Patent Application
20070089432
This application claims Paris Convention priority of DE 10 2005 029 151.1 filed Jun. 23, 2005 the complete disclosure of which is hereby incorporated by reference.
The invention concerns a cryostat configuration for keeping cryogenic fluids in at least one cryocontainer, comprising an outer shell and a neck tube in which a cold head of a cryocooler is installed, wherein at least the coldest cooling stage of the cold head of the cryocooler is disposed in such a manner that it does not contact the neck tube and the cryocontainer, wherein a cryogenic fluid is contained in the neck tube.
A cryostat configuration of this type is disclosed in US 2002/0002830 A1.
In cryostat configurations for keeping liquid cryogens, which are used e.g. in nuclear magnetic resonance (NMR) measuring apparatus, a superconducting magnet coil system is disposed in a first container containing liquid cryogen, usually liquid helium, which is surrounded by radiation shields, super insulation foils and optionally a further container with a cryogenic liquid, usually liquid nitrogen. The liquid containers, radiation shields and super insulation foils are accommodated in an outer container that delimits a vacuum chamber (outer shell, outer vacuum cover). The superconducting magnet is kept at a constant temperature by the surrounding evaporating helium. The elements surrounding the first cryocontainer thermally insulate the container to minimize the heat input into the container as well as the helium evaporation rate. Magnet systems for high-resolution NMR spectroscopy are usually so-called vertical systems, wherein the coil configuration axis and the opening for receiving the NMR sample extend in a vertical direction. The helium container of NMR spectrometers is usually connected to the outer vacuum cover via at least two thin-walled suspension tubes. The container is thereby mechanically fixed and the suspension tubes provide access to the magnet as is required e.g. for cooling and charging. Moreover, the waste gas is discharged via the suspension tubes, thereby cooling the suspension tubes and, in the ideal case, completely compensating for the heat input via the tube wall. A system of this type is described e.g. in DE 29 06 060 A1 and in the document “Superconducting NMR Magnet Design” (Concepts in Magnetic Resonance, 1993, 6, 255-273).
However, handling of the cryogens is difficult. They must be refilled at certain time intervals which often requires undesired interruption of the measurements. The dependence on liquid cryogens is also problematic if the infrastructure is inadequate such as e.g. in developing countries (India, China, etc.). Future cryogen price increases could render such cooling rather expensive.
Mechanical cooling apparatus, so-called cryocoolers, have recently been used to a greater extent for directly cooling superconducting magnet systems. In addition to cooling without cryogenic fluids (so-called dry cooling), there are conventional systems that contain at least one further cryogenic fluid that is, however, reliquefied by the cryocooler after evaporation. For this reason, none or nearly none of the cryogenic fluid escapes to the outside. The documents EP 0 905 436, EP 0 905 524, WO 03/036207, WO 03/036190, U.S. Pat. No. 5,966,944, U.S. Pat. No. 5,563,566, U.S. Pat. No. 5,613,367, U.S. Pat. No. 5,782,095, U.S. Pat. No. 5,918,470, US 2002/0002830 and US 2003/230089 describe such possible cooling of a superconducting magnet system using a cryocooler without losing cryogen.
The e.g. two-stage cold head of the cryocooler may be installed in a separate vacuum space (as described e.g. in U.S. Pat, No. 5,613,367) or directly in the vacuum space of the cryostat (as described e.g. in U.S. Pat. No. 5,563,566) in such a manner that the first cold stage of the cold head is attached to a radiation shield and the second cold stage is connected in a heat-conducting fashion to the cryocontainer either directly or indirectly via a fixed thermal bridge. The overall heat input into the cryocontainer can be compensated for through re-condensation of the helium, which evaporates due to heat input from the outside, on the cold contact surface in the cryocontainer, permitting loss-free operation of the system. Disadvantageously, the connection between the second cold stage and the cryocontainer has a thermal resistance, and vibrations of the cryocooler may be transmitted to the cryocontainer and the magnet even if “soft” heat transferring elements (e.g. braided wires) are used. In high-resolution NMR systems even small disturbances can be problematic and prevent meaningful measurements.
One way to avoid these disadvantages is to insert the cold head into a neck tube which connects the outer vacuum sleeve of the cryostat to the cryocontainer and is correspondingly filled with helium gas, i.e. a cryogenic fluid in the gaseous phase, as described e.g. in the document US 2002/0002830. The first cold stage of the two-stage cold head is in fixed conducting contact with a radiation shield. The second cold stage is freely suspended in the helium atmosphere and directly liquefies the evaporated helium. Since the cooling performance of the cryocooler is larger than the thermal input into the cryocontainer, an additional heating means evaporates a sufficient amount of helium to obtain a stationary state. The controlled variable for the heating means may e.g. be the pressure in the cryocontainer.
