WO2005015075A1

WO2005015075A1 - Cryostat

Info

Publication number: WO2005015075A1
Application number: PCT/GB2004/003303
Authority: WO
Inventors: David Teehan
Original assignee: Council For The Central Laboratory Of The Research Councils
Priority date: 2003-08-02
Filing date: 2004-07-30
Publication date: 2005-02-17
Also published as: GB0318147D0

Abstract

A cryostat comprises a tank thermally coupled to a sample holder, for boiling of a liquid to a gas for cooling of the sample holder. A liquid conduit is connected to the tank for providing the liquid to the tank. A gas outlet conduit is connected to the tank for removal of the gas from the tank. A jacket substantially encloses the sample holder. The gas outlet conduit comprises a heat exchanger for cooling of the jacket using the gas. The heat exchanger extends around the liquid conduit, at the position the liquid conduit passes through an aperture in the jacket.

Description

CRYOSTAT

The present invention relates to cryostats, and in particular cryostats that utilise the boiling of a liquid to a gas for cooling of a sample. A cryostat is an apparatus for maintaining a low temperature e.g. a temperature less than 0° Celsius. Cryostats are typically cooled by the boiling of a low temperature liquid into a gas. For instance, cryostats can utilise liquid nitrogen, which boils into gaseous nitrogen at -196°C. However, liquid helium is the standard coolant for devices operating at very low ("cryogenic") temperatures. Helium changes from the liquid to the gaseous phase at approximately -269°C. Figure 1 shows a cross section view of a typical helium cryostat 100. The cryostat 100 includes a tank 110 in thermal contact with a sample holder 114. In use, a sample is mounted upon the sample holder 114, and cooled by the cryostat 100. This cooling occurs by liquid helium 112 within the tank boiling to gaseous helium 130. Helium 120, 130 enters and leaves the cryostat 100 via the transfer line 104. Liquid helium 120 can be seen entering the transfer line 104 at the top right hand side of the diagram. Typically, the liquid helium 120 will be taken from a dewar containing liquid helium. The helium 120 enters the transfer line 104 via a conduit 122, commonly referred to as a capillary tube. The liquid helium 120 from conduit 122 is subsequently forced to enter a second conduit 124 in the cryostat 100, arranged to take the helium 120 to the tank 110 where it boils, cooling the sample. A gas outlet conduit 132 is connected to the tank 110, for removal of the gaseous helium 130 from the tank 110. Typically, the gas conduit 132 is of greater diameter than the inlet liquid conduit. The gas outlet conduit subsequently transfers the helium gas 130 to the gas return conduit 136 in the transfer line 104 via the internal volume 134 of the cryostat body 102. A seal 126 isolates the inlet flow and the return tubes, forcing the helium to flow through the cryostat circuit. A further seal 138 at the top of the cryostat body 102, between the cryostat body 102 and the transfer line 104, prevents the helium gas escaping from inside the cryostat body 134. A pump (not shown) acting on the helium gas return line 136 acts to draw the helium gas 130 through the gas return line 136, thus causing the flow described above. Importantly, the seal 138 also prevents air entering the helium circuit (this would interfere with the action of the pump in drawing helium through the cryostat). The returned gaseous helium 130 is usually recovered and stored (possibly to be re- liquefied and re-used), or simply vented into the atmosphere. Within the transfer line 104,the incoming liquid helium 120 is insulated from the outside world by the returning helium gas 130, with the return tube 136 surrounding the inlet conduit 122. Typically, the gas return tube 136 is in turn surrounded by several layers of reflecting/insulating film and a vacuum 137, to further insulate the helium from the ambient conditions. Normally, the cryostat is placed within a vacuum chamber 10. The vacuum (or at least very low pressure) 12 within the chamber 10 minimises the effects of heat being transferred from the ambient surroundings to the tank 110 and the sample holder 114. The cryostat 100 and the transfer line 104 enter the vacuum chamber 10 via an aperture 14. A seal 16 around the aperture 14 ensures an airtight fit between the aperture 14 and the cryostat body 102. Frequently, a jacket 140 in the form of a radiation shroud substantially encloses and shields the tank 110 and the sample holder 114 (and hence the sample when located on the sample holder) from the warmth (i.e. infra-red radiation) of the surrounding experimental chamber, thus helping to reduce the temperature of the sample. The amount of heat received by the sample via radiation from the surroundings (e.g. the walls of the vacuum chamber at room temperature, or a radiation shroud at a lower temperature) varies as the temperature of the surroundings T raised to the power of 4 (T⁴). Typically, the system shown in Figure 1 will cool the shroud to around -50°C. Typically, the radiation shroud will thus reduce the radiated heat reaching the sample by up to 67%. Typically, the shroud is mounted on (or at least coupled to) the cryostat body 102, such that gaseous helium 130 within the cryostat body 102 will slightly cool the shroud 140. If yet further cooling of the shroud is desirable, then it is known to provide a flow of liquid nitrogen to a tank thermally connected to the shroud, in order to further cool the shroud. Whilst liquid nitrogen can be utilised to cool the radiation shroud, it is a relatively inefficient way of cooling the shroud. In particular, an additional liquid supply must be provided and controlled, along with additional transfer lines. GB 1,210,624 describes how the radiation shroud can be cooled by providing an additional heat exchanger that utilises the return gas to cool the shroud. In other words, in the case of a helium cryostat, the helium gas removed from the tank is utilised to cool the shroud. It is an aim of embodiments of the present invention to provide an improved cryostat, that substantially addresses one or more of the problems of the prior art, whether referred to herein or otherwise.

