WO2001096020A1

WO2001096020A1 - Method and apparatus for providing a variable temperature sample space

Info

Publication number: WO2001096020A1
Application number: PCT/GB2001/002669
Authority: WO
Inventors: Jeremy A. Good
Original assignee: Cryogenic Limited
Priority date: 2000-06-15
Filing date: 2001-06-15
Publication date: 2001-12-20
Also published as: GB2379496A; GB2379496B; GB0014715D0; GB0229140D0

Abstract

A cryostat is provided with a sample space (12) in a chamber (10). A thermal syphon (18, 24, 28, 30) for controlling the temperature of a temperature regulating fluid is provided and forms a circuit together with the sample holder (12). A cryocooler (22) is located adjacent a condenser portion (24) of the thermal syphon for cooling the temperature regulating fluid and a heater (14) is provided to heat the fluid. A charcoal sorption pump is provided to reduce the pressure of the fluid and hence its temperature to reach temperatures of a few Kelvin and less.

Description

METHOD AND APPARATUS FOR PROVIDING A VARIABLE TEMPERATURE SAMPLE SPACE

The present invention relates to a method and apparatus for providing a variable temperature sample space for use with a superconducting magnet .

In recent years mechanical refrigerators often called cryocoolers using the Gifford Mc-Mahon principle or pulse tube techniques have provided access to temperatures of less than 4K. Previously, such temperatures could only be produced using liquid helium which is an expensive and finite natural resource. While in Europe and North America liquid helium is freely available at reasonable cost, in other parts of the world it is extremely expensive and difficult to procure. It evaporates easily so that it can not be readily stored or transported.

With temperatures of less than 4K now available using these mechanical refrigerators which typically require only 5 to 8 Kwatt of electrical power, it has become possible to cool superconducting magnets to around 4K providing fields typically up to 14 Tesla. It is also possible to use a cryocooler to cool a sample to low temperature. However, a difficulty arises if the same cryocooler is to be used to cool both the sample and the magnet . The magnet requires the bulk of the cryocooler refrigeration power especially when the magnetic field is being changed, as this imposes a heat load on the magnet .

At the same time, to cool the sample to 4K or below requires very good thermal contact between the sample and the cryocooler. On the other hand, it is necessary for many purposes in experimental science to vary the temperature of the sample in a controlled or reproducible way between the lowest temperature and room temperature or 30OK. To do this heat is applied and unless the thermal link to the cryocooler can be reduced, the heat required to reach a high temperature will overload the cryocooler and reduce the performance of the magnet .

There are some obvious ways to make and break the thermal contact between the cryocooler and the sample. One is to use a mechanical switch such as adjustable clamp and the other is to use exchange gas. The mechanical switch is of its nature an on/off device and a very unsatisfactory way of controlling heat flow and sample temperature in a controlled manner.

Exchange gas can be used as a more controlled method of heat transfer. To do this the sample is located in a closed vessel, the outside of which is kept at 4K. If helium gas is pumped away and the sample is surrounded by vacuum then it can be heated by application of electrical power. This is a satisfactory procedure for a sample temperature range of 4-100K. However, if taken up to 300K, radiation from the sample to the walls of the vessel imposes a considerable heat load on the cryogenerator if the sample has more than a few square centimetres of area. Each square centimetre will radiate from 1 to 40 milliwatts according to its emissivity and it would be desirable to keep the heat load below 100 milliwatts. A further problem is that precise control of the thermal conduction via exchange gas requires very careful control of the gas pressure .

The present invention seeks to provide improved heating and cooling of a sample space. According to an aspect of the present invention, there is provided apparatus for regulating the temperature of a sample as specified in claim 1.

Preferably, the apparatus includes a heat exchanger coupling the thermal syphon to the cryocooler.

The thermal syphon preferably includes a condenser portion disposed so as to allow downward flow of temperature regulating fluid being condensed.

Advantageously, a superconducting magnet is provided around the sample holder.

There is preferably provided a collection chamber at a bottom portion of the condenser portion for receipt of condensed temperature regulating fluid.

A heater is advantageously provided to heat temperature regulating fluid before passage to the sample holder for the purposes of temperature regulation.

A valve may be provided between the collection chamber and the sample holder for regulating the flow of temperature regulating fluid.

The sample holder, cryocooler and thermal syphon are preferably provided in a cryostat and a reservoir of temperature regulating fluid is preferably provided external to the cryostat.

The apparatus is advantageously provided with pressure adjustment means able to reduce the pressure of temperature regulating fluid within the syphon and sample holder circuit. The pressure adjustment means may include a charcoal sorption pump for pressure reduction and/or heating means for pressure increase. The pressure sorption pump and heating means are preferably heat insulated from the syphon and sample holder.

A radiation shield may be provided between the apparatus and a superconducting magnet provided in the apparatus.

An alternative method taught herein uses a thermal syphon. At its simplest, this is the reverse of a gravity fed domestic central heating system in which a boiler is used to heat water that rises into the radiators before cooling and flowing down a return pipe to the boiler.