A system of this type has, however, the disadvantage that the neck tube opens at the bottom into the helium container. To service the cryocooler, either the neck tube bottom must be closed via a special device to prevent the technician from getting injured in case of a magnet quench, or the magnet must be discharged, which causes an undesired downtime for the magnet system. Moreover, the asymmetric opening also exerts asymmetric forces on the helium container during normal operation, which must be compensated for by special centering elements. In case of a quench, the forces acting on the suspension tubes increase and the cryocooler is also subjected to the full quench pressure. For this reason, the cold head must be connected to the outer shell in a relatively rigid manner, which has an unfavorable effect on dampening of vibrations between the cold head and the outer shell. The neck tube may also have a comparably large wall thickness leading to large neck tube heat input. Since the neck tube and neck tube installations are subjected to the increased quench pressure in case the magnet quenches, these cryostat components are often also subject to more stringent guidelines imposed by the authorities, which, in turn, involves additional effort (e.g. proof of traceability of the components) and increased cost.
It is therefore the underlying purpose of the present invention to propose a cryostat configuration with integrated cryocooler which is of simple construction, wherein the heat transfer between the cooler cold head and the cryostat configuration is efficient and with little vibration, and which offers great safety during operation, in particular, also for maintenance work.
This object is achieved in accordance with the invention in that the neck tube is disposed between the outer shell of the cryostat configuration and at least one cryocontainer and/or the radiation shield, the neck tube being closed in a gas-tight manner at the end facing the cryocontainer and/or the radiation shield and coupled to one of the cryocontainers and/or a radiation shield which is disposed between the cryocontainers or between a cryocontainer and the outer shell, via a connection having a good thermal conductivity, wherein the neck tube has a fill-in device at its ambient temperature end for filling a cryogenic fluid into the neck tube.
The comparably simple construction requires no or hardly any additional expense compared to a cryostat configuration without cryocooler, especially to adopt the additional mechanical loads in case of a magnet quench, and to meet the high safety requirements and demands imposed by the authorities.
The neck tube housing the cold head of the cryocooler used for cooling is separated in a gas-tight manner from the cryocontainer, such that the neck tube is in its own cryogen atmosphere. The cryogenic region around the cryocooler is therefore completely separated from the cryocontainer. The heat is transferred from the cold head to the cryocontainer indirectly via the cryogen located in the neck tube (through evaporation and condensation of part of the liquid cryogen located in the neck tube) without any contact between the cryocooler cold head and the cryocontainer. No vibrations are therefore transmitted from the cold head to the cryocontainer and the magnet configuration.
Since the inventive cryostat configuration has no direct opening between the end of the neck tube and the cryocontainer, e.g. the helium container, the forces released by a magnet quench do not act on the cryocooler. This permits safe installation and removal of the cold head without interrupting operation of the system contained in the cryostat configuration. In this case, the cryocontainer has no opening to the outside except for the suspension tubes. In case of a quench, the forces acting on the cold head are no larger than during normal operation except for a purely static force due to the inner pressure which is slightly higher compared to the ambient pressure. For this reason, the connection between the cold head and the outer shell may be less rigid than for a neck tube which opens towards the cryocontainer. The inventive cryostat configuration therefore permits vibration-damped, softer support on the outer shell and/or the use of a neck tube with thinner walls to reduce the heat input into the cryocontainer. Moreover, the neck tube and further neck tube installations must no longer meet the strict guidelines and safety regulations for pressure containers imposed by the authorities, which reduces production costs.
The neck tube preferably contains the same cryogenic liquid as the cryocontainer. This is relevant, in particular, if the neck tube is thermally conductingly connected to the cryocontainer and not exclusively to the radiation shield. The cryogenic atmosphere in the neck tube may be supplied by the cryogen supply in the cryocontainer, as is described below.
Alternatively, the neck tube and the cryocontainer may contain different cryogenic fluids. This is possible by connecting the neck tube exclusively to the radiation shield in a heat-conducting manner or if the cryogenic fluid in the neck tube has a lower boiling point than the cryogenic fluid in the cryocontainer at the same pressure.