According to a first aspect, the present invention provides a cryostat comprising: a tank thermally coupled to a sample holder, for boiling of a liquid to a gas for cooling of the sample holder; a liquid conduit connected to the tank for providing the liquid to the tank; a jacket arranged to substantially enclose the sample holder; and a gas outlet conduit connected to the tank for removal of the gas from the tank, the gas outlet conduit comprising a heat exchanger arranged to cool the jacket using the gas, wherein the heat exchanger extends around the liquid conduit at the position the liquid conduit passes through an aperture in the jacket. The jacket may be a radiation jacket for shielding of the sample holder from ambient radiation. The heat exchanger may comprise a discrete conduit enclosed within the gas outlet conduit. The heat exchanger may be arranged to direct gas adjacent to the internal periphery of the jacket. The heat exchanger may comprise a coiled conduit. The heat exchanger may be connected to the jacket. The liquid and the gas may comprise helium. The jacket may be mounted on the heat exchanger. The cryostat may be arranged to be located within a vacuum chamber, with the liquid inlet conduit and the gas outlet conduit extending from the chamber via an aperture with an air tight seal, the heat exchanger being located closer to the tank than to the aperture. The cryostat may comprise a seal utilised to separate the liquid conduit and the gas outlet conduit, the heat exchanger being located between the tank and the seal. The heat exchanger may be located within 20cm of the tank. According to a second aspect, the present invention provides a method of manufacturing a cryostat, the method comprising the steps of providing a tank thermally coupled to a sample holder, for boiling of a liquid to a gas for cooling of the sample holder; providing a liquid conduit connected to the tank for providing the liquid to the tank; providing a jacket arranged to substantially enclose the sample holder; and providing a gas outlet conduit for removal the gas from the tank, the gas outlet conduit comprising a heat exchanger arranged to cool the jacket using the gas, wherein the heat exchanger extends around the liquid conduit at the position the liquid conduit passes through an aperture in the jacket. Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional view of a known cryostat; Figure 2 is a cross-sectional view of a cryostat in accordance with a first embodiment of the present invention; Figure 3 is a 3- dimensional exploded perspective view of a heat exchanger for use in the cryostat illustrated in Figure 2; and Figure 4 is a cross-sectional view of a cryostat in accordance with a second embodiment of the present invention. The present inventor has realised that the performance of a cryostat incorporating an additional heat exchanger to allow the return gas to cool the shroud can be further improved. In particular, by placing the heat exchanger so that it extends around the liquid conduit at the point which the liquid conduit passes through the shroud, the undesirable heat gain by the liquid conduit from the shroud is reduced. Consequently, liquid travelling along the liquid conduit can be utilised to cool the sample to a lower temperature. Preferably, both the heat exchanger and the liquid conduit pass through the same aperture in the shroud, with the liquid conduit not being in direct thermal contact with the shroud. If the tank 110 and the sample plate 114 are regarded as the first, primary heat exchanger, then the invention consists in the positioning of a secondary heat exchanger utilising the return gas to cool the shroud. In the case of a helium cryostat, this is particularly advantageous, as the return gas can act to cool the shroud down to approximately -250°C i.e. a much lower temperature than is achievable by cooling using liquid nitrogen (which boils at -196°C). Figure 2 shows a cryostat 200 in accordance with a first embodiment of the present invention. Identical reference numerals are used in this and successive figures to represent similar features to those shown in Figure 1. In this particular embodiment, the heat exchanger 250 is added to a standard cryostat at the point where the helium gas 130 is returned to the transfer line 104. In particular, the heat exchanger 250 is connected to the end of (and thus forms a part of) the gas outlet conduit 132. It will be seen that the cryostat 200 is generally similar to the cryostat 100 illustrated in figure 1. However, in this particular embodiment, the gas outlet conduit 132 comprises a heat exchanger 250. The heat exchanger 250 is arranged to direct gas adjacent to the internal periphery of the radiation shroud or jacket 140, and thus to cool the radiation jacket 140. In this particular embodiment, the heat exchanger 250 is in direct contact with the radiation jacket 140, though it will