Patent Application
20110271694
This application claims Paris Convention priority of DE 10 2010 028 750.4 filed May 07, 2010 the complete disclosure of which is hereby incorporated by reference.
The invention concerns a cryostat configuration, with at least one cryostat, which has at least one first chamber with supercooled liquid helium having a temperature of less than 4 K and at least one further chamber, which contains liquid helium at essentially atmospheric pressure having a temperature of approximately 4.2 K, wherein a Joule-Thomson valve is disposed in the first chamber, the first chamber being separated from the further chamber by a thermally insulating barrier, wherein helium from the first or the further chamber expands through the Joule-Thomson valve into a pump-off pipe, which is in thermal contact with the helium of the first chamber and supercools the latter, and wherein the pump-off pipe is directly or indirectly in thermal contact with the further chamber during its further progression and is then connected to the inlet of a pump.
Such a cryostat configuration is known from DE 40 39 365 C2 (U.S. Pat. No. 5,220,800).
Magnet systems for magnetic resonance equipment are subject to the highest achievable demands in terms of the magnetic field strengths and homogeneity.
At a resonance frequency of 600 MHz, a field strength of 14.1 T must be achieved. These high magnetic field strengths are technically best achieved using superconducting magnet coils having a superconducting short-circuiting switch.
The superconducting magnet coils only require energy during the charging phase and produce a high magnetic field in short-circuited operation for a long time after the power supply has been disconnected. Decay times until half the original field strength is reached are around 5000 years for modern superconducting magnets. This means that, in short-circuited operation over a period of hours and days, practically no change occurs in the magnetic field strength.
High stability over time is above all necessary in long-term measurements, especially in 2D and 3D measurements. This can only be achieved in superconducting short-circuited operation. The magnet coils are usually charged once and then produce a homogeneous magnetic field for many years once the power cables have been removed. In routine operation, typical intervals between helium refills of magnet systems are several months in the case of “low-loss” cryostats.
To obtain stronger homogeneous magnetic fields and a more stable superconductor, a publication by Williams et al. in “Rev. Sci. Instrum.” 52 (5), May 1981, American Institute of Physics, 649-656, proposes operating the superconducting magnet coil at a lower operating temperature than the normal temperature of liquid helium (T=4.2 K). This lower temperature is usually achieved by pumping off the liquid helium.
In the cited publication, a cryostat is proposed that has two concentric helium tanks, one nesting inside the other. The outer tank contains liquid helium at T=4.2 K under normal pressure (1 bar).
From this outer tank, a filling pipe for liquid helium leads to the inner tank so that the liquid helium can be moved from the outer to the inner tank. In the inner tank, in which the superconducting coil is located, the helium is pumped off down to a pressure of 40 mbar and thus cooled down to a temperature of 2.3 K.
One major disadvantage of this configuration is that the supercooled helium in the inner tank is under vacuum so that the electrical supply cables, especially those for charging the superconducting magnet coil, have to be routed through the cold vacuum system. This gives rise to sealing problems and also insulation problems due to the heat input into the cold vacuum reservoir through the supply cables brought in from an environment at room temperature and under normal pressure, which necessarily results in much reduced intervals between helium refills.
A further disadvantage is that no means are provided to lower the helium consumption required to operate this equipment, with the result that both enormous operating costs are incurred and only relatively short intervals between liquid helium refills are achieved, except in cases where it is in any event necessary to constantly fill the equipment with fresh helium during operation.
DE 40 39 365 C2 and U.S. Pat. No. 5,220,800 propose a system in which two temperature ranges are provided in a first and in a further chamber, wherein, in the first chamber, liquid helium, which enters from the further chamber under normal pressure and a temperature of T=4.2 K, is cooled by pumping off through a restrictor in a state of non-equilibrium.
This equalizes the pressure level in the first chamber with the pressure level in the further chamber. Because atmospheric pressure essentially prevails in the first chamber with the supercooled liquid helium, the problems associated with a vacuum bushing for the electrical supply cables to the superconducting magnetic coil do not occur.
Because of the vertical disposition of the first chamber underneath the further chamber, gravity counteracts flowback of the denser and therefore heavier supercooled helium from the lower cold reservoir into the upper warmer reservoir. This ensures defined flow conditions and there is no unwanted mixing of cold and warm helium in the upper reservoir.
A thermally insulating barrier not only prevents convection between the two chambers but, to a great extent, also heat transfer by thermal conduction from one chamber to the other. The barrier consists of two plates separated by a vacuum and consisting of a poorly thermally conducting material, such as stainless steel or plastic. The vacuum insulation prevents heat exchange between the upper and the lower reservoir.
To avoid unwanted cooling of the helium in the further chamber, an electric heating element is disposed in the further chamber usually in addition to the thermal insulation.
The vacuum is part of the single vacuum part of the cryostat, so that the barrier does not have to be separately evacuated.
In contrast to continuous tank systems, these measures cause a drastic reduction in the heat entering from the outside and are a precondition for a cryostat with low operating losses (“low loss”).
