Device and method for transferring liquid helium into an application cryostat

Abstract

A device (101) for transferring liquid helium (14) into an application cryostat (102), comprising: a storage dewar (1); a transfer line (9) comprising a first transfer line end (9a) in the storage dewar (1) and a second transfer line end (9b) for insertion into an application cryostat (102); a device (31) for generating a pressure difference between the storage dewar (1) and the application cryostat (102); is characterized by: a condensation heat exchanger (17) for condensing helium gas (12) to liquid helium (11), for insertion into the application cryostat (102); a cryocooler (29) for cooling the condensation heat exchanger (17); and a control device (8) with a measuring input (18a) for a pressure sensor (18) for measuring the gas pressure in the application cryostat (102) and a control output. (31a) for the device (31) for generating a pressure difference, wherein the control device (8) is programmed toThe device (31) is controlled to generate a pressure difference such that the volume of liquid helium (14) transferred per unit time through the transfer line (9) is approximately equal to the volume change of the helium that condenses from helium gas (12) to liquid helium (11) per unit time at the condensation heat exchanger (17). The device minimizes helium losses during the transfer of liquid helium in a simple manner.

Description

The invention relates to an apparatus for transferring liquid helium into an application cryostat comprisinga liquid helium supply dewar,a liquid helium transfer line for transferring liquid helium from the supply dewar to the application cryostat, comprising a first transfer line end arranged in the supply dewar and a second transfer line end for plugging into the application cryostat, andmeans for generating a pressure difference between the supply dewar and the application cryostat.Such a device is known from the company publication "NMR Magnet System Ultra Shield Magnets (English version) User Manual", version 006 (12.10.2004), chapter 12, the Bruker BioBlue AG, Duelanden, Switzerland.Superconducting magnets, for example for NMR spectroscopy (NMR=nuclear magnetic resonance) or magnetic resonance imaging, also referred to as MRI (MRI=magnetic resonance imaging), require cooling in order to ensure the superconducting state of the magnets. In many cases, the superconducting magnet is placed in an application cryostat in which liquid helium is stored, typically at a temperature of about 4.2 K, which corresponds to the boiling point of the liquid helium at atmospheric pressure. It is also known from other fields of application to carry out cooling with liquid helium.Since the application cryostat cannot effect perfect thermal insulation or heat is introduced into the application cryostat by the application, the liquid helium evaporates during operation. If the liquid helium level in the application cryostat has dropped too far for continued operation, liquid helium is replenished to the application cryostat.It should be noted here that helium is a short resource. Helium is a byproduct of natural gas production. Helium is available in the world market is becoming less and more expensive, and helium is being purchased, see D. Kramer, "Helium is against short in supply", Physics Today, April 4, 2022, American Institute of Physics, https: / / doi.org / 10.1063 / PT.6.2.20220404a.Therefore, efforts are being made to minimize the consumption of helium for the operation of application cryostats. Many consumers invest in helium recovery facilities that can collect and re-liquify evaporating helium.A considerable loss of helium occurs in the usual procedure for refilling liquid helium from a supply dewar into the application cryostat, as described in the aforementioned company publication "NMR Magnet System Ultra Shield Magnets (English version) User Manual". This usual procedure can be summarized as follows:Step a) The stock dewar ("transport dewar") is moved to a distance of a few metres from near the application cryostat, in said company publication of an NMR magnet.Step b) The warm transfer line is inserted into the transport dewar, so that its (first) end comes to lie below the liquid surface of the liquid helium contained.Step c) Helium vaporizes in the transport dewar due to the heat input of the warm transfer line. As a result, the pressure in the transport dewar rises and liquid helium is forced into the transfer line, which is thereby cooled. The helium escaping from the end of the transfer line during the cooling process is usually not collected and is lost to the atmosphere. For a typical transfer line of a few metres length, a few litres of liquid helium are required for cooling.Step d) Once the helium escaping from the (second) end of the conduit facing away from the transport dewar is sufficiently cold (i.e., the transfer conduit is sufficiently cooled), the transfer conduit is connected to the application cryostat (e.g., an NMR magnet), i.e., the second end is inserted into the application cryostat.Step e) After the transfer line is connected to the application cryostat, liquid helium flows from the transport dewar into the application cryostat. The mass flow is driven by the pressure difference existing between the transport dewar and the application cryostat. In the transport dewar, an overpressure has built up as mentioned above due to the insertion of the transfer line. As a rule, however, the pressure build-up in the transport dewar, which is established by the heat input during insertion of the transfer line, is not sufficient to transfer the desired quantity of liquid helium. Therefore, helium gas is introduced from a compressed gas cylinder into the conveying dewar via a pressure regulator, so that the pressure in the conveying dewar is kept constantly high. Typically, the overpressure set in the transport dewar is about 50-100 mbar. During the transfer of the liquid helium, gaseous helium corresponding to the volume of the incoming liquid helium is forced out of the application cryostat at an exit.For the sake of illustration: at the beginning of the transfer of the liquid helium into the application cryostats, the application cryostat or its helium tank is typically not empty, but still contains a small amount of liquid helium, and is otherwise filled with gaseous helium at a temperature of 4.2 K at a pressure of about 1 bar. As the helium tank slowly fills with liquid helium during the course of the transfer, the cold gaseous helium that was in the helium tank before the start of the transfer is successively forced out of the helium tank and escapes via the outlet of the application cryostat.Gaseous helium at 4.2 K and atmospheric pressure has a density of 16.5 g / l. Liquid helium at 4.2 K and atmospheric pressure has a density of 125 g / l. If, for example, 100 liters of liquid helium-i.e. 12.5 kg of helium-are transferred, 100 liters of gaseous helium-i.e. 1.65 kg-are accordingly forced out of the application cryostat. This corresponds to 13.2 liters of liquid helium or 13.2% of the amount transferred.In most cases, this helium quantity escapes via the outlet of the application cryostat simply into the atmosphere, which is not lasting and also makes the operation of the application cryostat more expensive.Alternatively, it is possible, for example, to install a gas balloon accumulator which is dimensioned sufficiently large to capture the helium (the "transfer losses") occurring during helium transfer at the outlet and to feed it to a high-pressure accumulator or condenser. In the pipe system leading to the gas balloon and in the gas balloon itself, the gaseous helium warms up to room temperature, which leads to a large volume increase of the gas. 100 litres of gaseous helium at atmospheric pressure and 4.2K correspond to about 10,000 litres (i.e. 10m 3) at atmospheric pressure and room temperature.In a typical helium transfer to an NMR magnet, between 100 and 400 liters of liquid helium are transferred depending on the type of magnet. A conventional transfer takes about one hour. In this time, therefore, between 10 and 40 cubic metres of gaseous helium are obtained at room temperature, which helium must be stored in a gas balloon or processed by a recovery system (e.g. compressed in pressure accumulators), corresponding to 13 l / h to 50 l / h of liquid helium (or between approximately 1.6 and 6.6 kg of helium per hour). Correspondingly large gas balloons or powerful recovery systems are very space-consuming and also very expensive.In the post-published German laid-open specification DE 10 2022 209 941 A1, it is proposed to supply the gaseous helium forced out at the application cryostat to the supply dewar via a return line during the transfer of liquid helium from the supply dewar into the application dewar.It should also be noted that it is also possible to actively cool application cryostats (e.g. containing superconducting magnets for NMR applications) with a cryocooler in continuous operation, cf. for example US 2002 / 0002830 A1. In this case, it is not necessary to replenish liquid helium, and the problem of helium loss when replenishing liquid helium is omitted.However, the active cooling system has various disadvantages compared to passive operation with a bath of liquid helium, in particular the introduction of vibrations through the cryo-cooler into the application cryostats, high energy consumption (approximately 8 kW in continuous operation), relatively high maintenance costs and relatively long downtime during the maintenance activities. In the case of cryogenically free actively cooled superconducting magnets (i.e. superconducting magnets which do not have a buffer volume of liquid helium), the very short time from any power failure to a breakdown of the superconductor in the superconducting magnet ("time-to-quench") is also added.U.S. Pat. No. 8,671,698 describes a helium re-condenser which is separate from the application cryostat and has a pulse tube cooler.US 2007 / 0107445 A1 has disclosed a retrofittable re-condenser for helium. The installation of such a device is very complicated. In addition, vibrations can also be introduced into the application cryostats here, and high energy and maintenance costs are created.U.S. Pat. No. 8,375,742 B2 describes a helium reflux condenser which is equipped with its own insulation jacket. Helium evaporating from an application cryostat is liquefied by the re-condenser and returned