APPARATUS AND METHOD FOR SUPER-COOLED OPERATION OF A CRYOSTAT WITH LOW QUANTITIES OF COOLANT

Abstract

A cryostat arrangement (1) having a vacuum container (2) and an object (4) to be cooled, which is arranged inside the vacuum container. A neck tube (8) leads to the object, and a cooling arm (10) of a cold head (11), around which a closed cavity (9) is formed, is arranged in the neck tube, which is sealed off fluid-tight in relation to the object and is filled with cryogenic fluid in normal operation. A thermal coupling element (15) couples the cryogenic fluid in the cavity to the object. A pump device (14), to which the cavity is connected via a valve (13) and with which the cavity is pumped out if the cold head fails. A monitoring unit (17) monitors the cooling function of the cold head, and activates the pump device to pump out the cavity if the cooling function of the cold head drops.

Claims

1. A cryostat arrangement, comprising: a vacuum container containing an object to be cooled, wherein the vacuum container has a neck tube which leads to the object, a cooling arm of a cold head at least partially arranged in the neck tube, a closed cavity which is sealed off fluid-tight with respect to the object and is formed around the cooling arm and at least partially filled with a cryogenic fluid in normal operation, and a thermal coupling element configured to thermally couple the cryogenic fluid in the cavity with the object, a pump device, to which the cavity is connected via an activatable valve and configured to pump the cavity out in the event of a drop in cooling function of the cold head, and a monitoring unit configured to monitor the cooling function of the cold head, and to activate the pump device in response to a drop in the cooling function of the cold head such that the cavity is pumped out.

2. The cryostat arrangement according to claim 1, wherein the object to be cooled comprises a superconducting magnetic coil system or a cryogen container.

3. The cryostat arrangement as claimed in claim 1, further comprising a pressure sensor connected to the cavity and configured to output an output signal to the monitoring unit, wherein the monitoring unit is configured to activate the pump device to pump out the cavity as soon as the output signal of the pressure sensor exceeds a predefined first threshold value P.sub.max.

4. The cryostat arrangement according to claim 3, wherein the pressure sensor is arranged in the cavity, and wherein 100 mbarP.sub.max500 mbar.

5. The cryostat arrangement as claimed in claim 3, wherein the pump device, following activation when exceeding the first threshold value P.sub.max, pumps out the cavity only until the output signal of the pressure sensor falls below a predefined second threshold value P.sub.min.

6. The cryostat arrangement as claimed in claim 5, wherein 75 mbarP.sub.min300 mbar.

7. The cryostat arrangement as claimed in claim 1, wherein the activatable valve is configured as a regulating valve.

8. The cryostat arrangement as claimed in claim 7, wherein the pump device is configured to operate at constant speed and/or constant pumping capacity.

9. The cryostat arrangement as claimed in claim 1, wherein the pump device is configured to operate with variable speed and/or variable pumping capacity.

10. The cryostat arrangement as claimed in claim 9, wherein the variable speed and/or the variable pumping capacity regulates pressure in the cavity.

11. The cryostat arrangement as claimed in claim 9, wherein the activatable valve is configured as an ON/OFF valve.

12. The cryostat arrangement as claimed in claim 1, wherein the pump device comprises an electrically operated suction pump buffered by an autonomous power source.

13. The cryostat arrangement as claimed in claim 12, wherein the electrically operated suction pump is buffered with a battery.

14. The cryostat arrangement as claimed in claim 1, wherein the pump device comprises a cryopump.

15. The cryostat arrangement as claimed in claim 14, wherein the cryopump is integrated into the cryostat arrangement and comprises pumping cold surfaces that are thermally coupled to the object.

16. The cryostat arrangement as claimed in claim 15, further comprising a connecting line that extends completely inside the vacuum container from the cavity to the pumping cold surfaces.

17. The cryostat arrangement as claimed in claim 1, further comprising a supply line connected to the cavity and configured to refill the cavity with cryogenic fluid after the cooling function of the cavity is put back in the normal operation.

