Automatic thermal decoupling of a cold head

Abstract

A cryostat has a cooling arm with a first thermal contact surface which can be brought into thermal contact with a second thermal contact surface on an object to be cooled. A hollow volume (2) between the inner side of the neck tube, the cooling arm, and the object is filled with gas and the cooling arm is pressurized by the inner pressure of the gas and also by atmospheric pressure. A contact device brings the first and the second contact surfaces into thermal contact below a threshold gas pressure and moves them away from each other when the threshold pressure has been exceeded such that a gap (13) filled with gas thermally separates the first and second contact surfaces. Operationally safe and fully automatic reduction of the thermal load acting on the object to be cooled is thereby obtained in case the cooling machine fails.

Claims

1. A cryostat comprising: a vacuum container having an outer shell, said vacuum container also having a chamber and at least one hollow neck tube which connects said chamber through said outer shell to a region outside of the cryostat; a cooling arm with a cold head disposed on a distal end of said cooling arm, said cold head having a first thermal contact surface; at least one object to be cooled, wherein said object to be cooled is disposed in said chamber, said object to be cooled having a second thermal contact surface; a contact device disposed on a proximal end of said cooling arm, wherein said cooling arm is at least partially disposed in said neck tube, said cooling arm being thermally connected to a refrigeration device, wherein said cooling arm is structured to be brought into thermal contact with said second thermal contact surface of said object via said first thermal contact surface of said cold head using said contact device; and a gas or gas mixture having internal pressure and a positive thermal expansion coefficient, said gas or gas mixture at least partially filling a hollow volume between an inner side of said hollow neck tube, said cooling arm and said object to be cooled, wherein said internal pressure of said gas or gas mixture surrounds part of said cooling arm, said contact device being surrounded by atmosphere having atmosphere pressure, wherein said cooling arm is disposed, structured, mounted and dimensioned for movement of said first thermal contact surface of said cold head within said hollow neck tube through a length of at least 5 mm towards and away from said second thermal contact surface of said object, said contact device being structured to bring or keep said first thermal contact surface of said cold head in thermal contact with said second thermal contact surface on said object to be cooled when said gas or gas mixture pressure is below a pre-determined low threshold pressure and said contact device moving said first thermal contact surface of said cold head away from said second thermal contact surface of said object to be cooled when said gas or gas mixture pressure has reached or exceeded a threshold pressure, thereby creating a gap filled with said gas or gas mixture, said gap thermally separating said first thermal contact surface of said cold head from said second thermal contact surface of said object, wherein said first thermal contact surface of said cold head is located completely or partially in liquid helium in an operating state below said pre-determined threshold pressure of said gas or gas mixture and, when said threshold pressure has been exceeded, said cooling arm emerges from said liquid helium into said surrounding gas or gas mixture due to movement of said first thermal contact surface of said cold head away from said second thermal contact surface of said object to be cooled.

2. The cryostat of claim 1, wherein said contact device comprises a bellows, a diaphragm and/or a radial seal by means of which said cooling arm is mounted in said hollow neck tube such that said cooling arm can be displaced in a linear direction along an axis thereof.

3. The cryostat of claim 1, wherein said contact device has a stop surface against which a counter surface of said cooling arm abuts during linear displacement along an axis thereof towards said object to be cooled, said counter surface being rigidly connected to said cooling arm, wherein relative positions of said stop surface and said counter surface are selected such that said first thermal contact surface of said cold head comes into thermally conducting contact with said second thermal contact surface on said object to be cooled when mechanical contact is obtained between said stop surface and said counter surface.

4. The cryostat of claim 1, wherein said contact device comprises a pretensioning device that generates an additional force acting on said cooling arm together with said pressure of said gas or gas mixture, said additional force thereby acting in a direction of movement of said cooling arm during linear displacement in said hollow neck tube along an axis thereof in a direction away from said object to be cooled.

5. The cryostat of claim 4, wherein said additional force on said cooling arm generated by said pretensioning device depends on a path of displacement of said cooling arm due to acting pressure of said gas or gas mixture, wherein said additional force becomes sufficiently large so that said first thermal contact surface of said cold head is lifted off said second thermal contact surface on said object to be cooled only when a predetermined threshold pressure of said gas or gas mixture is exceeded such that said gap filled with said gas or a gas mixture separates said first and said second thermal contact surfaces, wherein said gap quickly increases due to said additional force that acts on said cooling arm even when said pressure of said gas or gas mixture only slightly further increases.

6. The cryostat of claim 4, wherein said pretensioning device comprises one or more pretensioning springs generating said additional force.