via a transfer tube, which is also surrounded by the insulation jacket. In a variant, a connection for an external gas source is additionally provided.US 2009 / 0301129 A1 describes a helium re-condenser for retrofitting magnetic resonance systems, with which evaporating nitrogen and evaporating helium are to be re-condensed.EP 0 245 057 B1 and EP 0 396 624 B1 disclose condensation heat exchangers which are connected to a cold head via a cooling circuit and which are inserted into a cryostat with liquid helium.DE 10 2021 205 423 A1 also describes a device in which helium is purified and liquefied using a single cold head.DE 40 39 365 A1 describes an NMR magnet with a cryostat in which subcooled liquid helium is arranged in a first, lower chamber and liquid helium is arranged in an upper, second chamber at atmospheric pressure at 4.2 K, with a heat-insulating but pressure-permeable barrier being arranged between the chambers.At the website https: / / webbook.nist.gov / chemistry / fluid / thermophysical properties of various fluid systems, for example for helium, can be sought. This web page is operated by the National Institute of Standards and Technology (NIST), U.S. Department of Commerce.DE 10 2020 204 186 A1 discloses a mobile liquefaction plant for liquefying helium. This comprises a liquefying device for liquefying helium, a buffer for liquefied helium, a purifying device for helium and an additional collecting device for gaseous helium comprising a container with a flexible wall. With the liquefying device and the cleaning device helium gas stored at the location of an application cryostat, which is vaporized during operation, can be purified and liquefied and collected in the intermediate storage. When the application cryostat is filled with the liquid helium from the intermediate store, evaporated helium gas can be collected by the additional collection device.DE 699 26 087 T2 describes a device for recondensation of liquid helium, liquid helium being stored in a container. Gaseous helium vaporized in the container is conducted via a line to a cooling device arranged outside the container and liquefied there. The liquefied helium is conducted back into the container via a further line.Object of the InventionIt is an object of the invention to minimize helium losses in a simple manner during a transfer of liquid helium from a supply dewar into an application cryostat.DESCRIPTION OF THE INVENTIONThis object is achieved according to the invention by a device of the type mentioned at the beginning, which is characterized in that the device further comprisesa condensation heat exchanger for condensing helium gas to liquid helium for insertion into the application cryostat,a cryocooler for cooling the condensation heat exchanger, anda control device, having at least one measurement input for a pressure sensor for measuring the gas pressure in the application cryostat and a control output for the device for generating a pressure difference, wherein the control device is programmed to control the device for generating a pressure difference such that a volume of liquid helium transferred per unit time through the transfer line is approximately equal to the volume change of the helium, which condenses per unit time at the condensation heat exchanger from helium gas to liquid helium.With the device according to the invention, it is made possible, during a transfer of liquid helium from the supply dewar into the application cryostats, to carry out in situ liquefaction of gaseous helium to liquid helium in the application cryostat with the condensation heat exchanger which is inserted into the application cryostats. The cooling capacity of the condensation heat exchanger (or of the cryocooler) on the one hand and the transfer rate of liquid helium in the application cryostats on the other hand can be matched to one another such that the volume change (volume decrease) of the helium, which condenses per unit time in the application cryostat, at least substantially (and preferably exactly) corresponds to the volume of liquid helium, which is transferred by means of the device for generating a pressure difference from the supply dewar through the transfer line into the application cryostat ("equilibrium position"). By adjusting and maintaining the equilibrium position during the transfer of liquid helium, which is effected by the control device, it is achieved that during the transfer of liquid helium, no (or only very little) gaseous helium is forced out of the application cryostat by the inflowing liquid helium. Accordingly, the need to trap and process forced out gaseous helium for recovery is eliminated. In particular, a large gas balloon for collecting the pushed-out gaseous helium is no longer required for the transfer of the liquid helium.By means of the pressure difference between the supply dewar and the application cryostat set with the device for generating a pressure difference (wherein a higher pressure is provided in the supply dewar than in the application cryostat, usually with a pressure difference of 50-100 mbar), the liquid helium is forced through the transfer line into the application cryostat. The device for generating (or setting) a pressure difference is connected to the control output of the control device and is controlled by the latter. By increasing the pressure difference, the transfer rate of liquid helium can be increased, and by decreasing the pressure difference, the transfer rate of liquid helium can be decreased. It should be noted that within the scope of the invention, the pressure difference as such need not be known. Nevertheless, it is preferably provided that the control device additionally monitors the pressure in the supply dewar with a further pressure sensor.The (at least approximately) maintaining of the equilibrium position is preferably ensured in that the pressure (helium gas pressure) in the application cryostat is kept at least approximately constant, or at least remains in a predefined pressure interval. The control device accordingly typically uses the pressure in the application cryostat as an input variable (controlled variable) or at least as one of the input variables for the actuation of the device for generating a pressure difference.The device for generating a pressure difference is typically designed to change the pressure (gas pressure) in the feed dewar, for example by changing the current of an electric heater in the feed dewar or by changing the position of a control valve (inlet valve) in a helium gas line from a helium gas accumulator (in particular helium compressed gas accumulator) to the feed dewar. The cooling capacity at the condensation heat exchanger or the cooling capacity of its cryo-cooler typically remains constant. However, it is also possible, alternatively or additionally, to change the pressure in the application cryostat via a device for generating a pressure difference, for example by changing the current of an electric heater in the application cryostat or by changing the cooling capacity at the condensation heat exchanger.The cryo-cooler generally comprises a cold head and a compressor. The cryocooler can comprise in particular a Gifford-McMahon cooler or also a pulse tube cooler. The cold head is thermally coupled to the condensation heat exchanger in order to cool the latter, preferably via a cooling circuit. The cryocooler provides the cooling power required for the condensation of the gaseous helium in the application cryostat.If, for example, 100 liters of liquid helium are to be replenished in the application cryostats in the context of the invention, 100 liters of gaseous helium (which is at a temperature of 4.2 K) must be liquefied in the application cryostat during the transfer of the liquid helium in order to avoid the outflow of helium gas from the application cryostat. The density of gaseous helium at 4.2 K and 1 bar is 16.5 g / l; correspondingly, about 1.65 kg of helium must be liquefied. Helium has a latent heat of 20.6 kJ / kg, and accordingly, during the transfer of liquid helium, an energy of about 34 kJ at a temperature of 4.2 K must be absorbed by the cryo-cooler. For example, if a commercially available cryocooler having a cooling capacity of 2 watts is used, then this energy can be absorbed in about 4.7 hours. The transfer rate of liquid helium to be used within the scope of the invention is then approximately 21.3 l / h. In practice, the power to be absorbed will be slightly higher than said 34 kJ, since additional heat inputs into the system occur, for example, through the helium transfer line and through the inserted condensation heat exchanger (e.g. arranged on a cooling rod). As a result, the transfer time is extended or the transfer rate is reduced correspondingly slightly.The time required for the transfer of the liquid helium within the scope of the invention depends, as can be seen from the above example, in particular on the quantity of liquid helium to be replenished, on the cooling capacity of the cryo-cooler and on additional heat inputs by incomplete isolation. Depending on the transfer time to be expected, the transfer of the liquid helium can be planned, in particular also as an automated transfer "overnight", if no applications (e.g. NMR measurements) are undertaken with the application cryostat in any case. Typical transfer times within the scope of the invention are 1 h to 16 h, and preferably 2 h to 12 h. It should be noted that the transfer time can be reduced when subcooled liquid helium is transferred from the stock dewar to the application cryostat (see also further below).The stock dewar is typically configured to be transportable ("transport dewar"), preferably with rollers, so that it can be moved back and forth between different laboratories and can then be used particularly easily for refilling a plurality of application cryostats. The supply dewar is preferably combined with the cryocooler to form an integrated transportable assembly (e.g. arranged on a common platform with rollers), in particular if the supply dewar also serves as a helium condenser.Typically, (during the transfer of liquid helium through the transfer line), the volume of liquid helium transferred per unit time through the transfer line dV(LHe trans) / dt and the volume change of helium condensing per unit time at the helium gas-liquid helium condensation heat exchanger dV(He cond) / dt are as follows:In addition, the following typically also applies:Preferred Embodiments of the Device