An embodiment of the present invention is described below, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram in partial cross-section of an embodiment of variable temperature sample holding apparatus ; and

Figure 2 is a view of a charcoal sorption pump of the sample holding apparatus of Figure 1.

Referring to Figure 1, a sample is placed in a vertical chamber 10 containing helium gas. The bottom of this chamber 10, where the sample is situated, is copper plated to improve its thermal conductivity with an axial slot to avoid eddy currents being induced by changing magnetic fields. This region 12 where the sample is located is surrounded by an electrical heater 14 and incorporates a thermometer 16 to measure the sample temperature .

The chamber 10 and all the cold parts of the system are mounted in an outer vessel 32, the cryostat, which is highly evacuated so as to provide very good thermal isolation. The sample is normally placed in the centre of the superconductivity magnet (not shown) to provide a magnetic field on the sample. To maintain the sample at, say, a temperature of 50K, electrical heater power is applied to the sample. This heats the copper plated chamber 10 and the gas around the sample rises in the vertical chamber and passes through a side pipe 18 to a heat exchanger 20 on the cryocooler 22, where it is cooled.

Cryocoolers of the type normally used for superconducting magnets have two stages. The upper one runs at about 4OK and is used to cool radiation shields 48 around the magnet and the sample space. The shields 48 interrupt all heat loads from the outside of the cryostat flowing towards the magnet, which is cooled by the second stage to about 4K. Usually, the highest heat loads are due to the electrical leads used to power the magnet . The upper or first stage has a high cooling power, typically 40 watts at 40K, while the second stage provides, typically, 1 watt at 4K with a base temperature of 3K. Since the performance of a superconducting magnet improves as the temperature is reduced, it is desirable to keep the heat load from all sources to less than 1 watt and preferably less than % a watt.

The second stage at 4K also cools the magnet. When the magnet is ramped quickly, the second stage heats up for a short period, for example 1 to 10 minutes, to temperatures of 5 to 6K. If the sample is to stay at 4K or less, the chamber needs to be thermally isolated and of sufficient capacity (typically 100 cc) to hold enough liquid to supply the sample space with cold liquid for the short period while the magnetic field is changed.

The gas passes through the heat exchanger on the first stage and then, being colder and denser, sinks down the connecting pipe 24 to the second stage heat exchanger 26, which is at 4K or less. For this purpose, the cryocooler should preferably be mounted vertically or near vertically. The gas then cools and, if the pressure of gas is sufficient, condenses into liquid helium.

Beyond the second stage heat exchanger 26 there is provided a chamber 28. The chamber 28 is separate from the heat exchanger 26 and thermally isolated therefrom so as to provide a constant supply of cold liquid/gas through a needle valve 34 to the sample space 10. In use, chamber 28 at the bottom of the second stage heat exchanger will fill with liquid helium. From this a small tube 30 carries the liquid to the bottom of the sample space 12, where it is warmed by the heater 14 to the control temperature, in this case 50K.

The flow rate may be controlled by the needle valve 34 in the small tube 30, which is operated from outside the cryostat.

As will be apparent in Figure 1, the chamber 28 is slightly above the sample space 10 to allow liquid flow to be maintained. In one embodiment, the chamber is about the same in diameter as in height . The top of the sample space passes through the top of the cryostat and is sealed by an air lock and valve 36. The sample is loaded on a tubular sample rod through this airlock and all wiring to the sample passes down the centre of the rod 38.

There is always a heat load from the top of the sample space so circulation of gas is both maintained and required whatever the sample temperature is, but as the temperature gets higher, the heat load from room temperature is less so less circulation is required. Left to itself, the circulation rate will be fairly constant for all temperatures above 50K since the driving pressure, the column of liquid in the heat exchanger, is constant. This means that the heat load on the cryogenerator is also constant. However, adjustment of the valve 36 can be used to control and restrict the flow.

When the temperature is set for 4K and the heater turned off, liquid will flow into the sample chamber ensuring that the sample is at the lowest temperature. Indeed, the flow of liquid as opposed to gas in the small tube 30 will be more rapid when the tube itself is cold at both ends since the impedance of the tube to flow rises rapidly with temperature. So when the sample and this end of the tube is hot it will act as an impedance to flow. Mass transfer is faster the lower the temperature, thus increasing the cooling power.

The driving pressure of a column of liquid helium 160mm deep is 2 millibar. With suitable choice of tube sizes for the heat exchanger sample space 12 and connecting tubes, this is more than sufficient to maintain circulation. Since the pressure of gas in a chamber of constant volume is proportional to temperature and the system requires sufficient gas to permit condensation, it is preferable to fit a reservoir 40 external to the cryostat. This volume needs to be quite large compared to the cold space volume since 1 cc of liquid helium corresponds- to 650 cc of gas at NTP (normal temperature and pressure) and even the helium gas at 4K is about 75 times denser than the gas at 30OK. A volume of 50-100 litres of gas at NTP would be sufficient for a 25mm sample space.