7. The cryostat of claim 6, wherein said additional force exerted by said pretensioning springs on said cooling arm can be mechanically adjusted.

8. The cryostat of claim 1, wherein said cooling arm is mounted and said contact device is designed in such a fashion that said first thermal contact surface of said cold head inside said hollow neck tube can be moved by a length of at least 10 mm towards or away from said second thermal contact surface on said object to be cooled.

9. The cryostat of claim 1, wherein said chamber containing said object to be cooled is surrounded by a radiation shield inside said vacuum container.

10. The cryostat of claim 1, wherein a superconducting magnet coil is arranged in said chamber as said object to be cooled and said cryostat together with said superconducting magnet coil are part of an NMR, MRI or FTMS apparatus.

11. The cryostat of claim 10, wherein said NMR, MRI or FTMS apparatus comprises a high-resolution high field NMR spectrometer with a proton resonance frequency of between 200 MHz and 500 MHz.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1a shows a schematic vertical sectional view of an embodiment of an inventive cryostat of an NMR spectrometer, wherein the cooling arm of the cold head is spatially and therefore also thermally separated from the NMR magnet;

(2) FIG. 1b shows the configuration of FIG. 1a is but with physical and thermal contact between the cooling arm and the magnet;

(3) FIG. 2a shows a schematic vertical sectional view of a further embodiment with physical and thermal contact between the cooling arm and the object to be cooled, wherein the cooling arm is located in the area of its first thermal contact surface in a liquid helium bath;

(4) FIG. 2b shows a configuration as in FIG. 2b, wherein, however, the cooling arm is not in physical contact with the object to be cooled but the first contact surface is thermally connected to the second contact surface via a liquid helium bath; and

(5) FIG. 3 shows an embodiment of the inventive cryostat, in which the mechanical element that connects the cooling arm of the cold head in a flexible fashion to the neck tube of the cryostat, is designed as a vacuum-proof diaphragm.

DESCRIPTION OF THE PREFERRED EMBODIMENT

(6) FIGS. 1a, 1b, 2a and 2b each show a schematic vertical section of embodiments of the inventive cryostat 11; 11; 11; 11 comprising a vacuum container 9 which houses a chamber 12 containing at least one object 4 to be cooled (in particular a superconducting magnet coil in an NMR, MRI or FTMS apparatus), wherein the vacuum container 9 is provided with at least one hollow neck tube 10 which connects the chamber 12 through the outer shell of the vacuum container 9 to the area outside of the cryostat 11; 11; 11; 11, wherein the neck tube 10 comprises a cooling arm 1a; 1a; 1a; 1a of a cold head 1 which is thermally connected to a refrigeration device and can also be brought into thermal contact with a second thermal contact surface 3b; 3b; 3b on the object 4 to be cooled via a first thermal contact surface 3a; 3a; 3a on the cooling arm 1a; 1a; 1a; 1a.

(7) The chamber 12 containing the object 4 to be cooled is surrounded by a radiation shield 5 inside the vacuum container 9.

(8) The inventive cryostat 11; 11; 11; 11 is characterized in that the hollow volume 2; 2; 2 between the inner side of the hollow neck tube 10, the cooling arm 1a; 1a; 1a; 1a that is at least partially arranged therein, and the object 4 to be cooled is filled at last in part with a gas or a gas mixture with positive thermal expansion coefficient, wherein the inner pressure of the gas or gas mixture pressurizes part of the cooling arm 1a; 1a; 1a; 1a, whereas another part of the cooling arm 1a; 1a; 1a; 1a is directly or indirectly pressurized by atmospheric pressure, that the cooling arm 1a; 1a; 1a; 1a is mounted in such a fashion that it can be moved within the hollow neck tube 10 by a length of at least 5 mm with its first thermal contact surface 3a; 3a; 3a; 3a towards or away from the second thermal contact surface 3b; 3b; 3b, and that a contact device is provided which brings or keeps the first thermal contact surface 3a; 3a; 3a of the cooling arm 1a; 1a; 1a; 1a in thermal contact with the second thermal contact surface 3b; 3b; 3b on the object 4 to be cooled when the pressure of the gas or gas mixture is below a predetermined low threshold pressure, while the contact device moves the first thermal contact surface 3a; 3a; 3a of the cooling arm 1a; 1a; 1a; 1a away from the second thermal contact surface 3b; 3b; 3b of the object 4 to be cooled when the pressure in the gas or gas mixture has reached or exceeded the threshold pressure such that in this position, a gap 13 filled with gas or gas mixture thermally separates the contact surfaces 3a, 3b; 3a, 3b; 3a, 3b.