of the InventionIn a preferred embodiment of the device according to the invention, the control device is programmed to maintain the pressure in the application cryostat approximately constant during the transfer of liquid helium into the application cryostats. As a result, it is possible in a particularly simple manner to keep the transfer process in the equilibrium position, that is to say to carry out such that a volume of liquid helium transferred per unit time through the transfer line is approximately equal to the volume change of the helium which condenses per unit time at the condensation heat exchanger of helium gas to liquid helium. Typically, the measured pressure in the application cryostat is adjusted by the control device to a predefined, fixed target value. It should be noted that the target value should be above the ambient atmospheric pressure (e.g. around 5 to 50 mbar) in order to avoid sucking in air into the application cryostats. Alternatively, it is also possible, for example, to adjust the measured pressure to a target value which is tracked during the transfer, for example at a fixed distance from the surrounding atmospheric pressure.Furthermore, an embodiment in which the application cryostat is sealed against a discharge of gaseous helium is preferred, without damaging safety devices. This simplifies the construction of the entire system during the transfer of liquid helium, and no expensive helium is lost in the transfer process.In an advantageous embodiment, it is provided that the control device has a pressure sensor for measuring the surrounding atmospheric pressure, and that the control device is programmed to keep the pressure in the application cryostat above a surrounding atmospheric pressure at all times, in particular by a corresponding actuation of the device for generating a pressure difference. This prevents air from being drawn into the application cryostat; a suction of air can lead to freezing of air constituents (e.g. moisture) in the application cryostat and thus to a blockage of the gas flow. Preferably, the pressure can also be controlled directly in the application cryostat, e.g. via an electric heater attached to the condensation heat exchanger or elsewhere in the application cryostat. In the event of errors, it can thus be avoided that the pressure in the application cryostat falls unacceptably low (for example, when the transfer line is icing up and is completely blocked).In a preferred embodiment, the device for generating a pressure difference comprises a control valve in a helium gas line, with the helium gas line comprising a first helium gas line end connected to the supply dewar and a second helium gas line end for connection to a helium gas storage device, in particular a helium compressed gas storage device. With the control valve to the helium gas storage device, the pressure in the transport dewar can be changed in a simple and rapid manner. The control valve can be actuated and adjusted automatically by the control device, for example with an electric motor. Note that the helium to be introduced from the helium gas storage into the transport dewar should be prepurified so that contaminants (e.g., water vapor or nitrogen) cannot enter and freeze out the transport dewar and the application cryostat. The transport dewar is preferably configured such that gaseous helium introduced therein from the helium gas storage can be liquefied, preferably with the cold head of the cryocooler, which also serves for cooling the condensation heat exchanger. A common cryocooler is preferably used to operate a cold trap for cleaning the helium tunnel and to liquify the helium gas in the transport dewar; for this purpose, in particular a device as described in DE 10 2021 205 423 A1 can be used.In an advantageous embodiment, the device for generating a pressure difference comprises an electric heater in the supply dewar. By heating with the electric heater in the supply dewar (or in its helium container), stored liquid helium in the supply dewar is evaporated, which increases the pressure in the supply dewar, whereby the transfer of the liquid helium can be driven. This procedure is particularly simple.Particular preference is given to an embodiment in which the device further comprisesa closed cooling circuit of a coolant, comprising a flow line from a cold head of the cryocooler to the condensation heat exchanger and a return line from the condensation heat exchanger to the cold head of the cryocooler, wherein the cryocooler is configured to cool the coolant directly or indirectly with the cold head, in particular wherein the cold head of the cryocooler is arranged separately from the application cryostat. By means of the installation of the cooling circuit, the location of the actual refrigeration (at the cold head) can be spatially separated in a simple manner from the location of the helium condensation (at the condensation heat exchanger); the condensation heat exchanger can be installed in particular at the end of a rigid cooling rod which can be introduced in a simple manner into the application cryostat. The cold head does not need to be inserted into the application cryostat, which can save a great deal of space on the application cryostat and would also be impossible in many cases, since the openings in application cryostats are preferably formed with very small cross sections (significantly smaller than commercially available cold heads). Typically, the cold head is also arranged separately from the application cryostat, which simplifies the construction of the overall system. In addition, the input of vibrations to the application cryostats can be minimized. A part of the feed line and the return line and optionally also a part of the transfer line can be integrated into the cooling rod. It should be noted that the cooling circuit usually also comprises a compressor feed line from a separate compressor to the cold head and a compressor return line from the cold head to the compressor.In a preferred development of this embodiment, starting from the supply dewar, the feed line, the return line and the transfer line run as line bundles in a common insulation vacuum. This construction enables in a simple manner an isolation of the feed line, return line and transfer line. The line bundle is particularly easy to handle; in particular, the second transfer line end (outlet end) of the transfer line and the condensation heat exchanger (at the end of the feed line / start of the return line) can be inserted particularly easily jointly into the application cryostat. The line bundle is usually arranged in a rigid tube ("cooling rod") in the rear region, i.e. close to the second transfer line end of the transfer line and of the condensation heat exchanger, which further simplifies handling.A subvariant of this refinement is also preferred, in which the feed line and the transfer line are thermally coupled to one another in the common insulation vacuum, in particular by a multiplicity of coupling bridges. As a result, it is possible to precool the transfer line with the cooling circuit or with the coolant in the feed line before the start of the transfer of the liquid helium, so that the helium transfer line is already cold when liquid helium flows through it at the start of the transfer. As a result, no liquid helium is consumed (i.e., evaporated) for cooling the transfer line.An embodiment is furthermore preferred in which the device further comprises a helium gas storage device which is connected to the supply dewar via a helium gas line, in particular wherein the helium gas storage device is a helium compressed gas storage device. As a result, it is possible to introduce helium gas into the feed dewar, in particular in order to increase the pressure in the feed dewar (the helium reservoir is then usually a helium compressed gas reservoir) and / or in order to liquify the introduced gaseous helium in the feed dewar (for which purpose the cold head of the cryocooler typically protrudes into the feed dewar or its feed cryostat, see also below).An embodiment is particularly preferred in which a cold head of the cryo-cooler and a helium container of the Leaschdewar are arranged in a common Leaschrystat, wherein the cold head of the cryo-cooler is configured to liquify helium gas into liquid helium in the Leaschdewar. If helium can be liquefied in the Speardewar, no liquid helium needs to be transported to the site of the application cryostat over long distances (e.g. using a truck), which would be technically complicated, so that the operation of the application cryostat can be significantly reduced. Instead, gaseous helium that is easier to transport can be transported to the application cryostat (for example in compressed gas cylinders), and / or helium that vaporizes during normal operation of the application cryostat is liquefied (typically after intermediate storage) in the application cryostat, which is particularly lasting.A further development of this embodiment is also advantageous, in which the helium container and the cold head are configured to provide a liquid subcooled helium volume in the helium container. The subcooled helium has a temperature below the boiling point at the prevailing pressure in the helium vessel. Usually, the pressure in the helium vessel is about (but slightly above) 1 bar, corresponding to a boiling point of 4.2 K. The subcooled liquid helium then has a temperature of less than 4.2 K. Preferably, the subcooled liquid helium in the helium vessel has a temperature of 4.0 K or less, particularly preferably 3.8 K or less. However, the provision of subcooled liquid helium is more energy-consuming than the provision of liquid helium at the boiling point (4.2 K), the energy expenditure increasing sharply with decreasing temperature of the subcooled liquid helium. Therefore, the subcooled liquid helium in the helium vessel preferably also has a temperature of 3.7 K or more. When subcooled liquid helium is fed into the application cryostats, additional cooling capacity is available for the condensation of gaseous helium into liquid helium in the application cryostat, corresponding to the temperature difference to the boiling point and specific heat capacity of the liquid helium. As a result, the transfer of the liquid helium can take place more quickly with a given cooling capacity of the cryocooler than with liquid helium at the boiling point.A subvariant of this refinement is particularly preferred, in which a