GB2166535A - Cryostat for operation of a <3>He <4>He mixing unit - Google Patents

Abstract

This invention relates to a cryostat for operation of a <3>He-<4>He mixing unit. A preferred method of generating temperatures in the mK range consists in diluting liquid <3>He into liquid <4>He in a <3>He-<4>He mixing cryostat. In the familiar systems the mixing chamber of the <3>He-<4>He loop is located in a 1 K pot which, by means of <4>He contact gas is brought to 4 K and by evacuation of the <4>He to about 1 K level. Precooling, especially elimination of the contact gas, is very time consuming. Deviating from the familiar and usually employed cryostats, this invention substitutes a 2 K beaker 7 provided below the 4 K pool 4 for the usual 1 K pot and provides the mixing unit 8 with a precooler pipe 12. In the 2 K beaker 7 the helium is transformed into the superfluid state by an evaporator 19 and, consequently, cooled down to 2.17 K. <IMAGE>

Description

SPECIFICATION Cryostat for operation of a 3He 4He mixing unit The invention relates to a cryostat for operating a 3He-4He mixing unit according to the general definition contained in Claim 1.

Cryo-engineering is growing in importance, not the least as a result of superconduction engineering, inter alia in the medical field and in fusion technology. This gives rise to a number of research and measuring tasks to the performed in the cryogenic range near the absolute zero. In solid state physics and in nuclear physics important data are derived from experiments involving polarized nuclei. For this purpose, temperatures in the mK range as well as strong external magnetic fields are required.

The preferred method of producing temperatures in the mK-range consists in diluting liquid 3He into liquid 4He in a 3He-4He mixing cryostat. The 3He-4He cryostats supplied by Oxford Instruments and the S.H.E. Corporation are well known. The mixing unit proper, consisting of a 1K pot (or 1K-plate), a distiller, a heat exchanger and a mixing chamber, is installed within the evacuated 4 K space of an ordinary helium low-temperature cryostat. This mixing unit is initially brought to a temperature of 4 K by 4He contact gas and, after elimination of the contact gas through pumping off 4He from the 1 K pot, to a temperature between 1 and 2 K. At this temperature the 3He-4He mixture is introduced into the 3He-4He loop by condensation. By circulating 3He within this closed loop, 3He from a phase rich in 3He is converted into a phase poor in 3He by phase separation in the mixing chamber.Heat is withdrawn from the system due to the different enthalpies of 3He in the two phases. With this type of refrigerators temperatures down to 2 mK can be achieved under optimum conditions.

Precooling, especially elimination of the contact gas, is very time consuming and calls for a helium gas tight design of the 4 K space and of the 1 K pot. In addition, separate vacuum pumping systems are required for the 4 K space and the main isolating vacuum.

The tast underlying this invetion consists in shortening the cooling period of the sample in the cryostat mentioned at the beginning while reducing greatly expenditure in terms of engineering work.

The solution of this task is described by the features given in Claim 1.

In the other claims advantageous improvements and design versions of the invention are proposed.

The invention will be explained in more detail in Figs. 1 through 4 below by a practical example.

Fig. 1 shows the sectional view of a version of the entire cryostat system, consisting of an evacuated cryostat 1 in which from outside to the center the following systems are installed in a concentric configuration around the parts cooled to temperatures below 4.2 K: (LN2=liquid nitrogen) an LN2 bath 2 with thermally coupled LN2 shield 3, an LHe bath 4 (LHe=liquid helium) at the suspension 24, with likewise thermally coupled LHe shield 5, a 2 K thermal shield 6a surrounding the 2 K space 6, this 2 K thermal shield being mounted below the 2 K breaker 7 (see Fig. 3) so as to be a good heat conductor.

The 2 K beaker 7 is flanged to the bottom of the LHe bath 4 so as to be vacuum tight.

Within the 2 K thermal shield 6a the mixing unit 8 represented in Fig. 2 is installed at the mixing chamber 11 of which, where the temperature in the mK range required for sample 14 is generated, the sample 14 is fixed via a copper rod 13 having a good thermal conduction.

The sample 14 and the copper rod 13 are surrounded by an additional thermal shield 1 1a which is connected thermally with the mixing chamber 11. At the bottom of the LHe bath 4 a superconducting magnet 15 is provided above the suspension 25, this magnet surrounding cylindrically the sample 14 in its own helium bath outside the 2 K thermal shield and generating the strong magnetic field required for nuclear polarization.

