Method and device for precooling a cryostat

Abstract

A method is provided for precooling a cryostat having a hollow cold head turret into which a neck tube protrudes and connects an object to be cooled to the exterior of the cryostat, wherein a cold head having a cold head stage for cooling a cryogenic working medium may be introduced into the neck tube. During a condensation operation the cryogenic working medium flows through a heat pipe into an evaporator chamber which is thermally conductively connected to the object to be cooled. During a precooling phase a precisely fitting, thermally conductive short circuit block is inserted through the neck tube into the heat pipe to provide thermal conduction between the object to be cooled and a cooling device The short circuit block is removed from the heat pipe after the target temperature is reached, and heat is subsequently transmitted through the heat pipe during a condensation operation.

Claims

1. A cryostat that maintains an object to be cooled at a target temperature during an operating phase, and that reduces a temperature of the object to be cooled to the target temperature during a precooling phase, the cryostat comprising: a vacuum container which houses a chamber containing said object to be cooled, the vacuum container having at least one hollow cold head turret which includes a neck tube that connects the chamber through an outer shell of the vacuum container to an area outside of the cryostat; a two-stage cooler having a cold head and a cold head stage configured to reside in the neck tube for the operating phase, during which the cold head stage cools a cryogenic working medium, which condenses and flows or drips through a heat pipe into an evaporator chamber, and which cold head stage is thermally conductively connected to the object to be cooled via a thermal contact surface, so that the cooled cryogenic working medium is able to absorb heat from the object to be cooled and transport the heat to the cold head stage via the heat pipe; a high-power cooling device with a cooling power of at least 300W at 80K configured to be inserted into the neck tube; a short circuit block that is configured to be inserted through the neck tube into the heat pipe prior to the operating phase, and that resides therein during the precooling phase while the object to be cooled is precooled to the target temperature, the short circuit block having an outer circumference that fits within a circumference of the heat pipe with one free end external to the heat pipe and thermally connected to the high-power cooling device, and the other end of the short circuit block contacting the thermal contact surface, the short circuit block thereby operating as a material core to facilitate cooling of the thermal contact surface by the high-power cooling device; and a gas supply device for blowing the cryogenic working medium into a shared inner space of the neck tube, heat pipe, and evaporator chamber during removal of the short circuit block following the precooling phase and insertion of the two-stage cooler for the operating phase, the gas supply device configured to prevent air and/or moisture from entering the cryostat while providing the working medium to the heat pipe.

2. The cryostat according to claim 1, further comprising a sealing device which prevents gas exchange from the heat pipe to ambient air.

3. The cryostat according to claim 1, wherein the short circuit block is made of a material having a thermal conductivity of >200 W/m.Math.K.

4. The cryostat according to claim 1, wherein the cooling device is protected from external heat input by at least one of a vacuum device and an insulating jacket.

5. The cryostat according to claim 1, wherein the object to be cooled is not in contact with the working medium.

6. The cryostat according to claim 1, wherein the cooling device has a cooling power of at least 500 W at 80 K.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is illustrated in the drawings and is explained in greater detail with reference to exemplary embodiments, as follows:

(2) FIG. 1a shows a schematic vertical sectional view of one embodiment of a cryostat according to the invention, with a short circuit block during the precooling phase, for example a high-power Gifford-McMahon cooler, in an NMR spectrometer or NMR tomography device having a horizontal magnet system;

(3) FIG. 1b shows a configuration with a heat pipe according to the prior art, similar to the cryostat in FIG. 1a, but without the modifications according to the invention, and having a pulse tube cooler during continuous operation;

(4) FIG. 2a shows a schematic vertical sectional view of another embodiment of the cryostat according to the invention with functional features similar to FIG. 1a during the precooling phase, but in an NMR spectrometer or NMR tomography device having a vertical magnet system; and

(5) FIG. 2b shows a configuration similar to FIG. 2a, but in greater detail and during the intermediate phase after the target temperature is reached, when the short circuit block is removed from the heat pipe, in this case a gas supply device for blowing the cryogenic working medium into the shared inner space of the neck tube, heat pipe, and evaporator chamber being present.

DETAILED DESCRIPTION

(6) FIGS. 1a, 2a, and 2b each show a schematic vertical sectional view of embodiments of the cryostat according to the invention, while FIG. 1b shows a cooling system with a heat pipe according to the prior art.

(7) All cryostats illustrated in the drawings have a vacuum container which houses a chamber containing at least one object 10; 10 to be cooled, the vacuum container having at least one hollow cold head turret 12 into which a neck tube 4, which connects the chamber through the outer shell 8; 8 of the vacuum container to the area outside of the cryostat, protrudes, the neck tube 4 being geometrically configured in such a way that a cold head 1 may be introduced, the cold head having a (generally, second) cold head stage 7 via which a cryogenic working medium, which flows or drips through a heat pipe 5 into an evaporator chamber 6 during a condensation operation, may be cooled, and which is thermally conductively connected to the object 10; 10 to be cooled via a thermal contact surface 9; 9, so that the cooled cryogenic working medium is able to absorb heat from the object 10; 10 to be cooled and transport it to the cold head stage 7 via the heat pipe 5.

