Patent Application
20080083245
The present invention relates to a flow-cooled magnet system.
In many applications it is desirable to subject a “target” (including devices, other apparatus, an experimental sample or indeed an organism), to a controlled magnetic field. As is known, the strongest magnetic fields are at present generated electromagnets, cooled to cryogenic temperatures. Depending upon the application in question, the target may also be cooled to cryogenic temperatures (for example in the case of very sensitive devices), to an intermediate temperature or indeed the level of cooling may be minimal such that the target temperature is that of approximately ambient temperature.
The cooling systems required for maintaining such magnets at operative temperatures are often substantial. In addition, in order to cool the target, cooling systems of a different nature are used, often because of the different temperature requirements, the location of the target within the confines of a cryostat bore and in some cases due to the removable nature of the target from the system.
Known systems are often bulky and many require the warming of the magnet itself in order to exchange targets. There is therefore a need to improve upon known systems in order to address such deficiencies.
In accordance with the present invention we provide a flow-cooled magnet system comprising: —
We have realised that, by appropriate design, a system can be produced in which a common cooling system is used for cooling both the magnet and the target region. This is achievable by the use of a flow-cooled system in which a coolant flows between one and the other of the first and second parts. Since the first part is in thermal communication with the magnet, which generates the largest heat load, preferably the first part is upstream of the second part. Therefore the coolant fluid passes firstly through the first part and then moves downstream to cool the second part. Although the target region may comprise a detector, or other apparatus/devices, preferably the target region is adapted to retain a sample when in use. Such a sample may be retained within a sample holder. This sample holder may or may not also form part of the target region.
Typically the magnet is contained within a cryostat. Preferably such a cryostat comprises at least one radiation shield. The flow path in this case is preferably arranged to further comprise a third part in thermal communication with the at least one radiation shield. Thus the coolant fluid is preferably used to cool not only the magnet and the target, but also the at least one radiation shield. Typically the flow path divides from a common path within which the first part is located into at least two separate paths within one of which is located the second part and another one of which is located the third part. Thus some of the coolant flows within the second part and some within the third part, whereas all of the coolant flows within the first part.
The third part may also be used to cool further apparatus within the cryostat such as current leads for supplying electrical current to the magnet. It will be appreciated that such a magnet is typically a superconducting magnet operative at cryogenic temperatures, including 7 Kelvin and below, and particularly liquid helium temperatures (4.2 Kelvin).
Since not all of the fluid flows through each of the first, second and third parts, preferably the system further comprises a flow controller which is adapted to provide a predetermined flow rate of coolant within one or more of the first, second and third parts of the system. The flow controller may comprise a variable flow impedance although a static localised flow impedance may be used (such as a narrowed portion of conduit) where a predefined steady-state flow in each of the parts of the system is required.
Typically the second part comprises a heat exchanger with which a target, such as a sample holder, is in thermal communication. Heat exchange is therefore used to cool the sample via the sample holder. In order to provide enhanced control over the temperature within the target region, a heater may be provided within the target region such that the heat produced, in combination with the cooling effect of the coolant in the second part of the cooling system, causes the target region to be held at a predetermined temperature. The predetermined temperature may therefore lie in a range bounded by the coolant temperature and ambient temperature. This allows the target region to be maintained at a temperature substantially the same as that of the magnet itself, or operated at a higher temperature including room temperature.
In order to provide accurate control over the temperature within the target region, preferably the system further comprises a temperature sensor located within the target region, together with a temperature controller which is used to control the heater in response to signals received from the temperature sensor when in use. If a variable flow impedance is provided then the temperature controller might be used to control the flow impedance also.
The magnet is preferably located within a cryostat vacuum chamber and, when cooling a sample, the system preferably comprises a sample vacuum chamber in the target region. The sample vacuum chamber is preferably independent of the cryostat vacuum chamber and therefore the temperature and pressure of each of these chambers may be controlled independently of one another.
In most cases, the magnet is provided in the form of a solenoid magnet having a bore within which the target region is located. In some applications it is desirable to allow radiation (electromagnetic or particle beam) to be transmitted to and/or received from a sample when located within the target region and therefore in this case the system preferably further comprises at least a first window position so as to allow radiation to be transmitted and/or received from the sample. A second window located on the opposite side of the sample with respect to the first is also preferably provided so as to allow radiation to be received by the sample through the first window, to pass through the sample and then to pass through the second window for monitoring. Thus reflective and transmissive monitoring using electromagnetic radiation (including visible, ultra-violet, infra-red light, and other longer and shorter wavelengths) or particle beams can be achieved.
