of which is sealed (at said lower end) to the helium chamber.

The neck is also sealable at its other (upper) end at the exterior of the cryostat 100 with a cover 128. The neck therefore provides access to the direct cooling provided by the helium chamber. As the neck opens into the helium chamber at one end, but is sealable at the opposing end, port 120 is also connected to the neck to allow the neck, as well as the helium chamber, to be vented during operation. This provides a path for any helium gas that collects in the neck when the cryogen is in use to exit the cryostat, preventing gas build-up in the neck.

It is common to cool SPMs to below the boiling point of helium at standard pressure, since such low temperatures further reduce vibrations in a sample being analysed. To achieve this, a dilution refrigerator is commonly used and which is capable of cooling samples to below 1K. A dilution refrigerator 130 is mounted on a removable insert 132 located in the neck 126 of the cryostat 100 (as shown in Figure 1) such that at least the portion of the dilution refrigerator that applies cooling power is located in the bore of the superconducting magnet 124 in the helium chamber 102 and is immersed in liquid helium in use. The SPM (not shown) is held in a vacuum chamber 134 located at that end of the dilution refrigerator where the dilution refrigerator applies cooling power.

Figure 1 therefore represents the known arrangements where a liquid nitrogen filled radiation shield is used to provide good thermal performance with the detrimental effect that the nitrogen boil off causes unwanted vibrations.

We have realised that, with appropriate design, it is possible to avoid the use of liquid nitrogen by providing a thermal mass fixed during operation instead, which gives the required thermal performance for SPM experiments whilst additionally reducing vibrations in the SPM significantly. This reduction in the magnitude of vibrations causes a considerable improvement in the quality of data produced by the SPM, since vibrations are a source of noise in the SPM data.

Unlike known arrangements that use liquid nitrogen filled radiation shields where the liquid nitrogen boils off and is vented out of a cryostat to maintain the temperature of a shielded volume inside the cryostat within an operational temperature range, the disclosed thermal mass is fixed during operation. By this, we intend that the thermal mass is of fixed mass during operation. This is because the thermal mass is free of liquid cryogen (such as liquid nitrogen or liquid helium), and so maintains the same mass throughout operation of the cryostat, since there is no liquid cryogen that is vaporised and vented (and therefore lost from the cryostat) during operation of the cryostat to maintain the shielded volume within the operational temperature range.

An example of a cryostat in which this thermal mass is used is shown in Figure 2. In this example, the cryostat 1 has a central cryogen chamber 10, the walls of which are defined by a vessel 11. This cryogen chamber is adapted to contain helium, in particular liquid helium. In principle, the cryogen chamber may be adapted to contain other cryogens, either in gaseous or liquid form, depending on the requirements of the cryostat. Helium is preferred since it provides the ability to achieve the lowest operational temperatures.

The cryogen chamber 10 is suspended inside another chamber 12 by means of supports (not shown). The supports are made out of an insulating material, such as fibreglass. The chamber 12 in which the cryogen chamber is suspended is evacuatable in use, and is referred to hereafter as the “first evacuatable chamber”.

The walls 14 of the first evacuatable chamber 12 are formed from a hollow cylinder 16 to which end plates 18 are attached, for example by a welded joint. The walls form a radiation shield that insulates the cryogen chamber 10 from external thermal energy in use. A thermal mass 20 is thermally coupled to the radiation shield 14. Together, the thermal mass and the radiation shield 14 form a radiation shield system 22 that is adapted to insulate the cryogen chamber 10 in use. Since, the inner region surrounded by the radiation shield system is insulated by the system, the region is referred to as “the shielded volume”.

In the example shown in Figure 2, the thermal mass 20 is located on top of the radiation shield 14 (i.e. on an outwardly facing upper surface of an end plate of the radiation shield). It would be possible to locate the thermal mass in another position, such as a bottom surface (i.e. an outwardly facing surface at the base of the radiation shield), or a side surface, and still have it thermally coupled to the radiation shield. In an alternative embodiment, instead of having a separate thermal mass, the radiation shield can be used as the thermal mass. This is achieved by the radiation shield having walls with a greater thickness than those of a conventional radiation shield, which increases the thermal mass of the radiation shield.