thermal barrier is arranged in the helium container, with which thermal barrier the subcooled helium volume below the thermal barrier is separated from a liquid saturated helium volume above the thermal barrier. With this construction, a provision of subcooled liquid helium can be effected particularly efficiently and also comparatively easily. A thermal barrier which can be used in the context of this invention is described, for example, in DE 40 39 365 A1. The thermal barrier is thermally insulating but pressure permeable. The cooling head of the cryocooler can be arranged in a separate vacuum space which projects through the thermal barrier, so that a coldest stage of the cold head can provide cooling power below the thermal barrier.In a further subvariant, it is provided that a flow line of a cooling circuit for coolant of the cryo-cooler runs through the region of the liquid subcooled helium volume. For example, a large reservoir of "cold energy" is available at a temperature of less than 4.2 K for cooling the condensation heat exchanger. Due to the lower operating temperature, the heat transfer at the condensation heat exchanger can be configured particularly efficiently.In a further subvariant, the first transfer line end opens into the supply dewar in the region of the liquid subcooled helium volume, in particular close to a base of the helium container. Accordingly, the transfer line can easily convey subcooled liquid helium into the application cryostats.An embodiment is also advantageous in which the transfer line has a flushing valve in the region near the first transfer line end. Air present in the transfer line can be blown out through the flushing valve before the start of the transfer of liquid helium. For this purpose, some gaseous helium from the application cryostat, which is at a low overpressure with respect to the surrounding atmosphere, is conducted through the transfer line and through the flushing valve. This makes it possible to minimize the introduction of air into the application cryostats. In the simplest case, the flushing valve leads into the surrounding atmosphere, or alternatively to a helium recovery system (which is equipped with a cleaning function for separating air constituents).Inventive Application SystemsAlso included within the scope of the present invention is an application system comprising an apparatus according to the invention described above and an application cryostat, wherein the transfer line and the condensation heat exchanger are inserted into the application cryostats, in particular into a common access pipe of the application cryostat. In the application system according to the invention, a transfer of liquid helium from the supply dewar into the application cryostats can take place in a simple manner, wherein helium losses can be minimized. If the condensation heat exchangers (typically arranged on a cooling rod) and the transfer line for the liquid helium use the same access pipe (in particular suspension pipe of an NMR magnet), a sufficiently large outflow cross section can be provided in a simple manner for the case of a quench (sudden loss of the superconduction of the NMR magnet), namely at a second, unblocked access pipe (in particular suspension pipe). In addition, thermal coupling of the transfer line to a feed line of a coolant of the condensation heat exchanger is more easily possible.An embodiment of the application system according to the invention is particularly preferred, in which the application cryostat contains a superconducting magnet coil and an NMR probe head protrudes into a magnet bore of the magnet coil. The application cryostat can then be used for NMR measurements and the cost for NMR measurements on samples can be reduced due to comparatively low operating costs within the scope of the invention. Typically, the NMR probe head projects into a room temperature bore of the application cryostat which is coaxial with the magnet bore.Liquid Helium Transfer Method of the InventionAlso included within the scope of the present invention is a method for transferring liquid helium into an application cryostat, in particular using an apparatus according to the invention described above or an application system according to the invention described above, wherein a transfer line for liquid helium is connected with a first transfer line end to a supply dewar containing liquid helium and is inserted with a second transfer line end into the application cryostat, wherein liquid helium is transferred from the supply dewar via the transfer line into the application cryostat, wherein a flow of liquid helium through the transfer line is adjusted with a device for changing a pressure difference between the supply dewar and the application cryostat, characterized in that a condensation heat exchanger cooled with a cryocooler is provided, in the application cryostats, and gaseous helium is liquefied to liquid helium in the application cryostat, and a controller measures a pressure in the application cryostat and controls the pressure difference changing device so that a volume of liquid helium transferred per unit time through the transfer line is approximately equal to the volume change of helium condensing helium gas to liquid helium per unit time at the condensation heat exchanger. In the context of the method according to the invention, it is achieved that when liquid helium is replenished from the supply dewar, no helium gas or only a small amount of helium gas is forced out of the application cryostat by the liquid helium flowing in. This is achieved in that helium gas present in the application cryostat during the filling of the liquid helium into the application cryostat is liquefied in the application cryostat with the condensation heat exchanger, so that the associated volumes / volume changes approximately correspond ("equilibrium position"). Accordingly, no or only little helium gas is lost during the transfer of the liquid helium. Any helium recovery system for storing and re-liquefying the helium gas forced out is not necessary or can be designed to be comparatively small and cost-effective.A variant of the method according to the invention is particularly preferred, in which the control device maintains the pressure in the application cryostat approximately constant during the transfer of liquid helium into the application cryostats. By this procedure, it can be achieved in a simple manner that the volume of liquid helium transferred per unit time through the transfer line is approximately equal to the volume change of the helium, which condenses per unit time at the condensation heat exchanger from helium gas to liquid helium ("equilibrium position").In a preferred variant, no gaseous helium is discharged from the application cryostat during the transfer of liquid helium. Thus, helium losses during the transfer of the liquid helium can be completely eliminated without requiring a helium recovery system.A variant is also advantageous which provides that the control device measures a surrounding atmospheric pressure, and that the control device keeps the pressure in the application cryostat above the surrounding atmospheric pressure at all times, in particular by a corresponding control of the device for generating a pressure difference. This prevents air from being drawn in from the environment into the application cryostats. If there is a risk of the pressure in the application cryostat dropping too much, the transfer of liquid helium into the application cryostats is generally increased (by increasing the pressure in the supply dewar); if necessary (for example if the transfer line becomes clogged), an electric heater in the application cryostat can be switched on and / or turned on.In a preferred variant, it is provided that the control device actuates, as a device for changing a pressure difference, an adjusting valve in a helium gas line which leads from a helium gas storage, in particular helium compressed gas storage, to the supply dewar. This allows the pressure in the reservoir dewar to be increased directly and quickly to begin or increase the transfer of liquid helium.A variant is also advantageous in which the control device actuates an electric heater in the supply dewar as a device for changing a pressure difference. An electric heater is simple and inexpensive to install. Helium in the supply dewar can be vaporized by the heater, which increases the gas pressure in the supply dewar. A helium compressed gas reservoir is not required (but may nevertheless be provided).A variant is also preferred in which a coolant is guided in a closed cooling circuit through a feed line from a cold head of the cryocooler to the condensation heat exchanger and through a return line back to the cold head of the cryocooler, wherein the cold head of the cryocooler cools the coolant directly or indirectly, in particular wherein the cold head is arranged separately from the application cryostat. With the cooling circuit, the cold head can be spatially separated from the condensation heat exchanger, which saves installation space on the application cryostat (in particular in the region of the access pipes). In addition, the cold head can easily be used for further purposes, in particular for liquefying helium in the supply dewar.A variant is particularly preferred in which the transfer line between the supply dewar and the application cryostat is thermally coupled to the feed line. As a result, the transfer line for the liquid helium can be cooled via the cooling circuit.A further development of this variant provides that before the beginning of a transfer of liquid helium through the transfer line, the transfer line is first precooled with the coolant in the feed line. If liquid helium is then passed through the transfer line for a transfer for the first time, the transfer line is already cold and no or only little liquid helium evaporates on the way to the application cryostat. Liquid helium can thereby be saved.A variant is also preferred which provides that a cold head of the cryo-cooler and a helium container of the feed dewar are arranged in a common feed cryostat, and that before the start of the transfer of liquid helium gaseous helium from a helium gas storage, in particular helium compressed gas storage, is fed to the feed dewar and gaseous helium is condensed into liquid helium with the cold head in the feed dewar. In this variant, liquid helium can be generated on site in the supply dewar, and a complicated