The neck tube is advantageously produced from a material having a poor thermal conductivity, comparable to those of stainless steel. The heat input into the cryocontainer can thereby be reduced. The neck tube wall may be thinner than that of a neck tube which is open towards the cryocontainer due to the smaller load in case of a quench, which also reduces the heat input.
In a particular embodiment of the invention, at least sections of the neck tube are formed as bellows. The neck tube can therefore adapt to expansion or contraction of the cryocontainer. The amount of heat guided from the warm end to the cold end of the neck tube is less than with a straight tube.
In a preferred embodiment of the inventive cryostat configuration, the closed end of the neck tube directly contacts the cryocontainer or the radiation shield. The heat is transferred from the cryocontainer or radiation shield to the cryogen in the neck tube only via a wall separating the neck tube from the cryocontainer or radiation shield.
The thermal resistance between the neck tube and the cryocontainer or the radiation shield is thereby smaller than 0.05 K/W, preferably smaller than 0.01 K/W. To meet this demand, the surface area of the separating wall must be correspondingly large or the thickness of the separating wall must be correspondingly small and the material of the separating wall must have a good thermal conductivity.
Alternatively, the closed end of the neck tube may not directly contact the cryocontainer or the radiation shield but be connected thereto via rigid or flexible elements having a good thermal conductivity. This may be realized e.g. by copper wires braided into strands. In this case, the thermal resistance between the neck tube and the cryocontainer is larger than in case of a direct connection via a separating wall, but transmission of vibrations between the neck tube and the cryocontainer is reduced. Moreover, no additional asymmetric load acts on the cryocontainer even during normal operation, as is the case with direct contact between the neck tube and the cryocontainer. Centering elements in the form of tension or pressure centerings, which position the cryocontainer in its central position, are evenly loaded and may be designed like in cryostat configurations without an additional neck tube.
The thermal resistance between the neck tube and the cryocontainer or radiation shield is thereby less than 0.1 K/W, preferably less than 0.05 K/W. The number, length, diameter and material of the strands must be selected accordingly.
In an advantageous embodiment of the inventive cryostat configuration, the neck tube is connected to an external cryogenic fluid reservoir via the fill-in device. In this manner, cryogenic fluid can be constantly supplied to the neck tube during cooling of the cryostat configuration. The cryogenic fluid may also be a (high-pressure) gas at room temperature and be permanently connected to the neck tube via a pressure-reducing valve, i.e. also after cooling, such that even in case of a leakage in the neck tube region and the resulting gas leakage towards the outside, gas can always be supplied from the external reservoir.
In one particularly preferred embodiment, the cryocontainer is suspended at at least one suspension tube which is connected to the outer shell of the cryostat configuration, wherein the outer shell, the cryocontainer(s), the neck tube and the at least one suspension tube define an evacuated space, and wherein the neck tube is connected to at least one suspension tube of the cryocontainer via a connecting line which can be shut off and which comprises a rapid-action valve. During the cooling phase of the cryostat configuration, cryogen can be guided from the cryocontainer into the neck tube via the suspension tube and the connecting line which can be shut off. The cryogen escaping from the suspension tube in the form of gas at ambient temperature is thereby re-cooled on the neck tube wall and/or the tubes of the cold head of the cooler and even liquefied in the final cooling phase at the lower end of the neck tube or cold head. The rapid-action valve prevents a pressure increase in the region of the neck tube in case of a quench of the magnet configuration, when the shut-off valve is open and the pressure in the cryocontainer increases. As soon as a larger amount of (quench) gas flows through the connecting line, the rapid-action valve closes automatically to decouple the neck tube from the cryocontainer. The pressure in the region of the neck tube remains approximately the same. For this reason, the neck tube and further neck tube installations are no longer subject to the strict guidelines and safety regulations for pressure containers imposed by the authorities, and the production costs decrease. Installation work on the cooler and neck tube can be performed while the magnet is charged without endangering the technician in case of a quench.
Moreover, at least one suspension tube of the cryocontainer, which is connected to the neck tube in a heat-conducting manner, may additionally be connected to an additional line or exclusively to the additional line which is in thermal contact with the neck tube and terminates in the cryocontainer, wherein a shut-off device and/or a pump may be inserted in the additional line. The additional line does not connect the cryocontainer to the inside of the neck tube but is guided past it, forming a gas flow through the suspension tube and the additional line and back into the cryocontainer. The ascending gas thereby accepts heat from the wall of the suspension tube, cooling it, and being cooled again in the additional line in that region where it is connected to the neck tube. This reduces the heat input into the cryocontainer via the suspension tube(s).