Typical aluminium radiation shields have a thickness of between about 0.2mm and about 2.0mm. This gives a general mass per square metre (m2) of between about 0.54kg/m2 and about 5.40kg/m2. To be an effective thermal mass, the shield would need to have a general mass per square metre of between about 20.00kg/m2 and about 60.00kg/m2, which equates to an aluminium radiation shield with a thickness of between about 7.4mm and about 22.2mm.

The radiation shield system 22 is suspended within an outer casing 24 of the cryostat 1. As with the cryogen chamber 10, the radiation shield system is suspended within the outer casing by supports (not shown). The outer casing of the cryostat forms an outer chamber 26 that is evacuatable in use, and is referred to hereafter as “the second evacuatable chamber”. This provides further thermal insulation for the cryogen chamber, and provides insulation to the radiation shield system.

The first and second evacuatable chambers 12, 26 are in fluid communication with each other. This fluid communication is provided by internal gaps and holes 28 that allow gas to pass between the various chambers. The chambers are able to be evacuated by removing gas through a valve 30 in communication with the second evacuatable chamber. The valve is accessible from outside the cryostat 1 and is connectable to a vacuum pump (not shown) for removing gas from each chamber.

The cryogen chamber 10 is able to be filled through a port 32 that provides communication between the chamber and the exterior of the cryostat 1. The port also allows gas to pass out of the cryogen chamber in use, so as to vent boiled off cryogen.

The cryogen chamber 10 has a neck 34 that extends (in this case vertically upwards) from the vessel 11 to the exterior of the cryostat 1 providing communication between the exterior of the cryostat and the cryogen chamber. The neck is able to be sealed with a cover 36 at the exterior of the cryostat. It is beneficial for the thermal mass 20 to be located adjacent the neck since the neck is a source of thermal load. As the neck is sealable and opens into the helium chamber, the neck needs to be vented to prevent gas from building up in the neck due to helium boil-off in the cryogen chamber. This is achieved by the neck having a connection to port 32 that vents the cryogen chamber so that gas can pass out of the cryostat from the neck through the port.

The cryogen chamber contains a superconducting magnet 38. This has an axial bore which passes through it and is aligned with the neck. The neck 34 allows equipment to be positioned in the bore to apply a desired magnetic field. An example field strength for the magnetic field produced by the superconducting magnet in use is 18 Tesla (T), and an example range of field strengths that can be applied is between about 1 T and about 32 T.

In the example shown in Figure 2, a dilution refrigerator 40 is shown positioned in the bore of the superconducting magnet 38 and attached to a removable insert 41 that is located in the neck in use. In use, an SPM (not shown) is located in a vacuum chamber 44 mounted to the dilution refrigerator located in the bore of the superconducting magnet.

In the example shown, it can be seen that the port 32 for the cryogen chamber 10 and the neck 34 pass through the thermal mass. This can also be seen from Figure 3.

It is of course possible to have an arrangement whereby the port 32 and/or the neck 34 are accessible from the exterior of the cryostat 1 without passing through the thermal mass 20. However, there are advantages to at least one of these features passing through the thermal mass. For example, when the neck is directly thermally coupled to the thermal mass, this allows the thermal mass to absorb thermal energy that is conducted from the exterior of the cryostat towards the cryogen chamber along the neck when in use. Thus the thermal mass 20 sits along the path of heat flow and is positioned so as to intercept it. This reduces the amount of thermal energy reaching the cryogen chamber through the neck.

It is preferred therefore, as in the present example, that the port 32 for the cryogen chamber 10 is thermally coupled to the thermal mass 20 directly (by virtue of them passing through the thermal mass), so that thermal energy passing along them from the exterior of the cryostat 1 is absorbed by the thermal mass. As conventionally the neck 34 and port(s) extend upwardly from the chamber with which they communicate, the thermal mass shown in Figure 2 provides the best thermal performance when located on an upper surface of the radiation shield 14.