be appreciated that the heat exchanger 250 merely has to be thermally coupled to the radiation jacket 140. Figure 3 illustrates an exploded schematic view of the heat exchanger 250. The heat exchanger 250 in this embodiment takes the form of a ring or spool 252. Liquid helium 120 will flow unimpeded through the centre of the ring 250 via the normal liquid conduit 124. At least the outer periphery of the ring is formed of a thermally conductive material, such as stainless steel. A channel 254 is machined into the ring 252 to direct the relatively cold helium gas 130 returning from the tank 110 to circulate around it, close to the outer wall of the ring 252 i.e. close to the radiation shroud 140. The ring or spool 252 of the heat exchanger extends around the normal liquid conduit 124. The heat exchanger 250 in this embodiment is coaxial with the ring 252 of the heat exchanger 250. As both the heat exchanger 250 and the liquid conduit 124 enter the radiation jacket 140 via a common aperture in the surface of the jacket, the ease with which the jacket can be removed is greatly improved compared with prior art designs incorporating additional heat exchangers. Additionally, as the liquid conduit 124 passes through the coil provided by the heat exchanger 250, the aperture within the radiation shroud 140 can be of minimal size. Such a design improves the compactness of the overall cryostat design. It is envisaged that the radiation shroud is typically bolted onto a flange clamped to the heat exchanger 250. In such a configuration, the temperature of the radiation shroud has been measured at approximately -250°C, which is cooler even than that achievable by cooling the shroud with liquid nitrogen (i.e. to approximately -197°C) but without the inconvenience. This additional heat exchanger may be mounted between the tank 110 and the cryostat body 102. The above embodiment illustrates how a standard "straight through" cryostat can be adapted by providing an additional heat exchanger for the return gas. Preferably, the additional heat exchanger is located between the tank 110 and the seal 126 separating the inlet flow and the return tubes. More preferably, the heat exchanger is mounted adjacent to, or at least in the proximity of, the tank 110 and the sample holder 114. For instance, the heat exchanger may be mounted within 20cm of the outside adjacent surface of the tank 110. Figure 4 shows such an embodiment, with the heat exchanger 350 mounted adjacent to the tank 110. In particular, the heat exchanger 350 is mounted in series with the return line 132 by breaking into the return line 132 above the helium tank 110, such that the heat exchanger comprises part of the return line 132. The radiation shroud 340 is mounted on the heat exchanger 350, and extends around and substantially encloses both the tank 110 and the sample holder 114. Such an arrangement allows a smaller shroud 340 to be utilised, thus decreasing the total surface area which the heat exchanger 350 has to cool. It will be appreciated that the above embodiments are described by way of example only, and that various alternatives will be apparent to the skilled person as falling within the scope of the present invention. For instance, whilst the jacket has been described in terms of a radiation shroud, it will be appreciated that any cover that substantially encloses at least the sample holder could be utilised. For instance, a jacket could be provided arranged to be specifically cooled by the additional heat exchanger, with the jacket enclosing both the tank and the sample holder. An additional radiation shroud surrounding this jacket might also then be provided. It will also be appreciated that the heat exchanger could take any form that is suitable for arranging for the return gas to cool the jacket or shroud. For instance, whilst the above heat exchanger has been described as being formed by a channel within a ring, it will be appreciated that a helical heat exchanger could be utilised, or indeed any coiled conduit arranged for directing the gas flow adjacent to the jacket or shroud. Equally, whilst the heat exchanger 250 has been described as being formed of stainless steel, it will be appreciated that the heat exchanger may comprise any thermally conductive material. Compared with many metals, stainless steel is a relatively poor thermal conductor. However, based on the performance of the prototype, stainless steel appears to provide the performance to suit most applications. If improved performance is desired, then a better thermal conductor could be utilised e.g. OFHC (Oxygen Free High Conductivity) copper, which is a relatively good thermal conductor compared with many metals. Whilst the above embodiments have been described in terms of a helium cryostat, it will be equally appreciated that the present invention could be utilised for any cryostat that has a relatively cold return gas flow.