In an alternative embodiment of the inventive cryostat configuration, the cryocontainer is connected to the end of the neck tube, which is at ambient temperature, via at least one fluid line which is guided through the outer shell of the cryostat configuration, wherein the outer shell, the at least one cryocontainer, the neck tube and the at least one fluid line define an evacuated space, and wherein a rapid-action valve and a shut-off device are integrated in the fluid line in the region between the outer shell and the neck tube. This fluid line can guide cryogen from the cryocontainer into the neck tube even in cryostat configurations which are not suspended from suspension tubes.
In a preferred embodiment, the cryocooler is a pulse tube cooler or a Gifford-McMahon cryocooler having at least two cold stages. Two cold stages generate particularly low temperatures and can provide two different temperature levels.
A temperature of 77 K or less can be generated at one cold stage of the cold head of the cryocooler and at the same time liquid helium of a temperature of 4.2 K or less at another (second) cold stage.
At least one cold stage of the cryocooler cold head, which is not the coldest cold stage, is advantageously thermally coupled to a radiation shield or an additional cryocontainer which is not the coldest cryocontainer. In this manner, the heat input into the radiation shield is adsorbed or the cryogen loss in the additional cryocontainer is compensated for or at least reduced.
This thermal coupling between the at least one cold stage and the radiation shield or the additional cryocontainer is preferably a flexible and/or rigid solid connection which penetrates the neck tube wall. The connection between the cold stage and the neck tube wall may e.g. be rigid and the connection between the neck tube wall and the radiation shield or additional cryocontainer may be flexible in the form of copper strands.
In a particularly advantageous manner, a gas gap is provided between the at least one cold stage of the cold head of the cryocooler and the solid connection connected to the neck tube wall for thermal coupling of the at least one cold stage to the radiation shield or the additional cryocontainer. In this manner, the transmission of vibrations from the cold head to the radiation shield or the additional cryocontainer can be minimized, wherein the heat transmission is still sufficient. The solid connection may moreover be flexible or rigid.
The cryocooler of the inventive cryostat configuration may, however, also be a pulse tube cooler or a Gifford-McMahon cryocooler having one cold stage.
The cold stage can preferably produce a temperature of 77 K or less, e.g. for liquefying nitrogen, permitting cooling of a cryocontainer with liquid nitrogen or of a radiation shield.
In order to prevent unnecessary heat input from the neck tube into the tubes of the cold head of the cryocooler, the tubes of the cold head of the cryocooler are advantageously at least partially surrounded by a thermal insulation in the region of at least one cold stage.
In one preferred embodiment of the invention, at least one of the cryocontainers comprises an electric heating means to control the pressure in the cryocontainer.
This control is also possible when an electric heating means is provided in or on the neck tube.
An electric heating means may also be provided at at least one cold stage of the cryocooler, which is in contact with this cold stage. The pressure in the cryocontainer can also be indirectly kept constant via this heating means.
The invention can be utilized with particular advantage if the superconducting magnet configuration is part of an apparatus for magnetic resonance spectroscopy, in particular, magnetic resonance imaging (MRI) or nuclear magnetic resonance spectroscopy (NMR). The elimination of disturbances due to vibrations is particularly important for recording MRI or NMR data. Direct cooling by the cryocooler ensures long, continuous operation. Even maintenance and repair works on the cooler can be performed with great safety for the personnel, without discharging the magnet and causing long downtimes. The additional expense for apparatus and construction is within tolerable limits.
Further advantages of the invention can be extracted from the description and the drawings. The features mentioned above and below may be used individually or collectively in arbitrary combination. The embodiments shown and described are not to be understood as exhaustive enumeration but have exemplary character for describing the invention.
The system is self-regulating. At the start, the shut-off valve 7 in the connecting line 8 between the cryocontainer 1 and the neck tube 2 is open, and therefore the pressure in the neck tube 2 and in the cryocontainer 1 is the same. During cooling of the cryostat configuration, cryogen is continuously supplied from the cryocontainer 1 to the neck tube 2 during and after filling the cryocontainer 1 with cryogenic liquid with the cryocooler being switched on. If the lower ends of the neck tube 2 and of the cold head 3 of the cryocooler are sufficiently cold, and if the cooling power of the cryocooler is also larger than the heat input into the cryocontainer 1 and into the neck tube 2, the cold head 3 starts to liquefy cryogen which collects in the lower region of the neck tube 2 in the form of a cryogen bath 11. In subsequent operation, the cryocooler sucks in more and more cryogen from the cryocontainer 1 via the connecting line 8 such that the liquid level of the cryogen bath 11 in the neck tube 2 continuously rises. Moreover, the pressure in the neck tube 2 and also in the cryocontainer 1 decreases, since the amount of gas liquefied by the cryocooler is larger than the amount of evaporated liquid. The pressure in the neck tube 2 and in the cryocontainer 1 is the same, and therefore the temperature in both partial areas is also the same (=the boiling temperature associated with the prevailing (vapor) pressure). Since there is no temperature difference between the cryocontainer and the neck tube, there is no heat flow between the cryocontainer 1 and the cryogen bath 11 in the neck tube 2.