For such monitoring, the magnet system may be adapted for use upon an optical bench and is preferably provided with mountings for fitment to such an optical bench. In known magnet systems for use upon optical benches, a fixed orientation with respect to the bench is required due to the need for the cooling systems to be arranged in a particular orientation. This is particularly the case where liquid coolants are used within the system, such as when liquid helium is used as a coolant, not only due to its liquid state, but also since it may be within a superfluid state. In the present case however, the coolant fluid is preferably within the gaseous form (even if not exclusively so due to localised condensation), and the system may be adapted to be operational in vertical and horizontal orientations with respect to a nominal axis. This is advantageous since it allows greater versatility in terms of performing experiments in particular upon target samples. For example a microscope may be used to view the sample by placing the objective lens inside the bore of the magnet system and then ability to orient the system either horizontally or vertically allows greater ease of use of a microscope or indeed other equipment.
The flow of coolant fluid within the cooling system is preferably arranged to be continuous such that the system is a “continuous flow” system. However, a discontinuous flow in one or more parts of the system may be used in certain circumstances.
Due to the flow of coolant fluid, it is preferable that the magnet is cooled by thermal conduction rather than being placed within a large bath of liquid coolant. The magnet is preferably separated from the coolant by a high thermal conductivity member through which heat flows from the magnet. To achieve this, the magnet is preferably surrounded by an annular chamber which comprises the first part of the cooling system through which the coolant flows when in use. Thus the surface area of the high thermal conductivity member may be maximised so as to produce a strong cooling effect.
The target region may comprise a sample support for receiving a sample holder within which a sample is retained when in use. This support may be cooled using a heat exchanger, preferably in combination with a heater and temperature controller as described above.
Whilst, when in use, the coolant preferably flows continually, the system may be arranged such that the coolant is supplied to the flow path from a coolant reservoir. In this case the maximum operational period of the system is determined by the amount of coolant within the reservoir. In an alternative case, the cooling system may be a closed loop system such that, after passing through the flow path, the coolant is refrigerated and recirculated repeatedly through the flow path. A mechanical refrigerator could be used for refrigerating the coolant for recirculation although it will be appreciated that other cooling systems could be used to provide a similar effect.
Preferably the coolant is a cryogenic fluid, such fluids including gases or liquids at cryogenic temperatures (below 100 Kelvin). The coolant may therefore be helium or nitrogen gas for example.
Some examples of magnet systems according to the present invention will now be described with reference to the accompanying drawings, in which: —
With reference to
A cryostat 2 is provided, this containing a main vacuum chamber 3 within which is positioned a magnet 4. The cryostat 2 and magnet 4 are formed in a solenoid configuration so as to provide a bore 5 running substantially centrally through the magnet 4 and cryostat 2.
The magnet 4 is a superconducting solenoid magnet producing a magnetic field strength of about 5 Tesla. The magnet his energised using current leads 6 (shown as a dashed line). The magnet may operate in a “persistence” mode as is known in the art. The magnet 4 is bounded by walls 7 having a high thermal conductivity. In this case the walls are manufactured from high purity copper although of course the magnet windings are electrically insulated from the walls 7. Around the outer periphery of the magnet 4, beyond a dividing part of the walls 7, an annular chamber 8 is provided about the magnet periphery. In use coolant gas flows through the annular chamber 8 so as to cool the magnet 4. In the present example gaseous helium is used as the coolant. It is not essential that the helium is in its gaseous form to cool the magnet. Indeed the system may be arranged such that the annular chamber 8 is filled with liquid helium in an alternative embodiment. Gaseous helium is used in the present example since this allows the magnet to be oriented horizontally or vertically.
Within the bore 5, a sample vacuum chamber 9 is located. This can be thought of a separate cryostat within the cryostat 2. The vacuum chamber 9 is generally cylindrical in form such that outer wall of the cylinder conforms with the wall of the bore 5 (although it is separated therefrom by a small gap to provide thermal insulation). At least part of the inner wall is formed by a removable sample support 10, this also having generally cylindrical geometry including a central sample support bore 11. This bore 11 has a narrower diameter than that of the bore 5. At one end (the upper end in
The sample support 10 is positioned such that the sample holder 12 is located beneath a first window 13 (glass) in an upper central wall of the sample vacuum chamber 9. At the opposite end of the sample support 10 to the sample holder 12 the support bore 11 is sealed by a second window 14, also made from glass. The first and second windows 13, 14 allow electromagnetic radiation to pass from one side of the cryostat along the bores 5 and 11, through the sample 12 and to the other side of the cryostat. This allows monitoring of the sample in terms of optical observations or measurements of the optical behaviour of the sample within the sample holder 12. One such application is in making optical measurements upon “quantum dot” samples by external illumination with light emitting diode or laser devices.
Within the walls of the sample holder 12, a heat exchanger 15 is provided, this taking the form of a helical flow path formed by opposed walls cut as a meshing thread with the path defining a narrow helical gap between the walls. A heater 16 is also provided adjacent to the sample holder 12, together with a temperature sensor (not shown). The cooling system for the apparatus according to the invention is now described.
