US20110120147A1

US20110120147A1 - Pressurized Superfluid Helium Cryostat

Info

Publication number: US20110120147A1
Application number: US11/924,770
Authority: US
Inventors: Toshiyuki Shiino
Original assignee: Individual
Current assignee: Hitachi Ltd
Priority date: 2006-10-27
Filing date: 2007-10-26
Publication date: 2011-05-26
Also published as: JP2008109035A

Abstract

In a pressurized superfluid helium cryostat, a first flow rate control valve and a preliminary cooler are installed between a liquid helium inlet at a bottom portion of a 4.2K liquid helium bath and a J-T valve which is a second flow rate control valve. The preliminary cooler is cooled by depressurization by a vacuum pump. The liquid taken in from the 4.2K liquid helium bath is cooled to a temperature of the preliminary cooler by J-T expansion in the first flow rate control valve, and stored in the preliminary cooler. The liquid helium in the preliminary cooler can be cooled below 2.2K, and further subjected to J-T expansion in the J-T valve. By installation of the preliminary cooler, the efficiency of the superfluid cooler can be increased as compared to a conventional type.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a pressurized superfluid helium cooled cryostat used for cooling of a superconducting magnet for generating a high magnetic field.
2. Description of Related Art
In an NMR (nuclear magnetic resonance) spectrometer, an accelerator, a nuclear fusion reactor, an MRI (magnetic resonance imaging), and the like, a superconducting magnet is cooled by superfluid helium at a temperature of about 1.5 to 2K. A usual superconducting magnet is cooled using liquid helium at a temperature of 4.2K, however, when the superconducting magnet is cooled to a temperature of about 1.5 to 2K using the superfluid helium, the properties of the magnet are improved to allow a higher magnetic field to be generated.
As a method of cooling a superconducting magnet using superfluid helium, there are a saturated superfluid helium cooling method and a pressurized superfluid helium cooling method. Pressurized superfluid helium is superior to saturated superfluid helium in terms of a thermal transport property. In the saturated superfluid helium cooling method, there is a possibility of discharge in a low vapor pressure and vacuum leakage from the outside of a cryostat.
For this reason, the pressurized superfluid helium cooling method is generally used these days. Moreover, in the case of an impregnated close coiling type of superconducting magnet for generating a high magnetic field in an NMR spectrometer, it is considered that the lower the operation temperature is, the better.
As a pressurized superfluid helium cooled cryostat, there is known the one in which two helium baths composed of a 4.2K liquid helium bath and a superfluid helium bath in which a superconducting magnet is installed are installed in a low temperature bath, and a superfluid cooler is installed in the superfluid helium bath, so that liquid helium fed from the 4.2K liquid helium bath to the superfluid cooler is cooled preliminary by heat exchange with helium vapor exhausted to the outside of the cryostat from the superfluid cooler (see JP-B-60-4121 (FIG. 5 and the description thereof), and “The design and operation of a refrigerator system using superfluid helium”, Proc. Fifth International Cryogenic Engineering Conf. Kyoto (1974), pp. 265-267, G. Claudet, A. Lacaze, P. Roubeau, and J. Verdier.