18. A method for operating a cryostat arrangement comprising a vacuum container, an object to be cooled, and a thermal coupling element, wherein the object is arranged inside the vacuum container, wherein the vacuum container has a neck tube which leads to the object, wherein a cooling arm of a cold head is at least partially arranged in the neck tube, wherein a closed cavity, which is sealed off fluid-tight with respect to the object, is formed around the cooling arm, wherein the cavity is at least partially filled with a cryogenic fluid in normal operation, and wherein the thermal coupling element is configured to thermally couple the cryogenic fluid in the cavity with the object, comprising pumping the cavity out via a pump device such that the pressure in the cavity does not exceed a predefined first threshold value P.sub.max.

19. The method as claimed in claim 18, wherein the cavity is pumped out via the pump device such that the pressure in the cavity does not fall below a predefined second threshold value P.sub.min<P.sub.max.

20. The method as claimed in claim 18, further comprising using helium as a cryogenic fluid, and operating the pump device such that in the normal operation, the pressure in the cavity is between 100 mbar and 500 mbar.

21. The method as claimed in claim 20, wherein the pressure in the cavity is between 200 mbar and 300 mbar.

22. The method as claimed in claim 18 for operating a cryostat arrangement, wherein the pump device is connected to the cavity via an activatable valve, and wherein a monitoring unit monitors the cooling function of the cold head and/or the pressure in the cavity, further comprising activating the pump device via the monitoring unit if the cooling function of the cold head drops and/or if the pressure in the cavity exceeds the predefined first threshold value P.sub.max such that the cavity is pumped out to a pressure below the threshold value P.sub.max.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] The invention is illustrated in the drawing and will be explained in greater detail on the basis of exemplary embodiments. In the figures:

[0045] FIG. 1 shows a schematic cross section of a first embodiment of a cryostat arrangement according to the invention having a superconducting magnet coil directly in the vacuum as the object to be cooled;

[0046] FIG. 2 shows a schematic cross section of a second embodiment of the cryostat arrangement according to the invention having a cryogen container as the object to be cooled, wherein a superconducting magnet coil is contained in the cryogen container;

[0047] FIG. 3 shows a schematic cross section of a third embodiment of the cryostat arrangement according to the invention in the region of the neck tube having a pressure sensor connected to the cavity;

[0048] FIG. 4 shows a schematic detail of a fourth embodiment of the cryostat arrangement according to the invention having a cryopump; and

[0049] FIG. 5 shows a schematic time diagram of the heating of a cryostat arrangement according to the prior art in comparison to a cryostat arrangement according to the invention proceeding from an original operating temperature of 3 K after the failure of the refrigerating unit.

DETAILED DESCRIPTION

[0050] FIGS. 1 and 2 each show, in a schematic vertical sectional view, embodiments of the cryostat arrangement 1; 1; 1; 1 according to the invention having a vacuum container 2 and an object 4 to be cooled (in particular a superconductive magnet coil system 5 in an NMR, MRI, or FTMS apparatus), wherein the object 4 to be cooled is arranged inside the vacuum container 2, wherein the vacuum container 2 has a neck tube 8, which leads to the object 4 to be cooled, wherein a cooling arm 10 of a cold head 11 is arranged at least partially in the neck tube 8, wherein a closed cavity 9, which is sealed fluid-tight in relation to the object 4 to be cooled, is formed around the cooling arm 10, wherein the cavity 9 is at least partially filled with a cryogenic fluid in normal operation, and wherein a thermal coupling element 15 is provided, suitable for thermal coupling of the cryogenic fluid in the cavity 9 with the object 4 to be cooled.

[0051] The cryostat arrangement 1; 1; 1; 1 according to the invention is distinguished in that it comprises an activatable pump device 14, to which the cavity 9 is connected via an activatable valve 13 and with which the cavity 9 can be pumped out in the event of a drop of the cooling function of the cold head 11, and a monitoring unit 17 is provided, which monitors the cooling function of the cold head 11, and which is designed to independently activate the pump device 14 in such a way that the cavity 9 is pumped out if the cooling function of the cold head 11 drops.

[0052] FIG. 1 schematically shows a first embodiment of a cryostat arrangement 1 according to the invention, comprising a vacuum container 2, in the interior of which a vacuum is configured. A thermal radiation shield 3 (shown by dashed lines), which encloses a superconducting magnet coil system 5 as the object 4 to be cooled here, is arranged here in the vacuum container 2. The magnet coil system 5 is arranged here directly in the vacuum of the vacuum container 2.