In use, and before the SPM is used to analyse a sample, the radiation shield system 22 is cooled to an initial low temperature by a pre-cooling system 42. The pre-cooling system comprises a cryogen conduit 42a that is thermally coupled to the radiation shield system. The cryogen conduit is adapted to allow a cryogen (which may be a gas or liquid) to pass through it. In the present example liquid nitrogen is used. The cryogen conduit is supplied with cryogen through pipes 42b that extend from the radiation shield system to the exterior of the cryostat 1 and allow cryogen to be supplied as a continuous flow from the exterior of the cryostat through the cryogen conduit, until the radiation shield system has achieved a desired initial low temperature marking the completion of the pre-cooling.

In use, once the pre-cooling has concluded, the cryogen conduit 42a, and indeed the entire precooling system 42, is able to be evacuated, so that effectively no cryogen remains in the precooling system while the SPM is in operation. This evacuation is achievable by use of a vacuum pump (not shown) and valves 42c (shown in Figure 3 and Figure 4) on the pipes 42b that supply the cryogen conduit.

In the example shown in Figure 4, the cryogen conduit 42a is shown as being formed in a spiral on an upper surface of the thermal mass 20. This allows cooling to be applied to the entire circumference of the thermal mass.

Other arrangements of the cryogen conduit are possible, which may lead to quicker cooling of the thermal mass, and the radiation shield system 22. For example, the cryogen conduit may be arranged in a lattice structure so that a greater surface area of the thermal mass and/or radiation shield system is covered by the cryogen conduit. Alternatively, the cryogen conduit may be embedded in the thermal mass and/or the radiation shield system, allowing it to be cooled from within.

The cryogen conduit 42a may comprise a chamber formed of two concentric cylinders with sealed ends that form the sidewalls of the radiation shield. In this arrangement, to cool the radiation shield system, the cryogen conduit is filled with a cryogen, such as liquid nitrogen, to cool the radiation shield system. The conduit is then evacuated, and the cryostat is then able to operate with the desired low vibration function.

The valves 42c used to seal the cryogen conduit 42a and the pipes 24b can be seen in the example shown in Figures 3 and 4. To seal the cryogen conduit, the valves are closed. In figures 3 and 4, the valves shown have taps that are able to be turned so as to open and close the valves. However, other valves would also be suitable.

Figures 3 and 4, which show the example cryostat shown in Figure 2, show that the thermal mass 20 is positioned on an upper surface of the radiation shield 14, and the neck 20 and various ports 32 and/or valves 30 for the components contained within the radiation shield pass through the thermal mass from the interior of the shielded volume to the exterior of the cryostat, in an upper plate 44 which is shown in these figures.

Additionally, Figure 3 and Figure 4 show that there is a gap between the upper plate 44, which is part of the exterior of the cryostat 1, and an upper surface of the thermal mass 20. This provides the thermal mass with insulation from external thermal energy.

Figure 4 shows that the thermal mass 20 is generally cylindrical in shape. As an example of the dimensions of the thermal mass, this may have a diameter of 750 millimetres (mm), and a thickness/height of 150 mm. Of course, the dimensions of the thermal mass vary in accordance with the space available inside the cryostat, the material used for the thermal mass, and a number of other criteria.

The thermal mass 20 is made of aluminium (by this we mean either “pure” aluminium or an aluminium alloy, which is an alloy of various metals where aluminium is the largest constituent of the alloy), and has a mass of 150 kilogrammes (kg). When the thermal mass has a mass of 150kg, at a temperature of 77K, the thermal mass has an enthalpy of around 8.31 kilojoules per kilogramme (kJ/kg), equivalent to around 1247kJ for the total mass. At 200K this increases to about 84.80kJ/kg, which is equivalent to about 12720kJ for the total mass. This gives a thermal energy “cold” storage capacity of about 11473kJ.

When a heat load of 20 joules per second (J/s), that is 20 Watts (W), is applied, this would take about 159 hours (equivalent to about 6.6 days) for the thermal mass of 150kg of aluminium to warm from 77K to 200K, assuming a constant heat load of 20W and that the cryogen chamber is at a temperature of up to 77K. During this time, the radiation and thermal conduction heat loads in the cryogen chamber will be suppressed, and the helium boil-off will be much lower than a system without this thermal mass.