The electrical supply cables to the magnetic system and the supply cables for liquid helium are routed inside through the conduit passing through the tower or towers. This hollow conduit design results in a dual cryostat that can be used both at 4.2 K under normal pressure and in vacuum operation in the range, for example, from 1.8 K to 2.3 K.
In both operating modes, the cryostat has low-loss properties because, irrespective of the proportion of the helium flow that is pumped off and evaporates, the total enthalpy in both gas flows is essentially passed to the shield system of the cryostat.
Because the cryostat contains two chambers with helium at two different temperature levels, there are two exhaust gas flows at different pressure levels. One exhaust gas flow arises due to the helium evaporating from the further chamber at atmospheric pressure; the second exhaust gas flow is formed by the helium pumped off through the refrigerator under a pressure of approx. 40 mbar.
Depending on the operating state of the cryostat, the two exhaust gas flows have different strengths, and the exhaust gas flow from the further chamber may even cease altogether. For a low-loss cryostat, it is essential that the enthalpy contained in the exhaust gas be utilized as-completely as possible. For this purpose, it is necessary to distribute the two exhaust gas flows evenly among the various towers and the shields connected to them, whatever the strength of the two flows.
The object of this invention is to lower the helium consumption and therefore the operating costs still further as compared to prior art, while keeping the pressure in the first chamber as constant as possible.
The object is inventively achieved by fluidically connecting the outlet of the pump and/or an outlet for evaporating helium of the or of at least one of the cryostats through a cryogen pipe with the further chamber, and providing the cryogen pipe with a branch-off device, which returns a partial current of the helium located in the cryogen pipe into the further chamber.
Instead of releasing the total pumped-off helium into the atmosphere, as was previously the case, the inventive cryostat configuration leads part of the helium into the first chamber. The helium is re-condensed in the cryogen pipe, which becomes increasingly colder inside the cryostat. The thermal energy of the helium is brought into the further chamber, making a heating element superfluous.
The helium for re-condensation can originate from the same cryostat, into which the partial current is to be returned. This would be the case, for example, if only one cryostat were present. However, in a configuration having multiple cryostats, it is conceivable for the evaporated or pumped-off helium of one or more further cryostats to be input into one of the cryostats for re-condensation.
One especially preferred embodiment is characterized in that a pressure regulating device is provided, which keeps the pressure in the further chamber constant. This could be implemented, for example, using an actively or a passively regulated valve on the cryogen pipe. A constant pressure is indispensable for an even temperature distribution and especially important for highly sensitive NMR measurements.
In a further embodiment, a heating device is provided in the further chamber. Although the necessary heat input into the further chamber can only be achieved by the helium supplied to the cryostat, embodiments are conceivable in which pressure regulation is achieved by means of the evaporation rate of the helium from the further chamber.
It is advantageous if, in the embodiments stated above, the pressure regulating device sets the pressure in the further chamber to a settable target pressure that is greater than or equal to the ambient pressure of the cryostat configuration.
Alternatively, the pressure regulating device sets the pressure in the further chamber to a defined positive pressure above atmospheric pressure.
In a further embodiment, the cryogen pipe has at least one relief valve and/or at least one bursting disk. This ensures controlled pressure reduction in the event of an unexpected large increase in pressure.
An embodiment is also conceivable that is characterized in that the cryogen pipe contains a buffer vessel for the provision of an additional volume for the flowing helium. In this way, a reserve volume is constituted in case more helium has to be supplied to the cryostat. The buffer volume is also an additional means of keeping the pressure constant.
An embodiment is especially preferred in which the cryogen pipe has at least one filtering device for separating off impurities in the helium. Impurities that enter the first chamber can constitute significant heat input. Moreover, solids and frozen matter can be deposited, narrowing or even blocking pipes and valves. For that reason, the helium used must be of high purity.
It is preferred if the partial flow returned into the further chamber comprises between 20% and 80%, preferably between 25% and 60% of the total helium flow conveyed through the pump.
In a further conceivable embodiment, helium is input into the cryogen pipe from at least one further, physically separate cryostat. This embodiment is especially advantageous if multiple cryostats are installed in a place of work, such as a research institute. In this case, the evaporating helium from one cryostat can, for example, be input into another cryostat and cooled in the manner described.
A preferred embodiment is characterized in that a superconducting magnet coil is disposed in the first chamber.
In a variant of this embodiment, the cryostat configuration is part of NMR, MRI, or FTMS equipment.
In a further variant of the embodiment, the equipment comprises an ultrahigh-resolution high-field NMR spectrometer with a proton resonance frequency ≧800 MHz.
The first chamber and the further chamber can be disposed either one above the other or side by side.
Further advantages of the invention can be derived from the description and the drawing. The characteristics stated above and below can also be used individually or together in any combinations. The embodiments shown and described are not to be considered an exhaustive list but are intended as examples to explain the invention.
The invention is shown in the drawing and is explained in more detail using the example of the embodiments. The figures show:
In this way, the helium consumption of the entire cryostat configuration 20 is reduced from around 230 ml/h without helium return to around 170 ml/h.
Finally,
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2010 028 750.4 | May 2010 | DE | national |