and difficult transport of liquid helium to the application site can be avoided. If necessary, helium can be processed in a practically closed circuit at the application site, which is lasting and cost-effective.A variant is particularly preferred which provides that a liquid subcooled helium volume is provided in a helium container of the supply dewar and that the liquid helium transferred through the transfer line is taken from the liquid subcooled helium volume. The subcooled helium volume provides additional cooling capacity in the application cryostat that helps liquify helium gas in the application cryostat, and liquid helium transfer can be particularly rapid.A further development of the aforementioned variant is also advantageous, which provides that the condensation heat exchanger is cooled with a coolant which circulates in a closed cooling circuit, and that the coolant is guided in a feed line through the region of the liquid subcooled helium volume. As a result, a particularly large reservoir of cold energy at a temperature of <4.2 K can be used for liquefaction in the application cryostat. The condensation heat exchanger is highly efficient and powerful.A variant is also advantageous in which, before the start of a transfer of liquid helium, the transfer line is firstly flushed with gaseous helium from the application cryostat via a flushing valve which is arranged near the first transfer line end in the transfer line. This minimizes the entry of impurities (air constituents) into the application cryostats. The branch to the flushing valve is typically located in the region of the reservoir dewar. Typically, more than 3 / 4 of the length of the transfer line can be flushed via the flush valve.A variant is also preferred in which the application cryostat is used alternately in a normal mode for an application and is filled with liquid helium in a refill mode, wherein during normal mode the condensation heat exchanger and the second transfer line end are not inserted into the application cryostat, and during refill mode the condensation heat exchanger and the second transfer line end are inserted into the application cryostat. The condensation heat exchanger and the transfer line are inserted only when they are used. Heat inputs in the normal operation in the application cryostats can thus be minimized.A further development of the above-mentioned variant is also preferred, which provides that a superconducting magnet coil is arranged in the application cryostat, and during normal operation as an application with an NMR probe head NMR measurements are carried out on samples arranged in a magnet bore of the superconducting magnet coil. In the context of the invention, lasting and cost-effective NMR measurements on the samples are possible.Further advantages of the invention will become apparent from the description and the drawing. Likewise, the features mentioned above and those set out further below can be used according to the invention individually or together in any desired combinations. The embodiments shown and described are not to be understood as a final enumeration, but rather have exemplary character for describing the invention.DETAILED DESCRIPTION OF THE INVENTION AND DRAWINGFIG. 1 shows a schematic view of a first embodiment of an application system according to the invention, comprising an apparatus according to the invention for transferring liquid helium and an application cryostat, wherein a cryocooler has a separate cryocooler cryostat; FIG. 2 shows a line bundle of transfer line, feed line and return line for the invention; FIG. 3 shows a schematic view of a second embodiment of an application system according to the invention, comprising an apparatus according to the invention for transferring liquid helium and an application cryostat, wherein a cold head of the cryo-cooler and a helium container of the reservoir dewar are arranged in a common reservoir cryostat; FIG. 4 shows a structural form for the supply dewar for a device according to the invention for the transfer of liquid helium, with thermal barrier in the helium container of the supply dewar.FIG. 1 schematically shows a first embodiment of an application system 100 according to the invention, comprising a device 101 for transferring liquid helium and an application cryostat 102 during a transfer of liquid helium.The device 101 comprises a supply dewar 1 with a helium container 2 which is thermally decoupled from the environment 4 by an insulation vacuum 3. Room temperature (20° C.) and an atmospheric pressure p atm of 1.00 bar prevail here in the environment 4. One or more radiation shields or multi-layer superinsulation can be provided in the insulation vacuum 3 (not shown in more detail).Liquid helium 5 is stored in a lower region in the helium container 2, and gaseous helium 6 stands above the liquid helium 5 in an upper region. In the embodiment shown, the liquid helium 5 and the gaseous helium 6 have a temperature of about 4.2 K in the supply dewar 1, and a pressure p vorrat of about 1.08 bar prevails here in the supply dewar 1. In the embodiment shown, an electric heater 7 is also arranged in the supply dewar 1, which can be controlled by an electronic control device 8.A transfer line 9 for liquid helium projects into the supply dewar 1. A first transfer line end 9a opens into the liquid helium 5, close to the bottom of the helium container 2. The stock dewar 1 is configured to be transportable, here with rollers 37 ("transport dewar").A second transfer line end 9b of the transfer line 9 is inserted into the application cryostat 102 through an access pipe 15a, and opens into a helium tank 10 of the application cryostat 102. The helium tank 10 is thermally insulated from the environment 4 by an insulation vacuum 13. One or more radiation shields can be provided in the insulation vacuum 13 (not shown in more detail).Liquid helium 11 is located in a lower region in the helium tank 10 of the application cryostat 102, and gaseous helium 12 is located in an upper region. In the embodiment shown, the liquid helium 11 and the gaseous helium 12 in the helium tank 10 each have a temperature of about 4.2 K, and a pressure P anwend of about 1.03 bar prevails in the helium tank.The pressures generally apply to P atm< P anwend< p vorrat. (for the transfer of the liquid helium through the transfer line 9).Since the pressure p vorrat prevailing in the supply dewar 1 is somewhat greater than the pressure p anwend, prevailing in the application cryostat 102, liquid helium 5 is forced from the supply dewar 1 through the transfer line 9 into the application cryostat 102, cf. the liquid helium 14 flowing out at the second transfer line end 9 b. Liquid helium is thus transferred from the supply dewar 1 into the application cryostat 102 in a manner driven by a pressure difference between the supply dewar 1 and the application cryostat 102.Not only the transfer line 9 but also a cooling rod 16 protrude through the access pipe 15 ain this case into the application cryostat 102; the access pipe 15 ais therefore also referred to as common access pipe 15 a. At a lower end of the cooling rod 16, a condensation heat exchanger 17 is formed. With the condensation heat exchanger 17, gaseous helium 12 is continuously liquefied in the helium tank 10 during the transfer of liquid helium.The condensation heat exchanger 17 is cooled with a coolant which circulates in a cooling circuit 19. The cooling circuit 19 runs with a feed line 20 from a cold head heat exchanger 25 aon a cold head 25 to the condensation heat exchanger 17 and with a return line 21 from the condensation heat exchanger 17 back to the cold head heat exchanger 25 aon the cold head 25 Near the condensation heat exchanger 17 end regions of the feed line 20 and return line 21 run in the cooling rod 16. The coolant circuit 19 is driven by a compressor 28, wherein a compressor feed line 26 leads from the compressor to the cold head heat exchanger 25 aand to the feed line 20, respectively, and a compressor return line 27 leads from the return line 21 and from the cold head heat exchanger 25 ato the compressor 28, respectively. The cold head 25 and the compressor 28 form the cryo-cooler 29 of the device 101.The compressor 28 operates the cold head 25 here, wherein dedicated cold head lines 35, 36 are set up between the compressor 28 and the cold head 25, and furthermore ensures the coolant circulation in the cooling circuit 19 of the condensation heat exchanger 17. In this case, the cooling circuit 19 can be separately activated and deactivated via the cooling circuit valve 19 a, arranged here in the compressor feed line 26.The cold head 25 and the cold head heat exchanger 25 aare arranged here in a cryocooler cryostat 30, which is separate from the application cryostat 102 and from the supply dewar 1. The cryo-cooler 29 can be constructed in particular as a Gifford-McMahon cooler or pulse tube cooler. The cooling of the cooling rod 16 or of the condensation heat exchanger 17 is set up with a cryocooler 29 or a refrigerating machine outside the application cryostat 102.The control device 8 monitors the pressure p anwend in the application cryostat 102 by means of a pressure sensor 18, which is arranged here on a second access pipe 15 bof the application cryostat 102. Incidentally, the second access pipe 15 bis free, and can particularly well serve as an emergency discharge for helium gas (for example, in the case of quenching). The pressure sensor 18 is connected at a measurement input 18 ato the control device 8. Furthermore, in the embodiment shown, the control device 8 monitors the pressure p atm in the environment 4 with a pressure sensor 8 aintegrated in the control device 8.In the embodiment shown, the control device 8 controls the heating power of the electric heater 7 in the supply dewar 1 when performing the transfer of liquid helium. The pressure in the supply dewar 1, and thus also the pressure difference between the supply dewar 1 and the application cryostat 102, can be changed via the heating power at the heater 7. The heater 7 thus represents a device 31 for generating a pressure difference between the supply dewar 1 and the application cryostat 102. The heater 7 is connected to a control output 31 aof the control device 8.The heating power is adjusted and tracked by the control device 8 in such a way that the pressure p anwend there is adjusted to a predefined setpoint value p anwendsoll in the application cryostat 102. The predefined setpoint value