BRIEF SUMMARY OF THE INVENTION

A conventional type of pressurized superfluid helium cooled cryostat cools liquid helium fed to a superfluid cooler by heat exchange with helium vapor exhausted to the outside of a cryostat from the superfluid cooler, and is provided with a J-T (Joule-Thomson) heat exchanger for this purpose. As the J-T heat exchanger, a counter-flow type heat exchanger is used in general. However, since the J-T heat exchanger performs cooling with vapor, the preliminary cooling is limited up to about 2.2K.
Since the liquid helium is expensive, it is required to suppress the consumption thereof. The consumption of the liquid helium is determined by a heat leak from a room temperature part to a 4.2K liquid helium bath, a heat leak from the 4.2K liquid helium bath to a superfluid helium bath, and the cooling efficiency of a superfluid cooler for removing the heat leak. Accordingly, it is required to have a cryostat structure with low heat leak and a superfluid cooler with high cooling efficiency. Although the efficiency of the superfluid cooler is determined by the temperature of the liquid helium at an outlet of a J-T heat exchanger, since the J-T heat exchanger cools the liquid helium by vapor, there is a limit in ultimate temperature.
An object of the present invention is to provide a pressurized superfluid helium cooled cryostat designed so as to improve the cooling efficiency of a superfluid cooler thereof as compared to that of a cryostat in which liquid helium is cooled by a J-T heat exchanger.
Thus, the cryostat of the present invention is provided with a preliminary cooler with a system in which the temperature of the liquid helium is lowered by lowering vapor pressure, instead of the J-T heat exchanger in a conventional type of pressurized superfluid helium cooled cryostat. Specifically, there is provided a pressurized superfluid helium cooled cryostat including a first liquid helium bath for reserving liquid helium, a second liquid helium bath for reserving superfluid helium of which the temperature is lower than that of the liquid helium in the first liquid helium bath, a vacuum chamber containing the first liquid helium bath and the second liquid helium bath therein and insulating heat from the room temperature, and a superfluid cooler which is installed in the second liquid helium bath, communicates with the first liquid helium bath via a passage, and generates the superfluid helium to cool the second liquid helium bath, wherein a preliminary cooler is provided between the first liquid helium bath and the superfluid cooler for lowering the temperature of the liquid helium by lowering vapor pressure, and the first liquid helium bath and the superfluid cooler are communicated with each other via the preliminary cooler.
According to the present invention, it has became possible to enhance the cooling efficiency in a superfluid cooler, and to reduce the liquid helium consumption in a superconducting magnet cryostat using the superfluid helium cooling which requires long term operation.
Other objects, features and advantages of the invention will become apparent from the following description of an embodiment of the invention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a conceptual diagram of a pressurized superfluid helium cooled cryostat according to an embodiment of the present invention;

FIG. 2 is a detailed view in the vicinity of a preliminary cooler;

FIG. 3 is a conceptual diagram of a pressurized superfluid helium cooled cryostat of the type of cooling a superfluid cooler with a J-T heat exchanger; and

FIG. 4 is a temperature-enthalpy diagram of pressurized superfluid helium cooling.