[0053] The cryostat arrangement 1 is provided with a room temperature bore 6, through which a sample volume 7 in the center of the magnet coil system 5 is accessible. A strong, static, approximately homogeneous magnetic field B.sub.0 prevails in the sample volume 7, which can be employed for NMR measurements on a sample in the sample volume 7 using NMR resonators (not shown in greater detail).

[0054] A neck tube 8 leads through the vacuum container 2 to the object 4 to be cooled. In the embodiment shown, the neck tube 8 simultaneously forms the border of a cavity 9, which directly encloses a cooling arm 10 of a cold head 11 of an active cooling system of the cryostat arrangement 1.

[0055] The cavity 9 is connected via a pump line 12 and a valve 13, in particular a shut-off valve, to a pump device 14, with which the cavity 9 can be evacuated. A monitoring unit 17 is provided for activating the valve 13 and the pump device 14, which also receives (direct or implicit) items of temperature information from the cold head 11, and independently opens the valve 13 and activates the pump device 14 if a limiting temperature is exceeded.

[0056] During normal operation of the cryostat arrangement 1, the cavity 9 is at least partially filled with a cryogenic fluid (not shown in greater detail in the drawing), which couples the cooling arm 10 to the object 4 to be cooled via a thermal coupling element 15. The thermal coupling element 15 is an upper side of the object 4 to be cooled here, which simultaneously forms a part of the border of the cavity 9. In the case of a disturbance of the active cooling of the cooling arm 10, the cavity 9 is pumped out using the pump device 14 and the cryogenic fluid in the cavity 9 is thus cooled.

[0057] FIG. 2 shows a second embodiment of a cryostat arrangement 1 according to the invention, which substantially corresponds to the first embodiment of FIG. 1; therefore, only the essential differences will be explained.

[0058] In the second embodiment, the object 4 to be cooled is designed as a cryogen container 20, inside of which a superconducting magnet coil system 5 is arranged. Furthermore, a second cryogenic fluid, partially liquid and partially gaseous helium here (again not shown in greater detail in the drawing) is arranged in the cryogen container 20. The superconducting magnet coil system 5 is typically immersed at least partially in the liquid helium. The thermal coupling unit 15 is formed here by a part of the upper side of the cryogen container wall, which simultaneously delimits the cavity 9.

[0059] FIG. 3 shows a third embodiment of the cryostat arrangement 1 according to the invention in the region of the neck tube 8. In thispreferredembodiment, a pressure sensor 30 is connected to the cavity 9. In refinements which are not shown separately in the drawing, the pressure sensor can in particular be arranged directly in the cavity 9. The output signal of the pressure sensor 30 is fed into the monitoring unit 17, which activates the pump device 14 to pump out the cavity 9 as soon the output signal of the pressure sensor 30 exceeds a predefined first threshold value P.sub.max, wherein in particular 100 mbarP.sub.max500 mbar. After its activation because the first threshold value P.sub.max is exceeded, the pump device 14 only pumps out the cavity 9 until the output signal of the pressure sensor 30 falls below a predefined second threshold value P.sub.min, wherein in particular 75 mbarP.sub.min300 mbar.

[0060] In this embodiment of the cryostat arrangement 1 according to the invention, the pump device 14 comprises an electrically operated suction pump which is buffered by an autonomous power source, preferably with a battery 31.

[0061] Furthermore, this embodiment of the cryostat arrangement according to the invention comprises a device for refilling the cavity 9 with the cryogenic fluid. This device comprises a storage container 32 having the cryogenic fluid and a supply line 33, which connects the container 32 to the cavity. Using this device, the most autonomous operation possible can also be ensured after the failure of the cooling device, since the possibility exists of refilling the loss of cryogenic fluid. It is advantageous if the supply line 33 is equipped with a pressure reducer 34. The filling procedure can thus be carried out in a controlled manner without an overpressure arising.

[0062] FIG. 4 shows a fourth embodiment of the cryostat arrangement 1 according to the invention. The pump device comprises a cryopump 40 hereinstead of an electrically operated pumpwhich is preferably integrated into the cryostat arrangement 1. The advantage of a cryopump 40 is that it can be embodied as completely passive and therefore very reliable.