In the present example the coolant is gaseous helium at ambient pressure and at a temperature of approximately 4.2 Kelvin. Lower temperature helium could be used (such as 2 Kelvin) although to retain the gaseous state the system would need to be operated at low pressure. The gaseous helium is provided from a helium reservoir 20 shown schematically in
At a branch point 23, downstream of the annular chamber 8, the exit conduit divides into a cryostat conduit 24 and a sample conduit 25. Although the branch point is shown in
The purpose of the cryostat conduit is to take some of the coolant gas and use its cooling effect to cool further parts of the cryostat apparatus. In the present case, the cryostat conduit 24 is placed in thermal communication with the cryostat radiation shields generally indicated at 26. Thereafter, further downstream, the coolant is brought into thermal communication with the current leads 6 of the magnet 4, as indicated at 27. In each case suitable heat exchangers may be used to maximise the cooling effect. Having cooled the current leads 6, the cryostat conduit 24 passes out of the cryostat and through a check valve 30. The check valve provides a localised flow impedance and therefore controls the relative amounts of fluid flowing in each of the cryostat and sample conduits 24, 25. The check valve is not essential if the natural flow impedance of the cooling system as a whole is arranged to provide the correct division of coolant flow rates between the cryostat conduit 24 and sample conduit 25. As is known, the impedance of the conduits is a function of their length and cross section and one or each of these can be arranged accordingly.
Returning once more to the branch point 23, the sample conduit 25 (which represents a “second part” of the flow path), is arranged to open through a port into the helical threaded space defining the heat exchanger 15 within the sample support 10. The helium coolant gas therefore flows through the helical path of the heat exchanger and cools the sample support 10 and, by conduction, the sample holder 12. A venting portion 28 of the sample conduit carries the coolant gas from a port in a distal part of the heat exchanger 15 with respect to the position of the entry port, out of the cryostat 2 to a venting location.
The sample 12 is therefore cooled by the operation of the heat exchanger 15 as a result of the large surface area provided by the helical flow path. The temperature of the sample 12 is a function of the temperature and flow rate of the coolant gas, this being controlled at least in part by the check valve 30. In order to provide more accurate control of the temperature of the sample in the sample holder 12, the heater 16 is used in conjunction with the cooling effect of the heat exchanger 15 using an appropriate control system and temperature sensor (not shown) so as to ensure that the sample is subjected to a predetermined temperature or temperature cycle. This can be performed using a microprocessor and a feedback loop.
As is illustrated in
The cooling of the apparatus using the cooling system (comprising the conduits, heat exchangers and so on) will now be described. Helium gas is pumped through the conduit by a pumping system which may be provided either within a common part of the flow path such as within the supply conduit 21, or in each of the cryostats and sample conduits 24, 25 respectively. As shown in
The conduits 21 and 22 through which all of the coolant flows may be thought of as a common part of the flow cooling system, this containing the first part in which the thermal communication of the magnet occurs. In the second part, within the sample conduit branch, the thermal communication with the sample holder 12 occurs. Part of the coolant branches along the cryostat conduit 24 and this performs heat exchange with the radiation shields 26 and the current leads 6 before exiting the cryostat through the check valve 30. The parts of the flow path within the cryostat conduit 24 that are in thermal communication with the radiation shields and current leads comprise a third part of the flow path. As for the venting via the venting portion 28 of the sample conduit, downstream of the check valve 30, the helium gas may also be collected for further use.
We turn now to a second embodiment of the present invention, this being illustrated within
As will be appreciated from a consideration of the example described above in association with
Referring now to
The reservoir of the first example is replaced in this example by a vessel 35 as shown in
In this example, the vessel 35 is sealed from the external environment and thermally insulated therefrom also, and the pump 37 is operative to supply gaseous helium along the supply line 21. As before, the valve 30 controls the relative flow between the two conduits 24, 25. Unlike in the previous example however, where each of these conduits was vented either to the external atmosphere or to a collection system, in the present example the gas is returned to the vessel 35 in each case where it is cooled again by the mechanical refrigerator 36 which maintains the temperature of the vessel such that the helium entering the supply conduit 21 is at a sufficiently low temperature. As in the case of
In a practical system, as would be appreciated by one of ordinary skill in the art, greater thermal efficiency can be achieved by techniques such as using the cooling effect of the helium within the “exit” path so as precool the “entry” path of the gas flowing into the cryostat. This may be achieved for example by providing the venting portion 28 in
The abovedescribed examples have been discussed in connection with the use of helium, this is due to the present desired use of liquid helium for cooling superconducting magnets and performing certain low temperature experiments. However, the system may also be used with other cryogenic coolants such as nitrogen at for example 77 Kelvin rather than 4.2 Kelvin. The systems are also described using approximately ambient pressures, whereas it will be appreciated that by using non-ambient pressures within the cooling system, different cooling temperatures may be achieved. Whilst the apparatus described in the examples is in relation to an experimental magnet for optical experiments, it will be appreciated that the principle underlying how the system is cooled may be applied to magnet systems including nuclear magnetic residence and magnetic residence imaging systems.
The present examples are particularly useful for performing magneto-optical experiments on an optical bench and for this reason in each example, as is shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| 0619607.5 | Oct 2006 | GB | national |