DETAILED DESCRIPTION OF THE INVENTION

The pressurized superfluid helium cooled cryostat according to the present invention is provided with a preliminary cooler of the type of lowering the temperature by lowering vapor pressure between a 4.2K liquid helium bath which is a first liquid helium bath and a superfluid cooler installed in a superfluid helium bath which is a second liquid helium bath. In the present invention, it is desirable that a passage which allows a liquid helium inlet from the 4.2K liquid helium bath to communicate with the preliminary cooler is equipped with a first flow rate control valve for J-T (Joule-Thomson) expansion, and a passage which allows the preliminary cooler to communicate with the superfluid cooler installed in the superfluid helium bath is equipped with a second flow rate control valve for J-T expansion.
The preliminary cooler is cooled by depressurizing it with a vacuum pump. At that time, the temperature of the preliminary cooler is kept to a temperature slightly higher than that of the superfluid cooler. The reason for this is to cause the liquid helium to flow from the 4.2K liquid helium bath to the superfluid cooler. The liquid taken in from the 4.2K liquid helium bath is subjected to the J-T expansion at the first flow rate control valve to be cooled to the temperature of the preliminary cooler, and reserved in the preliminary cooler. Since the liquid helium in the preliminary cooler can be cooled to a temperature lower than 2.2K, the liquid helium is further subjected to the J-T expansion at the second flow rate control valve and supplied to the superfluid cooler. Thus, the efficiency of the superfluid cooler can be increased more than the case where a J-T heat exchanger is installed. This is considered to be advantageous particularly in the case of cooling a superconducting magnet requiring a low temperature.
A pressurized superfluid helium cooling method in a cryostat using a conventional J-T heat exchanger will be described with reference to the conceptual diagram of a cryostat shown in FIG. 3 and the temperature-enthalpy diagram shown in FIG. 4.
In the cryostat, the 4.2K liquid helium bath 2 and the superfluid helium bath 3 in which the superconducting magnet 4 is installed are arranged in a vacuum chamber 1. Moreover, the superfluid cooler 10 is installed in the superfluid helium bath 3. The superfluid cooler 10 is depressurized by a vacuum pump 15 via a pumping line 11 for the superfluid cooler so as to be cooled. The liquid helium in the 4.2K liquid helium bath 2 is introduced into the superfluid cooler 10 from the liquid helium inlet 6. Between the liquid helium inlet 6 and the superfluid cooler 10, the J-T heat exchanger 18 is provided so as to thermally contact with the pumping line 11 for the superfluid cooler. By the J-T heat exchanger 11, the liquid helium taken in from the liquid helium inlet 6 is cooled. The liquid helium cooled by the J-T heat exchanger 18 is subjected to the J-T expansion at a J-T valve (Joule-Thomson valve) 9 so as to be further cooled, and fed to the superfluid cooler 10.
Although it is not shown in FIG. 3, it is general that an 80K thermal shield cooled by liquid nitrogen in order to prevent heat leak due to thermal radiation and thermal conduction and a 4K thermal shield cooled by liquid helium are generally installed in the vacuum chamber as shown in FIG. 1 described below. Between the 4.2K liquid helium bath 2 and the superfluid helium bath 3, a cold safety valve 5 is installed. The cold safety valve 5 operates as a liquid helium feed passage from the 4.2K liquid helium bath 2 to the superfluid helium bath 3, and has a role to ensure that the pressure in the superfluid helium bath 3 becomes the atmospheric pressure. Moreover, the cold safety valve 5 becomes a pressure discharge passage when the superconducting magnet 4 is quenched.
The cooling process will be described with reference to FIG. 4. In FIG. 4, the temperature of the superfluid helium bath 3 is assumed to be 1.8K. In the J-T heat exchanger 18 thermally contacting with the pumping line 11 for the superfluid cooler, the liquid helium introduced from the liquid helium inlet 6 provided in the 4.2K liquid helium bath 2 is cooled to about 2.2K by evaporated helium gas forcedly exhausted from the superfluid cooler 10. This is the process of (1) to (2) in FIG. 4. The cooled liquid helium is cooled to 1.8K by the J-T expansion by the J-T valve 9, and accumulated in the superfluid cooler 10. This is the process of (2) to (3) in FIG. 4. By forcedly evaporating the liquid helium accumulated in the superfluid cooler 10 by depressurizing it to a vapor pressure of 0.0017 MPa by the vacuum pump 15 so as to be exhausted, the heat is taken away from surroundings by an amount of the enthalpy difference in the process of (3) to (4) shown in FIG. 4. The forcedly evaporated helium gas cools the liquid helium taken in, is returns to a room temperature part, and subsequently exhausted outside the vacuum pump 15. This is the process of (4) to (5) in FIG. 4.
FIG. 4 shows the case of the J-T expansion from 4.2K, and the case (3a) of being preliminary cooled to an ideal temperature of 1.8K, which are processes of (1) to (3a). From FIG. 4, it is found that the lower the temperature before the J-T expansion is, the larger the enthalpy difference is, which removes a large amount of heat. Accordingly, the larger the efficiency of the J-T heat exchanger 18 is, the larger the heat which can be removed by the superfluid cooler 10 becomes. This means that even if the temperature of the superfluid cooler 10 and the heat leak into the superfluid helium bath 3 are equivalent, the heat can be removed with fewer liquid helium consumption in the superfluid cooler 10, and therefore the efficiency of the superfluid cooler 10 increases. However, in the case of the conventional type of pressurized superfluid helium cooled cryostat, since the liquid helium introduced from the 4.2K liquid helium bath 2 is cooled by heat exchange with the vapor of the superfluid cooler 10, there is a lithe preliminary cooling is limited up to about 2.2K.
Next, an example of the present invention will be described with reference to FIGS. 1 and 2.