Furthermore, it is attractive to be able to obtain temperatures below 4K. To do this, it is necessary to reduce the pressure on the liquid helium to below 1 bar. For a temperature of 1.5K a pressure of a few millibar is necessary. Although this can be done with an external pump, a more convenient way is to use a charcoal sorption pump.

For this purpose, a tube leads from the sample space 12 to a chamber 42 filled with charcoal (Figure 2) .

Charcoal, when cooled by the cryocooler to about 4K or less, absorbs helium strongly. A charge of charcoal will absorb >3% of its weight of helium and achieve a low pressure in the sample space. Pumping on the gas and liquid helium surrounding the sample by this means will reduce the temperature of the sample which is immersed in the liquid to 1.5K or below. After a period of time the liquid will all be pumped away into the charcoal and it is necessary to regenerate the system.

In order to be able to heat the charcoal above 4K to about 6OK to expel the helium and regenerate the charcoal for the next cycle it is preferable to have a thermal on/off contact to the cryocooler. The most convenient way to achieve this is to use exchange gas. A chamber 42 (Figure 2) containing the charcoal sorption pump material is partly or wholly enclosed in an outer chamber 44 containing exchange gas helium. A fixed charge of gas is used so the vessel can be sealed off . The outside of the outer exchange gas chamber 44 is copper plated and thermally anchored to the second stage 26 of the cryogenerator at 4K. As an alternative to charcoal, a sorption pump can use molecular sieve material such as Alumina.

When heating the charcoal to regenerate the Helium gas it is necessary to prevent this heat reaching the cryocooler. This is done by removing the exchange gas in the outer chamber 44. A second small charcoal pump 46 attached the exchange gas chamber 44 is used for this purpose.

The second small charcoal pump 46 is mounted on a brass or similar metal pillar or tube of medium thermal conductivity connected to the outer wall at 4K. The pump consists of a small (approximately 5cc) cup containing the charcoal, the open end of which is sealed with a metal gauze to prevent the charcoal coming out. The cup is heated electrically and its temperature monitored by a thermometer. A very small amount of heat, around 5 to 10 milliwatts, is needed to heat the charcoal to 40-50K, at which temperature the exchange gas is released from the charcoal and causes the main charcoal pump inside the inner chamber to be cooled to about 4K.

Allowing the small cup to cool to 4K by conduction along the tube causes the charcoal in the cup to absorb the exchange gas so that the main pump can be heated conveniently for regeneration.

By means of this apparatus and method it is possible to operate the superconducting magnet together with a sample where its temperature may be varied from 30OK to 1.5K in a convenient manner with accurate control of temperature (5mK) . The system applies convection to drive the gas flow to cool a sample in a controlled manner over a range of temperatures from room temperature at 30OK down to 4K while not placing a high heat load on the 4K stage of the fridge. The skilled person will appreciate the advantage of having flow control (the pre-set valve to control flow) and also of having the second heat exchanger to cool the gas to 40k before it reaches the 4K stage. These features allow the charcoal pump to operate successfully to cool from 4K to around IK.

It will be apparent that the system provides for heating and cooling of the charcoal pump without any mechanical valves or heat switches and thus avoids overloading the 4K stage of the cryogenerator.

Furthermore, it should be pointed out that surrounding the sample space is a radiation shield 48 cooled by the first stage of the cryocooler to approximately 4OK and this isolates the magnet from any radiation from the sample space when it is hot.

Claims

1. Apparatus for regulating the temperature of a sample including a sample holder, a cryocooler and a thermal syphon connected to the sample holder and providing with the sample holder a temperature regulating fluid circuit in which fluid flow is by convection.

2. Apparatus according to claim 1, including a heat exchanger coupling the thermal syphon to the cryocooler.

3. Apparatus according to claim 1 or 2 , wherein the thermal syphon includes a condenser portion disposed so as to allow downward flow of temperature regulating fluid being condensed.

4. Apparatus according to claim 3, including a collection chamber at a bottom portion of the condenser portion for receipt of condensed temperature regulating fluid.

5. Apparatus according to claim 4, including a valve between the collection chamber and the sample holder for regulating the flow of temperature regulating fluid.

6. Apparatus according to any preceding claim, including a heater for heating temperature regulating fluid before passage to the sample holder.

7. Apparatus according to any preceding claim, wherein the sample holder, cryocooler and thermal syphon are located in a cryostat and a reservoir of temperature regulating fluid is provided external to the cryostat .

8. Apparatus according to any preceding claim, including pressure adjustment means able to reduce the pressure of temperature regulating fluid within the syphon and sample holder circuit .

9. Apparatus according to claim 8, wherein the pressure adjustment means includes a charcoal sorption pump for pressure reduction and/or heating means for pressure increase.

10. Apparatus according to claim 9, wherein the pressure sorption pump and heating means are heat insulated from the syphon and sample holder.

11. Apparatus according to any preceding claim, wherein a radiation shield is provided between the apparatus and a superconducting magnet provided with the apparatus.

12. A method of regulating the temperature of a sample including providing a sample holder, providing a cryocooler, providing a temperature regulating fluid in the sample holder, and applying convection to drive the temperature regulating fluid to cool the sample.