(9) The cooling arm 1a; 1a; 1a; 1a is advantageously mounted in such a fashion and the contact device is designed in such a fashion that the first thermal contact surface 3a; 3a; 3a of the cooling arm 1a; 1a; 1a; 1a can be moved within the hollow neck tube 10 by a length of at least 10 mm, preferably at least 20 mm, in particular at least 50 mm towards or away from the second thermal contact surface 3b; 3b; 3b on the object 4 to be cooled.

(10) The contact device may comprise a bellows and/or a diaphragm and/or, as illustrated in the figures of the drawing, a radial seal 6 by means of which the cooling arm 1a; 1a; 1a is mounted in the hollow neck tube 10 in such a fashion that it can be displaced in a linear direction along its axis.

(11) The contact device has a stop surface 14a against which the cooling arm 1a; 1a; 1a; 1a in the hollow neck tube 10 can abut with its counter surface 14b that is rigidly connected to the cooling arm 1a; 1a; 1a; 1a during linear displacement along its axis in the direction towards the object 4 to be cooled, wherein the relative positions of the surfaces are selected such that in case of mechanical contact between the stop surface 14a and the counter surface 14b, the first thermal contact surface 3a; 3a; 3a of the cooling arm 1a; 1a; 1a; 1a also comes into thermally conducting contact with the second thermal contact surface 3b; 3b; 3b on the object 4 to be cooled.

(12) The contact device moreover comprises a pretensioning device which generates an additional force in addition to the pressure of the gas or gas mixture acting on the cooling arm 1a; 1a; 1a; 1a, which additional force acts in a direction of movement of the cooling arm 1a; 1a; 1a; 1a during linear displacement in the hollow neck tube 10 along its axis in a direction away from the object 4 to be cooled. The pretensioning device comprises one or more pretensioning springs 7, wherein the additional force that the pretensioning springs 7 exert on the cooling arm 1a; 1a; 1a; 1a can be mechanically adjusted by means of one or more adjustment screws 8.

(13) In the embodiment of the inventive cryostat 11 illustrated in FIGS. 1a and 1b, the overall hollow volume 2 comprises only gas or a gas mixture but no liquid.

(14) Thermal decoupling between the cooling arm is and the object 4 to be cooled is achieved by generating the gas-filled gap 13 due to the gas pressure-driven movement of the cooling arm is when the predetermined threshold pressure has been reached or exceeded by heating of the gas or gas mixture. This operating state is illustrated in FIG. 1a.

(15) In contrast thereto, FIG. 1b shows an operating state of the cryostat 11 below the threshold pressure, in which the first thermal contact surface 3a of the cooling arm 1a is in direct physical and therefore also thermal contact with the second thermal contact surface 3b on the object 4 to be cooled.

(16) The embodiments of the inventive cryostat 11; 11 illustrated in FIGS. 2a and 2b are characterized in that the first thermal contact surface 3a, 3a of the cooling arm 1a; 1a is located completely or partially in liquid helium in an operating state below the predetermined threshold pressure of the gas or gas mixture and when the threshold pressure has been exceeded, it emerges from the helium bath 20; 20 into the surrounding gas or gas mixture hollow volume 2; 2 due to the movement away from the second thermal contact surface 3b; 3b of the object 4 to be cooled.

(17) In the embodiment illustrated in FIG. 2a, the first thermal contact surface 3a of the part of the cooling arm 1a that is immersed into the helium bath 20 in the operating state below the predetermined threshold pressure of the gas or gas mixture is in physical contact with the second thermal contact surface 3b on the object 4 to be cooled.

(18) FIG. 2b, however, shows an embodiment of the invention in which the cooling arm 1a is not in physical contact with the object 4 to be cooled even in an operating state below the threshold pressure but the first contact surface 3a is thermally connected to the second contact surface 3b via the helium bath 20.

(19) When the predetermined threshold value has been reached or exceeded through heating of the gas or gas mixture and the accompanying increase in inner pressure, the cooling arms 1a; 1a of the embodiments of FIGS. 2a and 2b are each caused to move away from the object 4 to be cooled. The contact devices of these embodiments are designed such that the first thermal contact surface 3a; 3a of the cooling arm 1a; 1a emerges from the helium bath 20; 20 in such an operating state and a gap is again formed towards the second thermal contact surface 3b; 3b on the object 4 to be cooled which is filled with thermally insulating gas or gas mixture.

(20) In the embodiment of the inventive cryostat 11 illustrated in FIG. 3, the contact device comprises a vacuum-proof diaphragm 15 by means of which the cooling arm 1a is mounted in the hollow neck tube 10 such that it can be displaced in a linear direction along its axis.