The cryostat 1 accommodates, along the axis of symmetry, the pipework and ductwork systems required for operation, e.g. the helium lift 25 needed to fill the cryostat 1 and the respective offgas line 26 with the rupture disk 27 required for safety reasons.

The evaporator 19 with the needle valve 20 represented in Fig. 4 and the pump tube 21, the pump tube outlet 22 and the needle valve drive 23 being recognizable at its end, open into the 2 K beaker 7.

Along the axis of symmetry the 3He evacuation line 16 of variable diameter can be clearly recognized. It runs through the 2 K beaker 7 up to the mixing unit 8 and ends at the distiller 9. Along this 3He evacuation line 16 and surrounding it, beginning at the distiller 9, the 3He condensation line 1 7 is provided with many windings of different pitches, some of the designed as a heat exhanger. From top, and likewise running through the 2 K beaker 7, the precooler pipe 12 leads to the mixing unit 8, parts of it being in the required good contact with the heat exchanger.

Fig. 2 shows a version of the mixing unit 8 consisting of round disk shaped parts mounted along the axis at varying spacings.

Seen from top to bottom the distiller 9, the heat exchanger 10 and the mixing chamber 11 are arranged below each other. The 3He loop needed for cooling by means of 3He phase separation consists of the 3He loop line 17 provided at the distiller 9 and the 3He evacua tion line 16. For instance, for the precooler pipe 12 a bifilar attachment at the circumference of the disk shaped parts has been chosen although it can be connected with the parts in a different appropriate way, e.g., designed as a disk on the surface or integrated in the parts themselves by suitable channels or bores. To achieve a good thermal contact this joint can be made by soldering, welding or pressing or integrated into the parts.

Fig. 3 shows the 2 K beaker 7 with the precooler pipe 12 passing through it, with the 3He condensation line 17 made as a heat exchanger 18, and the 3He evacuation line 16.

The direct access to the helium bath 4 which is required for helium replacement can be clearly recognized. This configuration which is open towards the helium bath 4 allows at any time and preferably in the cooled down condition of the entire system, to remove and reinstal the evaporator 19 which ends in the 2 K beaker 7 and has been described in more detail in Fig. 4.

Fig. 4 shows the configuration of the individual parts of the evaporator 19. The needle valve 20 is a hollow body of rotation with a lateral inlet 29 for helium. The cavity is so designed that the outlet 31 can be reduced in a defined way by the tip of a needle 30. The end opposed to the top of the needle 30 is connected via a driving rod 28 with the needle valve drive 23 working on the principle of a micrometer screw. The outlet 31 ends in the expansion volume 32 which is evacuated via several thin tubelets 33 leading to the pump tube 21. The helium flowing through the inlet 29 is brought to evaporate in a specific way via the outlet 31 narrowed by the needle 30 in the steadily evacuated expansion volume 32.

Deviating from the known and normally used cryostats, the invention (see Fig. 1) accordingly substituted a 2 K beaker 7 below the 4 K bath 4 for the usual 1 K pot and provided the mixing unit 8 with a precooler pipe 12, with the 2 K space 6 connected to the general vacuum system because flooding with nitrogen and hydrogen or helium gas is no longer necessary. Consequently, the separate vacuum system and the long evacuation time of the 2 K space 6 can be suppressed.

An evaporator 19 consisting of a needle valve 20 between the liquid helium and a pump tube 21 leading to the outside opens into the 2 K beaker 7 whose helium is in direct contact with the helium pool 4. When the needle valve 20 is opened while pumping, liquid helium evaporates into the expansion volume 32. As a result, the temperature of the surrounding helium decreases. On account of its higher density the colder helium falls down to the bottom of the 2 K beaker 7. The process continues until the helium has attained its maximum density and hence the superfluid condition at a temperature of 2.17 K. Up to a certain level, which depends on the refrigeration capacity of the evaporator 19, the superfluid helium is in an equilibrium state at 2.17 K.The desired goal is the level of the 2 K beaker 7 which, via the heat exchanger 18, serves to precool the 3He to be supplied into the mixing unit 8. With the highly sensitive needle valve 20 the refrigeration capacity of the evaporator 19 is set to the required accuracy. This is achieved by a very precise needle valve drive 23 located outside the cryostat 1 and connected with the needle valve 20 through a long driving rod 28 so that the evaporator capacity can be set with an accuracy better than 5 mW.

Maintenance of the evaporator 19 is feasible at any moment, even in the cooled down condition, because the needle valve 20 is connected with the needle valve drive 23 via the driving rod 28 and the pump tube 25 surrounds the driving rod 28 in a concentric configuration. The sole operation required is to withdraw the whole unit, similar to a pipe, from the cryostat 1.