p anwendsoll is 1.03 bar here and is selected such that it is slightly (here by 0.03 bar) above the pressure p atm of the environment 4. In the simplest case, the predetermined setpoint value p anwendsoll is constant during the entire duration of the transfer of liquid helium. Should the weather, and thus the air pressure in the environment 4 or the pressure p atm, monitored by the sensor 8 a, change greatly during the transfer, the predefined setpoint value p anwendsoll can also be changed if necessary in order to remain at a desired distance (minimum distance and / or maximum distance) from the pressure p atm of the environment with the pressure p anwend in the application cryostat 102, in particular in order to avoid an intake of ambient air into the application cryostat 102 and / or in order to prevent triggering of overpressure fuses (overpressure valves, rupture disks, not shown in more detail) at the application cryostat 102.If, during the delivery of liquid helium into the application cryostat 102 through the transfer line 9, the pressure p anwend remains substantially at p anwendsoll then substantially the volume of liquid helium dV(LHe trans) / dt delivered per unit time corresponds to the volume change of the helium dV(He cond) / dt which condenses per unit time at the condensation heat exchanger 17 ("equilibrium position"). In this case, during the supplying of the liquid helium to the application cryostat 102, no gaseous helium exits from the application cryostat 102.If the pressure p anwend falls below p anwendsoll the control device 8 increases the heating power at the heater 7, so that additional helium is vaporized in the supply dewar 1, the pressure p vorrat in the supply dewar rises, the flow of liquid helium through the transfer line 9 is increased, and the gas pressure in the application cryostat 102 rises. If the pressure P anwend rises above p anwendsoll the control device 8 lowers the heating power at the heater 7 or completely switches it off, so that less or no helium at all is evaporated in the supply dewar 1, the pressure in the supply dewar 1 (also as a result of the trailing discharge of liquid helium 5) falls, the flow of liquid helium through the transfer line 9 is lowered, and the gas pressure in the application cryostat 102 falls.In the embodiment shown, the cooling capacity of the cryo-cooler 29 is kept constant during the transfer of liquid helium.In the embodiment shown, a superconducting magnetic coil 32 (also referred to as a magnet for short) is arranged in the application cryostat 102. In normal operation, an NMR probe head 33 protrudes into a room temperature bore (not shown in detail) of the application cryostat 102, so that a measurement sample 34 can be subjected to an NMR measurement in the magnetic field of the magnet 32 in its magnet bore. Note that, in normal operation, the transfer pipe 9 and the cooling rod 16 are drawn out from the access pipe 15 a, and only in replenishment operation, the transfer pipe 9 and the cooling rod 16 are inserted into the access pipe 15 a(the latter illustrated in FIG. 1 ).In the application cryostat 102 and in the supply dewar 1, fill level sensors can also be provided, which are read out by the control device 8 (not shown in more detail).FIG. 2 illustrates a rear section of a line bundle 40, which can be used within the scope of the invention on a device according to the invention for transferring liquid helium (cf. also FIG. 3 in this respect).In the line bundle 40, the feed line 20 and the return line 21 of the cooling circuit for the condensation heat exchanger 17 as well as the transfer line 9 for the liquid helium 14 which flows out into the application cryostat at the second transfer line end 9 brun. The line bundle 40 forms a common insulation vacuum 41 for the lines 9, 20, 21. Between the transfer line 9 and the feed line 20, a plurality of coupling bridges 42 made of material of good thermal conductivity, for example of high purity copper, are set up, by means of which a thermal coupling is set up between the feed line 20 and the transfer line 9. As a result, it is possible in particular to precool the transfer line 9 by means of the feed line 20 before the start of the transfer of liquid helium.The vertically extending part of the line bundle 40 forms here an easily manipulated cold rod 16. It should be noted that the line bundle beyond the cooling rod is preferably flexible. The line bundle may have a so-called super-insulation. In addition, spacers of low thermal conductivity are provided in order to keep the lines spaced apart from the outer insulation sheath.FIG. 3 schematically shows a second embodiment of an application system 100 according to the invention, comprising a device 101 for transferring liquid helium and an application cryostat 102. The application system 100 corresponds largely to the application system of FIG. 1, so that only the essential differences are explained below. For the sake of simplicity, the compressor and parts of the lines carrying the coolant are not shown in detail in FIG. 3.In the embodiment of FIG. 3, a common supply cryostat 50 is configured, in which both the cold head 25 and the helium container 2 are arranged. The cold head 25 is arranged in a receiving region 51 which is open toward the helium container 2, so that the gaseous helium 6 is distributed in the upper part of the helium container 2 and in the receiving region 51. The supply cryostat 50 here has a radiation shield 58, which is thermally coupled to a warmer cooling stage of the cold head 25 (coupling not shown in more detail).From a helium gas storage 52 which is designed as a helium compressed gas storage 53, a helium gas line 54 leads into the supply dewar 1. a first helium gas line end 54 aprojects into the upper part of the helium container 2, and a second helium gas line end 54 bis connected to the helium compressed gas storage 53. The helium compressed gas storage device 53 can be configured to be transportable, for example with rollers (not shown in more detail).The helium gas line 54 has, near its second helium gas line end 54 b, an adjusting valve 55, which can be actuated automatically with the control device 8, here with an electric motor (not shown in more detail).The helium gas line 54 leads past the cold head 25 through the receiving region 51 and is thermally coupled to the two cold stages of the cold head 25 by means of a helium supply heat exchanger 56. Helium gas 6 aflowing through the helium gas line 54 into the feed dewar 1 (or even helium gas 6 already present in the feed dewar 1) can be liquefied by means of the cold head 25.It should be noted that the helium gas 6a should be purified before liquefaction, for example by means of a cold trap (not shown in detail); here, for example, the apparatus described in DE 10 2021 205 423 A1 can be used.In the embodiment shown, a pressure sensor 57 is also provided, with which a pressure p vorrat in the supply dewar 1 is measured and monitored by the control device 8; the control device 8 ensures that p vorrat remains above p atm at all times (see also below), whereby an intake of ambient air into the supply dewar 1 is prevented. In addition, the control device 8 also monitors the pressure P anwend in the application cryostat 102 with the pressure sensor 18.The control device 8 can here use the heater 7 (see above) or also the control valve 55 as a device 31 for generating a pressure difference between the supply dewar 1 and the application cryostat 102. By increasing the opening cross section of the control valve 55, additional helium (helium gas and / or liquefied helium) can be introduced into the supply dewar 1, which increases the pressure p vorrat in the supply dewar 1. By reducing the opening cross section of the control valve 55 or by closing the control valve 55, the pressure p vorrat in the supply dewar 1 can be lowered (with a trailing discharge of liquid helium through the transfer line 9 into the application cryostat).The feed line 20, the return line 21 and the transfer line 9 extend here between the supply dewar 1 and the application cryostat 102 as a line bundle 40 in a common vacuum insulation 41 (cf. FIG. 2 ). In addition, the transfer line 9 has, close to its first transfer line end 9 a, a branch 60 to a flushing valve 59, wherein the branch 60 is located here within the common supply cryostat 50. Through the flushing valve 59, the transfer line 9 between the second transfer line end 9 bto the branch 60 can be flushed with helium gas 12 (which originates from the application cryostat 102), which takes place before the start of the transfer of liquid helium through the transfer line 9; it should be noted that p anwend > p atm for this purpose. To completely eliminate helium losses, the outlet of the purge valve 59 can be connected to a helium recovery system; the air that is purged into the helium recovery system can be separated from the recovery system in an upstream cold trap during the re-liquefaction of the helium (helium recovery system and cold trap not shown in any more detail).Inventive Process ProcedureIn the following, the sequence during the refilling of liquid helium from the supply dewar 1 into the application cryostat 102 is explained by way of example within the scope of a method according to the invention. The exemplary sequence can take place in particular on an application system 100 as illustrated in FIG. 3. Steps 2 to 6 can be assigned to a transfer operation, and step 7 to a normal operation of the application cryostat 102. Step 1 may take place in parallel with normal operation or as part of a transfer operation.Step 1) Helium gas is supplied from the helium compressed gas storage 53 to the supply dewar 1 via the helium line 54, wherein it is liquefied by the cold head 25. Liquid helium 5 collects in the helium container 2.Step 2) Once liquefaction is complete (e.g., because helium canister 2 is full or helium pressurized gas reservoir 53 is empty), transfer of liquid helium 5 may be prepared. For this purpose, the user first inserts the cooling rod 16 with the transfer line 9 and the condensation heat exchanger 17 into the application cryostat 102 into the access pipe 15 a.Step 3) The purge valve 59 is opened, and helium gas from the application cryostat 102 forces air out of the transfer line 9 through the purge valve 59. When the existing air has leaked, the purge valve 59 is closed.Step 4) Then, the refrigerant circuit valve (Bzz. 19 ain FIG. 1 ) for the coolant, whereby a portion of