(8) The chamber containing the object 10; 10 to be cooled is surrounded by a radiation shield 2; 2 inside the vacuum container.

(9) A connection 11 having good thermal conductivity is provided between the radiation shield 2; 2 and a first stage of the cold head 1.

(10) The object 10; 10 to be cooled is generally a superconducting NMR coil system mounted on or in a coil body 3.

(11) According to the invention, in the cryostats shown in FIGS. 1a, 2a, and 2b, during a precooling phase the object 10; 10 to be cooled is precooled to a target temperature of the working range of the cryogenic working medium in which the heat pipe 5 is able to operate efficiently, wherein for the precooling, a precisely fitting short circuit block 14 with good thermal conductivity is inserted through the neck tube 4 into the heat pipe 5, one free end of the short circuit block being thermally connected to a high-power cooling device 13, and its other end contacting the thermal contact surface 9; 9. In an intermediate phase the short circuit block 14 is removed from the heat pipe 5 after the target temperature is reached, and during an operating phase, heat is subsequently transmitted through the heat pipe 5 during a condensation operation.

(12) The cryogenic working medium is blown into the inner space of the neck tube 4 during the removal of the short circuit block 14 from the neck tube 4 in the intermediate phase, and during insertion/placement of the cooling device 13 or the cold head 1 through the neck tube 4 at the start of the operating phase, so that the heat pipe 5 is loaded with the cryogenic working medium.

(13) The cryogenic working medium is selected from helium, neon, nitrogen, hydrogen, or carbon dioxide, depending on the temperature range to be reached in the operating phase.

(14) In the intermediate phase, the high-power cooling device 13 having the short circuit block 14 is replaced by a two-stage cryocooler having a cold head stage 7.

(15) The cryostat according to the invention is characterized in that a gas supply device 15 for blowing the cryogenic working medium into the shared inner space of the neck tube 4, heat pipe 5, and evaporator chamber 6 is present, so that a gas flow in the direction of the neck tube may be provided which prevents air and/or moisture from penetrating into the cryostat during the removal of the short circuit block 14 from the neck tube 4 during the intermediate phase, and during insertion of the heat pipe 5 through the neck tube 4 at the start of the operating phase, and which at the same time loads the heat pipe 5.

(16) A sealing device is generally present which prevents gas exchange from the heat pipe 5 to the ambient air.

(17) The short circuit block 14 is made of a material having a thermal conductivity of >200 W/m.Math.K, in particular copper or pure aluminum, so that its outer circumference makes a precise geometric fit with the outer circumference of the heat pipe 5.

(18) The high-power cooling device 13 has a cooling power of at least 300 W, preferably at least 500 W, at 80 K, and is protected from undesirable external heat input by a vacuum device and/or an insulating jacket 16, since the cooling device protrudes from the neck tube, and in this area must be shielded from lateral heat input from the surroundings.

(19) In the embodiments of the invention illustrated in the drawings, a tank containing liquid helium is present, with which the thermal contact surface 9; 9 may be cooled with respect to the object 10; 10 to be cooled, but the object 10; 10 to be cooled itself is not situated in a tank containing liquid helium.

(20) In the embodiment shown in FIG. 1a, the cryostat according to the invention is part of an NMR apparatus having a horizontal magnet system.

(21) In comparison, FIG. 1b shows a cryostat according to the prior art, as described, for example, in patent application DE 10 2014 218 773, unpublished at the time of filing of the present invention.

(22) The embodiments of the cryostat according to the invention illustrated in FIGS. 2a and 2b are in each case part of an NMR apparatus having a vertical magnet system.

(23) FIG. 2b shows one embodiment of the invention in which a gas supply device 15 for the cryogen is provided when the short circuit block 14 (FIG. 1a) having the high-power cooling device 13 is exchanged with a pulse tube cooler. The direction of flow of the cryogen during the exchange is indicated by arrows.

(24) In one advantageous design, the high-power cooling device 13 is a Stirling cooler, not a Gifford-McMahon cooler. These coolers are even more efficient, but require a dedicated compressor and are more costly.

(25) The high-power cooling device 13 and the pulse tube cooler are preferably operated with the same compressor unit.

LIST OF REFERENCE NUMERALS:

(26) 1 Cold head 2; 2 Radiation shield 3 Coil body 4 Neck tube 5 Heat pipe 6 Evaporator chamber 7 Cold head stage (second stage) 8; 8 Outer shell 9; 9 Thermal contact surface 10; 10Object to be cooled 11 Radiation shield-cold head connection (first stage) 12 Cold head turret 13 Cooling device 14 Short circuit block 15 Gas supply device 16 vacuum device and/or insulating jacket