Precooling of the mixing unit 8 is performed in the absence of contact gas by forced flow of refrigerant through a precooler pipe 12 rigidly connected with the various parts of the mixing unit 8.

The thermal capacity supplied to the various parts of the mixing unit 8 by heat conduction of the precooler tube 12 in normal operation following precooling can be neglected.

The required temperature of the sample 14 in the mK range is obtained via the cooper rod 13 being a good thermally conducting connection to the mixing chamber 11 of the mixing unit 8. The low temperature of down to 2 mK of the mixing chamber 11 is achieved by phase separation of 3He in a forced 3He flow via the heat exchanger 10 and the distiller 9 connected with a 3He evacuation line 16 and 3He condensation line 17. The condensation line 17 is supplied via the other heat exchanger 18, which is iocated in superfluid helium in the 2 K beaker 7, with compressed 3He cooled in this 2 K breaker 7 to a temperature S3 K.

The modification in the cryostat design resulting from this invention increases in a particularly favorable manner the effectivity and safety of operation of the cryostat.

The total duration of precooling of the mixing unit 8 is shorter than the time normally needed in order to cool down to 4 K the cryostat 1. This means that as soon as the cryostat 1 has been filled, also precooling of the mixing unit 8 is completed.

Reference List: 1 Cryostat pressure vessel 2 LN2 bath 3 LN2 shield 4 LHe bath 5 LHe shield 6 2 K space 6a 2 K thermal shield 7 2 K beaker 8 Mixing unit 9 Distiller 10 Heat exchanger 11 Mixing chamber 1 1a Thermal shield 12 Precooler pipe 13 Cooper rod 14 Sample 15 Superconducting magnet 16 3He evacuation line 17 3He condensation line 18 Other heat exchangers 19 Evaporator 20 Needle valve 21 Pump tube 22 Pumpe tube outlet 23 Needle valve drive 24 Suspension of LHe bath 25 Suspension of magnets 26 Helium gas line 27 Burst disk 28 Driving rod 29 Inlet 30 Needle 31 Outlet 32 Expansion volume 33 Tubelet

Claims (9)

1. Cryostat for operation of a 3He-4He mixing unit wherein a distiller, one or several heat exchangers, and a mixing chamber, the latter in thermal contact with the sample, are arranged within an evacuated 4 K space, with the mixing unit initially coolable to a temperature of 4 K and subsequently, by 3H circulation within a closed 3He loop, to a temperature down to 2 mK, comprising a) part of the 4 K bath made as a 2 K beaker (7) without physical separation in which a temperature around 2.2 K can be achieved by means of an evaporator (19); b) the 3He loop (16, 17) for the mixing chamber (11) routed through this 2 K beaker (7) as a heat exchanger (18); and c) for precooling of the mixing unit (8) to a temperature of about 4 K, a cryo-line (12) for carrying a liquid and/or gaseous coolant firmly attached on the distiller (9), the heat exchangers (10), and the mixing chamber (11) of the mixing unit (8).

2. Cryostat as defined in Claim 1 wherein the 2 K beaker (7) is arranged as a holdup tank for the 2.2 K-He as an extension below the 4 K bath (4).

3. Cryostat as defined in Claim 1 or in one of the following claims wherein the cryo-line (12) is made bifilar.

4. Cryostat as defined in Claim 1 or in one of the following claims wherein the cryo-line (12) is routed through the 2 K beaker (7).

5. Cryostat as defined in Claim 1 or in one of the following claims wherein the 2 K thermal shield (6a) for the mixing unit (8) is attached to the 2 K beaker (7).

6. Cryostat as defined in Claim 1 or in one of the following claims wherein the evaporator (19) consists of an expansion volume (32) and an adjustable needle valve (20) integrated in the pump tube (21) and determining the refrigeration capacity.

7. Croystat as defined in Claim 1 or in one of the following claims, wherein the needle valve (20) can be adjusted via a long drive rod (28) guided within the pump tube (21) and operated on the principle of a micrometer screw.

8. Cryostat as defined in Claim 1 or in one of the following claims wherein the needle valve drive (23) is outside the cryostat (1) and combined with the evaporator (19) in such a way that it can be removed from the cooled down cryostat (1) for maintenance work and reinstalled thereafter.

9. Cryostat for operation of a 3He-4He mixing unit substantially as herein described with reference to the accompanying drawings.