The shut-off valve 7 in the connecting line is then closed (
If the cooling power of the cryocooler is higher than is required for condensing the evaporated cryogen, further gas from the neck tube 2 is liquefied and the pressure therein drops. This again causes a temperature drop in the cryogen bath 11 in the neck tube 2 with the result that the heat flow through the separating wall 5 or elements 6 having a good thermal conductivity increases and more cryogen evaporates from the cryogen bath 11 of the neck tube 2. Conversely, more vapor is condensed in the cryocontainer 1, such that the pressure therein also decreases unless external measures are taken. The pressure in the cryocontainer 1 is preferably controlled to a value which is larger than the ambient pressure, which can be realized via a controller PIC which controls a heating means 12 in the cryocontainer 1 in such a manner that the excess power of the cryocooler 1 is compensated for and the pressure remains constant. An equilibrium is quickly established, wherein the amount of cryogen evaporating in the cryogen bath 11 in the neck tube 2 is the same as the amount that can be reliquefied by the cryocooler 1.
The temperature difference and therefore also the pressure difference between the cryocontainer 1 and the neck tube 2 depend on the efficiency of heat transfer, the size of the heat transfer surface, as well as the thickness and the material of the separating wall 5. The better the heat transfer, the larger the heat transfer surface, the thinner the separating wall 5 and the higher the thermal conductivity of the wall 5, the smaller are the temperature and pressure differences.
The thermal resistance of the flexible elements 6 having a good thermal conductivity of
The configuration of
The proposed configuration functions even when the connecting line 8 between the suspension tube 9 and the neck tube 2 remains open, as is shown in
If the second cold stage 27 of the cold head 3 has excess power, its temperature will drop below the boiling temperature of the cryogen associated with this pressure, and the liquid in the cryogen bath 11 close to the flange 14 of the second cold stage 27 will sub-cool.
Additional heat is transferred through thermal conduction between the separating wall 5 and the second cold stage 27. Since liquid cryogen, such as e.g. liquid helium, has relatively poor heat conducting properties, the heat flow can generally be neglected considering the small temperature difference and the usually available exchange surfaces and distances between the separating wall 5 and the second cold stage 27.
It must be noted that the cooling power is lower due to the lower temperature of the second cold stage 27 of the cryocooler. Moreover, it can also decrease due to the changed ambient conditions (liquid instead of gas) around the tubes 13 of the second cold stage 27 of the cryocooler compared to a configuration with closed connecting line 8. As soon as the heat input into the cryogen bath 11 of the neck tube 2 via the separating wall 5 and the other contributions to the heat input (e.g. via the wall of the neck tube 2) have reached the magnitude of the cooling power of the cryocooler at the lower temperature, an equilibrium state is reached.
Operation of the inventive cryostat configuration with open connecting line 8 (
Moreover, the entire liquid bath 11 in the neck tube 2 could evaporate and an underpressure generated in the neck tube 2.
To control the pressure in the cryocontainer, the embodiment of
In all embodiments, the heat transfer between the second cold stage 27 of the cryocooler and the cryocontainer 1 is completely contact-free. Transfer of vibrations is therefore largely prevented.
When using a two-stage cryocooler (
To prevent or at least reduce undesired heat input from the neck tube 2 into the tubes 13 of the cold head, the tubes 13 of the cold head are surrounded by thermal insulation 24 in the region of the first cold stage and possibly also in the region of further cold stages. The tubes above the first cold stage of the cold head have temperatures between room temperature and the temperature of the first cold stage.
In general, the cryocontainer 1 cannot be cooled by a one-stage cold head 25 due to the excesssively low temperature, e.g. when it contains liquid helium. It is, however, feasible to cool either a further cryocontainer (in most cases a container with liquid nitrogen) or a radiation shield 20 in an analog manner as described above.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2005 029 151.1 | Jun 2005 | DE | national |