As the heat load is the sum of the thermal energy to which the cryostat as a whole is exposed during operation, the thermal mass heat load will be lower, as this takes into account the amount of energy lost from the thermal mass to the cryogen chamber. As such, it is unlikely that the thermal mass will be exposed to a thermal mass heat load of 20J/s once the cryostat is in operation, as this corresponds to the heat load from the environment surrounding the cryostat to which the cryostat is exposed.

Instead, the thermal mass heat load will be lower. However, when the thermal mass (and consequently the radiation shield) is at a temperature of 77K, thermal energy loss from the thermal mass to the cryogen chamber is about 0.05W (0.05J/S), so is almost negligible.

When the thermal mass and radiation shield warm to temperatures of greater than or equal to 100K, the thermal energy loss from the thermal mass to the cryogen chamber increases rapidly as the temperature increases. Due to helium vapour cooling of the cryogen chamber that occurs until a natural equilibrium is reached between the temperature of the thermal mass and radiation shield and the temperature of the cryogen chamber, the exact thermal energy loss from the thermal mass to the cryogen chamber, and thereby the thermal mass heat load, is difficult to predict.

Example thermal mass heat loads of efficient cryostats are 14W at around 77K, which drops to around 12W at 200K. Accordingly, the period of time it takes the thermal mass to reach 200K from 77K will be extended as the amount of thermal energy absorbed by the thermal mass is reduced. The thermal mass heat load decreases as the cryostat warms because it is gradually approaching thermal equilibrium, at which point the thermal mass heat load tends to zero.

The radiation shield system can be cooled below 77K, however the relationship between thermal capacity and temperature for aluminium is such that the thermal capacity reduces significantly below 150K and particularly below 100K, reaching almost zero at around 20K. Accordingly, whilst the benefit of keeping the cryogen chamber cool for an extended period of time without the use of a liquid nitrogen shield is still achieved, the period is not significantly extended by cooling the thermal mass below about 100K.

The radiation shield to which the thermal mass is thermally coupled is usually made of aluminium or copper, and the cryogen chamber is usually made of stainless steel. This is because the radiation shield should be a material with a high conductivity so that any heat absorbed by the shield is spread across the shield; and the cryogen chamber should have a lower conductivity, as the cryogen chamber is in contact with liquid helium in use and hot spots in the chamber raise the rate of boil-off less than if the chamber has a higher overall temperature.

As mentioned, in an alternative example, the radiation shield system may comprise a thermal mass with two concentric cylinders sealed at each end by a plate with a cavity between the cylinders. Such a radiation shield may be made of aluminium (notably distinct from stainless steel which has a much lower heat capacity), and may have a mass of at least 100kg. To cool this radiation shield system a pre-cooling system is used that comprises filling the cavity with a cryogen (usually liquid nitrogen) and allowing the radiation shield system to cool by thermal conduction. The cryogen can then be evacuated from the cavity. As for the previous example, this radiation shield will warm to a temperature less than 250K from an initial temperature of 77K in a period longer than 72 hours with a 20W heat load.

When aluminium is used for such a radiation shield system, care needs to be taken, as eddy currents can be induced in the aluminium by the magnetic field should the field quench causing the magnetic field to breakdown. This can cause an aluminium radiation shield to deform because the eddy currents cause heating in the radiation shield and generate forces in the radiation shield due to coupling with the magnetic field as the magnetic field breaks down. The use of thicker radiation shields can reduce the likelihood of the radiation shield deforming, but eddy currents can cause damage to the cryogen chamber as any movement or deformation of the shields can cause the magnet to move towards the centre of the shield. Further, the use of aluminium inside the magnet is avoided because in very low temperature areas, such as those at about 0.1 K, eddy currents produced when the magnet is energised can cause unwanted heating.

Figure 5 shows an example process used to cool the cryostat. In use, the evacuatable chambers of the cryostat are firstly evacuated (S1).

The pre-cooling system is then used to cool the radiation shield system (S2). This may be achieved by passing gaseous or liquid cryogen through the cryogen conduit. In this case the cryogen is liquid nitrogen, which cools the radiation shield system to 77K by thermal conduction.

The cryogen chamber is then filled with cryogen (S3). In this case, the cryogen is liquid helium. After this, the pre-cooling system is evacuated (S4), and the dilution refrigerator, which is already located in the bore in the superconducting magnet, is used to cool the SPM to an operating temperature (S5). Typical operating temperatures may be up to 0.1 K, 50 milliKelvin (mK) or lower.