[0063] A suitable adsorption material 41for example, activated charcoalis cooled in a cryopump. At a temperature of 4.2 K, even helium can thus be pumped. As shown in FIG. 4, the cryopump 40 can be integrated directly into the magnet cryostat. The pump is operated using a small quantity of liquid helium, which is indicated in the drawing below the adsorption material 41. The helium can evaporate into the atmosphere (1000 mbar), since it has a temperature of 4.2 K. The cryopump 40 cannot be thermally coupled well to the cold head, since heat is released during the pumping of helium using the cryopump 40, which would otherwise contribute to the heating of the magnet coil system 5.

[0064] One essential purpose of the present invention is to provide a cryomagnet which is operated, for example, at 3 K (corresponding to a pressure of approximately 240 mbar), in contrast to the conventional operation at 4.2 K (corresponding to the boiling point of helium at normal pressure). Due to this slight reduction of the temperature, the current carrying capacity rises in the superconductor, for example, NbTi, so that the design of a significantly more compact magnet is possible. According to simulations (not shown here), a magnet having a flux density of 9.4 T can thus be reduced from approximately 900 kg to 600 kg superconductor, because it can be charged with higher current densities.

[0065] It is disadvantageous in this case that the superconductor is more sensitive to an introduction of heat, and quench already occurs at lower temperature (approximately 4-5 K). A safety unit is thus required, with which to ensure that, in the event of failure of the cold head, the temperature of the cryostat can be kept as long as possible at 3 K.

[0066] It can be seen by way of example in the diagram shown in FIG. 5 how the temperature curve of helium develops upon failure of the cold head as a function of time. The MCV cryostat from the prior art cited at the outset (DE 10 2014 218 773 A1) can be used as an example (line having rhomboid symbols). After the failure, the temperature rises up to the boiling point at normal pressure, where the temperature has a plateau phase until the liquid component of the helium has passed into the gas phase. The temperature continually rises thereafter up to the quench.

[0067] Because of the present invention (line having square symbols), the temperature only rises until the limiting pressure (300 mbar/3.2 K) is reached. The pump subsequently begins to pump out gas in order to keep the bath at this temperature by vaporization. If all of the helium has vaporized, the temperature rises and a quench becomes unavoidable if the cold head fails for an even longer time.

[0068] In order to keep the temperature at 3 K, which is mentioned by way of example, in this manner, a pressure sensor 30 is necessary, as shown in FIG. 3. This pressure sensor 30 measures, in the case of rising temperature, the pressure increase in the cavity 9 linked thereto, where the cryogen (helium) is located, and relays the measured value to the monitoring unit 17, which activates the pump device 14 upon exceeding a threshold value P.sub.max, which then pumps out the cavity 9 to remove the cryogen from the gas phase until the desired (partial) vacuum is reached. In the case of accurate regulation of the partial vacuum, a regulating valve 13 (or a pump having variable speed) is advantageous.

[0069] During the charging or after failure of the cold head, the battery-buffered pump device 14 switches on upon exceeding the settable pressure threshold value P.sub.max in the neck tube 8 and keeps the pressure stable through regulation, pressure sensor 30, and regulating valve 13 by pumping out helium. The pressure is somewhat above the pressure in normal operation. For example, 180 mbar in normal operation. Regulating pressure 300 mbar (corresponds to 3.2 K).

LIST OF REFERENCE NUMERALS

[0070] 1; 1; 1; 1 cryostat arrangement [0071] 2 vacuum container [0072] 3 thermal radiation shield [0073] 4 object to be cooled [0074] 5 superconducting magnet coil system [0075] 6 room temperature borehole [0076] 7 sample volume [0077] 8 neck tube [0078] 9 cavity [0079] 10 cooling arm [0080] 11 cold head (=cold head) [0081] 12 pump line [0082] 13 valve [0083] 14 pump device [0084] 15 thermal coupling element [0085] 17 monitoring unit [0086] 20 cryogen container [0087] 30 pressure sensor [0088] 31 battery [0089] 32 storage container [0090] 33 supply line [0091] 34 pressure reducer [0092] 40 cryopump [0093] 41 adsorption material

REFERENCE LIST

[0094] Publications considered for the judgment of patentability: [0095] [1] DE 10 2014 218 773 A1 [0096] [2] U.S. Pat. No. 8,729,894 B2 [0097] [3] US 2007/089432 A [0098] [4] US 2010/298148 A [0099] [5] US 2007/022761A [0100] [6] DE 10 2004 012 416 B4 [0101] [7] US 2007/051115 A [0102] [8] U.S. Pat. No. 8,950,194 B2