Claims

1. A cryostat comprising: a tank thermally coupled to a sample holder, for boiling of a liquid to a gas for cooling of the sample holder; a liquid conduit connected to the tank for providing the liquid to the tank; a jacket arranged to substantially enclose the sample holder; and a gas outlet conduit connected to the tank for removal of the gas from the tank, the gas outlet conduit comprising a heat exchanger arranged to cool the jacket using the gas, wherein the heat exchanger extends around the liquid conduit at the position the liquid conduit passes through an aperture in the jacket.

2. A cryostat as claimed in claim 1, wherein the heat exchanger extends in a coil around the liquid conduit.

3. A cryostat as claimed in claim 1 or claim 2, wherein the heat exchanger is coaxial with the liquid conduit.

4. A cryostat as claimed in any one of the above claims, wherein the heat exchanger and the liquid conduit pass through the aperture in the jacket.

5. A cryostat as claimed in any one of the above claims, wherein the jacket is a radiation jacket for shielding of the sample holder from ambient radiation.

6. A cryostat as claimed in any one of the above claims, wherein the heat exchanger comprises a discrete conduit enclosed within the gas outlet conduit

7. A cryostat as claimed in any one of the above claims, wherein the heat exchanger is arranged to direct gas adjacent to the internal periphery of the jacket.

8. A cryostat as claimed in any one of the above claims, wherein the heat exchanger comprises a coiled conduit.

9 A cryostat as claimed in any one of the above claims, wherein the heat exchanger is connected to the jacket.

10. A cryostat as claimed in any one of the above claims, wherein the liquid and the gas comprise helium.

11. A cryostat as claimed in any one of the above claims, wherein the jacket is mounted on the heat exchanger.

12. A cryostat as claimed in any one of the above claims, wherein the cryostat is arranged to be located within a vacuum chamber, with the liquid inlet conduit and the gas outlet conduit extending from the chamber via an aperture with an air tight seal, the heat exchanger being located closer to the tank than to the aperture.

13. A cryostat as claimed in any one of the above claims, wherein a seal is utilised to separate the liquid conduit and the gas outlet conduit, the heat exchanger being located between the tank and the seal.

14. A cryostat as claimed in any one of the above claims, wherein the heat exchanger is located within 20cm of the tank.

15. A method of manufacturing a cryostat, the method comprising the steps of: providing a tank thermally coupled to a sample holder, for boiling of a liquid to a gas for cooling of the sample holder; providing a liquid conduit connected to the tank for providing the liquid to the tank; providing a jacket arranged to substantially enclose the sample holder; and providing a gas outlet conduit for removal of the gas from the tank, the gas outlet conduit comprising a heat exchanger arranged to cool the jacket using the gas, wherein the heat exchanger extends around the liquid conduit at the position the liquid conduit passes through an aperture in the jacket.

16. A cryostat substantially as described herein, with reference to Figures 2 to 4 of the accompanying drawings.

17. A method of manufacturing a cryostat, substantially as described herein with reference to Figures 2 to 4 of the accompanying drawings.