The superconducting magnet is then switched on (S6), and the SPM is then able to be operated (S7) in low vibration conditions in a magnetic field produced by the superconducting magnet. An SPM experiment is then run while the cryostat operates (S8). During the experimental run time, the shielded volume, , warms up, which increases helium boil off, as cooling power is no longer being applied to the shield.

In an alternative embodiment, the SPM can be loaded into the cryogen chamber, the evacuatable chambers can then be evacuated, followed by the thermal mass being pre-cooled and evacuated with the cryogen chamber being filled last before the SPM experiment is run.

Claims (18)

1. A cryostat for use with a Scanning Probe Microscope (SPM), the cryostat comprising: a cryogen chamber for containing a cryogen, with a superconducting magnet positioned therein; a radiation shield system at least partially surrounding the cryogen chamber configured to insulate the cryogen chamber from external thermal energy thereby providing a shielded volume, the radiation shield system comprising a thermal mass; and a pre-cooling system configured to be capable of cooling the radiation shield system to an initial temperature, wherein during operation of the cryostat, in the absence of cooling applied from outside of the shielded volume, the thermal mass is adapted to maintain the radiation shield system at a temperature of less than an upper temperature limit of 250 Kelvin (K) for a period of at least 72 hours when the initial temperature is 77 Kelvin when the temperature of the cryogen chamber is less than or equal to the initial temperature.

2. The cryostat according to claim 1, wherein the pre-cooling system comprises a cryogen conduit thermally coupled to the radiation shield system.

3. The cryostat according to claim 2, wherein the cryogen conduit is thermally coupled to the thermal mass.

4. The cryostat according to claim 2 or claim 3, wherein the cryogen conduit is evacuatable.

5. The cryostat according to any one of the preceding claims, wherein the radiation shield system further comprises a radiation shield to which the thermal mass is thermally coupled.

6. The cryostat according to claim 5, wherein the thermal mass is positioned outside the shielded volume.

7. The cryostat according to any one of the preceding claims, wherein the thermal mass made of aluminium or an aluminium alloy.

8. The cryostat according to any one of the preceding claims, wherein the thermal mass has a mass of at least 50 kilogrammes (kg).

9. The cryostat according to any one of the preceding claims, further comprising a neck between the cryogen chamber and an exterior of the cryostat, wherein the thermal mass is thermally coupled to the neck.

10. The cryostat according to claim 9, wherein the neck has a portion that extends from the thermal mass towards an exterior of the cryostat.

11. The cryostat according to any one of the preceding claims, wherein the thermal mass is adapted to maintain the radiation shield system at a temperature between the upper temperature limit and the initial temperature for a period of at least 72 hours on exposure to a heat load of up to 30 Joules per second (J/s).

12. The cryostat according to any one of the preceding claims, wherein the cryogen chamber is adapted to contain helium.

13. A cryostat substantially as described herein, with reference to and as illustrated in the accompanying drawings Figure 2 to Figure 4.

14. A method of shielding a cryogen chamber in a cryostat for use with an SPM, comprising the steps: cooling, with a pre-cooling system, a radiation shield system comprising a thermal mass to an initial temperature, wherein the radiation shield system at least partially surrounds a cryogen chamber for containing a cryogen to insulate the cryogen chamber from external thermal energy thereby providing a shielded volume, the cryogen chamber having a superconducting magnet positioned therein; and operating the cryostat, wherein in the absence of cooling applied from outside of the shielded volume, the thermal mass maintains the radiation shield system at a temperature of less than an upper temperature limit of 250K over a period of at least 72 hours when the initial temperature is 77K.

15. The method according to claim 14, wherein the cooling step includes the step of passing a cryogen through a conduit that is in thermal contact with the radiation shield system.

16. The method according to claim 15, wherein the cooling step further includes the step of evacuating the cryogen from the conduit after the radiation shield system has been cooled to a predetermined temperature.

17. The method according to any one of claims 14 to 16, wherein the cryostat is a cryostat according to any one of claims 1 to 13.

18. A method substantially as described herein, with reference to and as illustrated in the accompanying drawing Figure 5.