the refrigerant discharged from the compressor (Bzz. 28 in FIG. 1 ) and the circulation of the coolant in the cooling circuit 19 or through the condensation heat exchanger 17 is activated. Since the feed line 20 is thermally connected to the transfer line 9, the transfer line 9 cools down.Step 4) Once the condensation heat exchanger 17 and the transfer line 9 are cold, the transfer of liquid helium ("helium transfer") is started. This can either occur automatically if the pressure in the helium tank 10 of the application cryostat 102 falls due to the condensation that starts, or the pressure in the supply dewar 1 is actively increased, for example with the heater 7 or with helium gas 6 asupplied from the helium compressed gas storage means 53.Step 5) The helium transfer is now controlled by the control device 8 such that the transfer rate of the liquid helium through the transfer line 9 corresponds to the rate at which gaseous helium 12 condenses in the application cryostat 102, so that no helium gas 12 escapes from the application cryostat 102 during the helium transfer. Accordingly, according to the invention, the helium transfer can be carried out in such a way that no storage balloon is necessary in order to carry out the helium transfer without loss.Step 6) Once helium transfer is complete (e.g., because the application cryostat 102 is completely filled with liquid helium 11 or the liquid helium 5 in the reservoir dewar 1 is exhausted), the user removes the cooling rod 16 from the application cryostat 102 with the transfer line 9 and the condensation heat exchanger 17.Step 7) Now, using the application cryostat 102, and in particular the NMR magnet (Bzz contained in the application cryostat 102. 32 in FIG. 1 ) NMR measurements are continued only a few hours after the start of helium transfer. Compared to a conventional helium transfer with escape of gaseous helium through one of the access pipes 15 a, 15 b, the stabilization of the application cryostat 102 in the context of the invention takes place much more quickly, since no large amounts of cold gas have leaked through the access pipes (suspension pipes) 15 a, 15 b(the latter usually leads to a strong cooling of the access pipes during a conventional helium transfer, and during the return to the thermal equilibrium, the magnetic field is unstable).FIG. 4 shows an alternative design of the stock dewar 1 for the invention, which can be used, for example, in the embodiment of the application system of FIG. 3. Only the essential differences from the design of FIG. 3 will be explained.In the design shown in FIG. 4, a thermal barrier 70 is arranged in the helium container 2 of the supply dewar 1. The thermal barrier 70 is pressure-permeable (i.e. through which liquid helium can flow), but thermally insulating. The thermal barrier 70 can be formed in particular as described in DE 40 39 365 A1.The cold head 25 is arranged in a closed vacuum container 71, wherein the vacuum container 71 reaches beyond (below) the thermal barrier 70. The lowermost (coldest) cooling stage 73 of the cold head 25 can thereby cool the liquid helium 74 below the thermal barrier 70. This liquid helium 74 below the thermal barrier 70 is subcooled and has a temperature of about 3.7 K here. The subcooled liquid helium 74 is also referred to in its entirety as subcooled liquid helium volume 74. Above the thermal barrier 70, saturated liquid helium 75 is at a temperature of about 4.2 K; the saturated liquid helium 75 is also referred to in its entirety as saturated liquid helium volume 75. The gaseous helium 6 above it likewise has a temperature of about 4.2 K and is under the pressure p vorrat of about 1.08 bar.The transfer line (conduit tube) 9 protrudes to just short of the bottom of the helium container 2 deeply into the region of the subcooled liquid helium 74 In the section in which the transfer line 9 passes through the region of the saturated helium volume 75, the transfer line 9 is thermally insulated from the saturated helium volume 75 (not shown in detail). Accordingly, as part of the helium transfer, subcooled liquid helium 74 is conveyed through the transfer line 9 into the application cryostat.The subcooled liquid helium 74 conveyed into the application cryostat can remove thermal energy from the application cryostat as the subcooled liquid helium re-heats to 4.2K. Less cooling power must then be applied to the condensation heat exchanger in order to condense gaseous helium at the condensation heat exchanger. If, for example, 100 l of liquid helium (12.5 kg) are transferred at a temperature of 3.7 K, approximately 26.3 kJ of additional cold energy are available, which can be used for the required approximately 34 kJ for the condensation heat (see above). However, the lower the temperature of the subcooled liquid helium 74, the more energy-consuming it becomes.The feed line 20 of the cooling circuit for the condensation heat exchanger in the application cryostat runs through the region of the subcooled liquid helium 74.It should be noted that subcooled helium for the invention can also be effected, for example, by means of releasing helium in a throttle to low pressure (not shown in more detail)In summary, the invention relates to a device (101) for transferring liquid helium (14) into an application cryostat (102), comprisinga stock dewar (1),a transfer line (9) comprising a first transfer line end (9a) in the supply dewar (1) and a second transfer line end (9b) for insertion into an application cryostat (102),a device (31) for generating a pressure difference between the supply dewar (1) and the application cryostat (102), characterized bya condensation heat exchanger (17) for condensing helium gas (12) to liquid helium (11) for insertion into the application cryostat (102),a cryocooler (29) for cooling the condensation heat exchanger (17), anda control device (8), having a measurement input (18a) for a pressure sensor (18) for measuring the gas pressure in the application cryostat (102) and a control output (31a) for the device (31) for generating a pressure difference, wherein the control device (8) is programmed to control the device (31) for generating a pressure difference such that a volume of liquid helium (14) transferred per unit time through the transfer line (9) is approximately equal to the volume change of the helium which condenses per unit time at the condensation heat exchanger (17) of helium gas (12) to liquid helium (11). Helium losses during the transfer of liquid helium are easily minimized with the device.List of reference characters1 Feed Dewar 2 Helium container 3 Insulation vacuum (at the feed Dewar) 4 Environment / surrounding atmosphere 5 Liquid helium (in the feed Dewar) 6 Gaseous helium (in the feed Dewar) 6 aGas helium (from helium gas storage) 7 Electric heater (in the feed Dewar) 8 Electronic control device 8 a Pressure sensor for the surrounding atmosphere 9 Transfer line for liquid helium 9 aFirst transfer line end (inserted into the feed Dewar) 9 bSecond transfer line end (inserted into application cryostats) 10 Helium tank 11 Liquid helium (in the application cryostat) 12 Gaseous helium (in the application cryostat) 13 Insulation vacuum (at the application cryostat) 14 (outflowing / transferred) liquid helium 15 aFirst / common access pipe 15 bSecond access pipe 16 cooling rod 17 condensation heat exchanger 18 pressure sensor for application cryostats 18 ameasuring input (for pressure sensor 18) 19 cooling circuit 19 acooling circuit valve 20 flow line for coolant 21 return line for coolant 22 vacuum insulation (flow line and return line) 23 vacuum insulation (transfer line) 25 cold head 25 a cold head heat exchanger (for cooling circuit) 26 compressor flow line 27 compressor return line 28 compressor 29 cryocooler 30 cryocooler cryostat 31 device for generating a pressure difference 31 acontrol output (for device 31) 32 superconducting magnet coil / magnet 33 NMR probe head 34 measurement sample 35, 36 cold head lines 37 rollers 40 line bundle 41 common insulation vacuum 42 coupling bridges 50 common storage cryostat 51 receiving region 52 helium gas storage 53 helium compressed gas storage 54 helium gas line 54 afirst helium gas line end 54 bsecond helium gas line end 55 control valve 56 helium supply heat exchanger 57 pressure sensor for storage dewar 58 radiation shield 59 flushing valve 60 branch for flushing valve 70 thermal barrier 71 vacuum container 73 lowermost / coldest cooling stage 74 subcooled liquid helium / helium volume 75 saturated liquid helium / helium volume 100 application system 101 liquid helium transfer device 102 application cryostat

Claims

Device (101) for transferring liquid helium (14) into an application cryostat (102), comprising - a supply dewar (1) for liquid helium (5), - a transfer line (9) for liquid helium (14) for transferring liquid helium (5) from the supply dewar (1) into the application cryostat (102), comprising a first transfer line end (9a) arranged in the supply dewar (1) and a second transfer line end (9b) for insertion into the application cryostat (102), - a device (31) for generating a pressure difference between the supply dewar (1) and the application cryostat (102), characterized in that, the device (101) further comprising - a condensation heat exchanger (17) for condensing helium gas (12) to liquid helium (11) for insertion into the application cryostat (102), - a cryocooler (29) for cooling the condensation heat exchanger (17) and - a control device (8) having at least one measurement input (18a) for a pressure sensor (18) for measuring the gas pressure in the application cryostat (102) and a control output (31a) for the device (31) for generating a pressure difference, wherein the control device (8) is programmed to control the device (31) for generating a pressure difference such that a volume of liquid helium (14) transferred per unit time through the transfer line (9) is approximately equal to the volume change of the helium, which condenses per unit time at the condensation heat exchanger (17) from helium gas (12) to liquid helium (11).Device (101) according to claim 1, characterised in that the control device (8) is programmed to keep the pressure (p anwend) in the application cryostat (102) approximately constant during the transfer of liquid helium (14) into the application cryostat (102).Device (101) according to any one of the preceding claims, characterized in that the application cryostat (102) is sealed against a discharge of gaseous helium (12), without any damage to any safety devices.Device (101) according to one of the preceding claims, characterized in that the control device (8) has a pressure sensor (8a) for measuring the surrounding atmospheric pressure (p atm) and in that the control device (8) is programmed to keep the pressure (p anwend) in the application cryostat (102) above a surrounding atmospheric pressure (p atm) at all times, in particular by a corresponding actuation of the device (31) for generating a pressure difference.Device (101) according to one of the preceding claims, characterized in that the device (31) for generating a pressure difference comprises an adjusting valve (55) in a helium gas line (54), with the helium gas line (54) comprising a first helium gas line end (54a) connected to the supply dewar (1) and a second helium gas line end (54b) for connection to a helium gas storage device (52), in particular a helium compressed gas storage device (53).Device (101) according to one of the preceding claims, characterized in that the device (31) for generating a pressure difference comprises an electric heater (7) in the supply dewar (1).Device (101) according to one of the preceding claims, characterized in that the device (101) further comprises - a closed cooling circuit (19) of a coolant, comprising a feed line (20) from a cold head (25) of the cryocooler (29) to the condensation heat exchanger (17) and a return line (21) from the condensation heat exchanger (17) to the cold head (25) of the cryocooler (29), wherein the cryocooler (29) is configured to cool the coolant directly or indirectly with the cold head (25), in particular wherein the cold head (25) of the cryocooler (29) is arranged separately from the application cryostat (102).Device (101) according to Claim 7, characterized in that, starting from the feed dewar (1), the feed line (20), the return line (21) and the transfer line (9) run as a line bundle (40) in a common insulation vacuum (41).Device (101) according to Claim 8, characterized in that, in the common insulation vacuum (41), the feed line (20) and the transfer line (9) are thermally coupled to one another, in particular by a multiplicity of coupling bridges (42).Device (101) according to one of the preceding claims, characterized in that the device (101) further comprises a helium gas store (52) which is connected to the supply dewar (1) via a helium gas line (54), in particular wherein the helium gas store (52) is a helium compressed gas store (53).Device (101) according to one of the preceding claims, characterized in that a cold head (25) of the cryo-cooler (29) and a helium container (2) of the storage dewar (1) are arranged in a common storage cryostat (50), wherein the cold head (25) of the cryo-cooler (29) is configured to liquify helium gas (6, 6a) into liquid helium (5) in the storage dewar (1).Device (101) according to claim 11, characterised in that the helium container (2) and the cold head (25) are configured to provide a liquid subcooled helium volume (74) in the helium container (2).Device (101) according to claim 12, characterised in that a thermal barrier (70) is arranged in the helium container (2), with which thermal barrier the subcooled helium volume (74) below the thermal barrier (70) is separated from a liquid saturated helium volume (75) above the thermal barrier (70).Device (101) according to either of Claims 12 and 13, characterized in that a feed line (20) of a cooling circuit (19) for coolant of the cryocooler (29) runs through the region of the liquid subcooled helium volume (74).Device (101) according to one of Claims 12 to 14, characterized in that the first transfer line end (9a) opens out in the feed dewar (1) in the region of the liquid subcooled helium volume (74), in particular close to a base of the helium container (2).Device (101) according to one of the preceding claims, characterized in that the transfer line (9) has a flushing valve (59) in the region close to the first transfer line end (9a).Application system (100) comprising a device (101) according to any one of the preceding claims and an application cryostat (102), wherein the transfer line (9) and the condensation heat exchanger (17) are inserted into the application cryostat (102), in particular into a common access pipe (15a) of the application cryostat (102).The application system (100) according to claim 17, characterized in that the application cryostat (102) includes a superconducting magnet coil (32), and an NMR probe head (33) is projected into a magnet bore of the magnet coil (32).Method for transferring liquid helium (14) into an application cryostat (102), in particular using an apparatus (101) or an application system (100) according to one of the preceding claims, wherein a transfer line (9) for liquid helium (14) is connected by a first transfer line end (9a) to a supply dewar (1) containing liquid helium (5) and is inserted by a second transfer line end (9b) into the application cryostat (102), wherein liquid helium (14) is transferred from the supply dewar (1) via the transfer line (9) into the application cryostat (102), wherein a flow of liquid helium (14) through the transfer line (9) is adjusted by means (31) for changing a pressure difference between the supply dewar (1) and the application cryostat (102), characterized in that a condensation heat exchanger (17), which is cooled by a cryo-cooler (29), is inserted into the application cryostat (102) and liquifies gaseous helium (12) to liquid helium (11) in the application cryostat (102), and in that a control device (8) measures a pressure (p anwend) in the application cryostat (102) and controls the device (31) for changing a pressure difference such that a volume of liquid helium (14) transferred per unit time through the transfer line (9) is approximately equal to the volume change of the helium, which condenses from helium gas (12) to liquid helium (11) at the condensation heat exchanger (17) per unit time.Method according to claim 19, characterised in that during the transfer of liquid helium (14) into the application cryostats (102), the control device (8) keeps the pressure (p anwend) in the application cryostat (102) approximately constant.Method according to claim 19 or 20, characterised in that during the transfer of liquid helium (14) no outflow of gaseous helium (12) from the application cryostat (102) takes place.Method according to one of Claims 19 to 21, characterized in that the control device (8) measures a surrounding atmospheric pressure (p atm) and in that the control device (8) keeps the pressure (p anwend) in the application cryostat (102) above the surrounding atmospheric pressure (p atm) at all times, in particular by a corresponding actuation of the device (31) for generating a pressure difference.Method according to one of Claims 19 to 22, characterized in that the control device (8) actuates, as a device (31) for changing a pressure difference, an actuating valve (55) in a helium gas line (54), which valve leads from a helium gas store (52), in particular helium compressed gas store (53), to the supply dewar (1).Method according to one of Claims 19 to 23, characterized in that the control device (8) actuates an electric heater (7) in the supply dewar (1) as a device (31) for changing a pressure difference.Method according to one of Claims 19 to 24, characterized in that a coolant in a closed cooling circuit (19) is guided through a feed line (20) from a cold head (25) of the cryocooler (29) to the condensation heat exchanger (17) and through a return line (21) back to the cold head (25) of the cryocooler (29), wherein the cold head (25) of the cryocooler (29) cools the coolant directly or indirectly, in particular wherein the cold head (25) is arranged separately from the application cryostat (102).Method according to claim 25, characterised in that the transfer line (9) is thermally coupled between the supply dewar (1) and the application cryostat (102) to the forward line (20).Method according to claim 26, characterised in that before the beginning of a transfer of liquid helium (14) through the transfer line (9), the transfer line (9) is first precooled with the coolant in the feed line (20).Method according to one of Claims 19 to 27, characterized in that a cold head (25) of the cryocooler (29) and a helium container (2) of the supply dewar (1) are arranged in a common supply cryostat (50), and in that, before the start of the transfer of liquid helium (14), gaseous helium (6a) is fed from a helium gas store (52), in particular helium compressed gas store (53), to the supply dewar (1) and gaseous helium (6, 6a) is condensed into liquid helium (5) by means of the cold head (25) in the supply dewar (1).Method according to one of Claims 19 to 28, characterized in that a liquid subcooled helium volume (74) is provided in a helium container (2) of the supply dewar (1), and in that the liquid helium (14) transferred by the transfer line (9) is taken from the liquid subcooled helium volume (74).Method according to claim 29, characterised in that the condensation heat exchanger (17) is cooled with a coolant which circulates in a closed cooling circuit (19), and in that the coolant is guided in a feed line (20) through the region of the liquid subcooled helium volume (74).Method according to one of Claims 19 to 30, characterized in that, before the start of a transfer of liquid helium (14), the transfer line (9) is firstly flushed with gaseous helium (12) from the application cryostat (102) via a flushing valve (59) which is arranged near the first transfer line end (9a) in the transfer line (9).Method according to one of claims 19 to 31, characterised in that the application cryostat (102) is used alternately in a normal mode for an application and is filled with liquid helium (14) in a refilling mode, wherein during normal mode the condensation heat exchanger (17) and the second transfer line end (9b) are not inserted into the application cryostat (102), and during refilling mode the condensation heat exchanger (17) and the second transfer line end (9b) are inserted into the application cryostat (102).Method according to claim 32, characterized in that a superconducting magnet coil (32) is arranged in the application cryostat (102), and during normal operation as an application with an NMR probe head (33), NMR measurements are performed on samples (34) arranged in a magnet bore of the superconducting magnet coil (32).

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