EXAMPLE

FIG. 1 is a schematic view showing a pressurized superfluid helium cooled cryostat according to the example of the present invention. The cryostat has the structure in which a 4.2K liquid helium bath 2 which is a first liquid helium bath for accumulating 4.2K liquid helium, and a superfluid helium bath 3 which is a second liquid helium bath for accumulating 1.5 to 4.2K liquid helium and in which a superconducting magnet 4 for generating a high magnetic field is installed are installed in a vacuum chamber 1. As a superconducting magnet 4 installed in the superfluid helium bath 3, there are a high sensitive NMR spectrometer, an MRI, an accelerator, and the one for a nuclear fusion reactor.
In order to reduce thermal radiation from the room temperature part outside the vacuum chamber 1, a 80K thermal shield 14 cooled by liquid nitrogen is placed between the vacuum chamber 1 and the 4.2K liquid helium bath 2. The 80K thermal shield 14 is cooled by the liquid nitrogen accumulated in a liquid nitrogen reservoir 13. Further, a 4K thermal shield 17 cooled by the liquid helium at 4.2K is installed between the 4.2K liquid helium bath 2 and the superfluid helium bath 3. In order to reduce heat leak due to thermal conduction from a pumping line, an electrical lead, etc., it is preferable to thermally contact the pumping line, the electrical lead, etc. with the 80K thermal shield 14 and the 4.2K thermal shield 17. Further, in order to reduce heat leak due to thermal radiation, it is preferable to cover the outside of the 80K thermal shield 14 with a laminated heat insulator.
Between the 4.2K liquid helium bath 2 and the superfluid helium bath 3, a cold safety valve 5 is installed. Since the thermal conductivity of superfluid helium existing in a gap between this cold safety valve 5 and a valve seat is very large, it is preferable to set the gap to be 10 μm or less to make the heat leak small. The gap of the safety valve can be made 10 μm or less by polishing it using alumina powders and the like after mechanical processing.
The liquid helium introduced from a liquid helium inlet 6 of the 4.2K liquid helium bath 2 is subjected to J-T expansion in a J-T valve 9 corresponding to a second flow rate control valve, and lowered to a temperature of a superfluid cooler 10. It is preferable that the position of the liquid helium inlet 6 is near the bottom portion of the 4.2K liquid helium bath 2 possibly, in order to cause the fluid refilling interval of the liquid helium to be long. However, if refilling is not enough at a time of initial cooling so that air, liquid nitrogen, water and the like remain in the cryostat, those will be solidified and accumulated in the bottom portion of the 4.2K liquid helium bath 2. If these impurities enter the liquid helium inlet 6, plugging will occur in a passage communicating to the superfluid cooler 10, which make the cooling impossible. For this reason, it is preferable that a filter made of a sintered material of fine particles is installed at the liquid helium inlet 6.
In order to cool the inside of the superfluid helium bath 3, the superfluid cooler 10 is installed within its storage bath. The superfluid cooler 10 is a container which can accumulate liquid helium, and the saturated vapor pressure is lowered by depressurizing the inside of the container with a vacuum pump 15 installed in the room temperature part, so that the temperature is lowered. A rotary pump is suitable for the vacuum pump 15 for example, and the pumping speed of the rotary pump is high at a pressure of about 0.0017 MPa which is the saturated vapor pressure of the 1.8K superfluid helium. When a large pumping capacity is required, a mechanical booster pump may be used. Moreover, when a low temperature of about 1K is required, it is also possible to use a turbo-molecular pump which can change its rotating speed depending on load.
Since the superfluid cooler 10 requires taking heat from the surrounding liquid helium, it is desirable to use a material having high thermal conductivity. However, since the material having high thermal conductivity has small electric resistance in general, there is the danger that damage may be caused by eddy current generated when the superconducting magnet 4 is quenched. For this reason, it is desirable to install the cooler at a place apart from the superconducting magnet 4 as far as possible. As the shape of the superfluid cooler 10, a liquid reservoir type and a tube type are well known, however, the liquid reservoir type is preferable because a large evaporation area can be obtained and a liquid level position can be easily detected and easily controlled. Since the pumping line 11 for the superfluid cooler communicating the superfluid cooler 10 with the vacuum pump 15 reaches to the room temperature part, stainless steel having low thermal conductivity is suitable, for example. As the J-T valve 9, a needle valve is suitable, of which flow rate is readily controlled.
As described above, as the temperature of fed liquid helium before the J-T expansion decreases, the cooling efficiency of the superfluid cooler 10 increases. Therefore, a first flow rate control valve 7 and a preliminary cooler 8 are installed on a near side of the J-T valve. The detailed view around the preliminary cooler 8 is shown in FIG. 2. The preliminary cooler 8 is a container in which the liquid helium is accumulated, and the temperature therein is lowered by depressurizing and exhausting the inside of the container by a vacuum pump 16 installed in the room temperature part through a pumping line 12 for the preliminary cooler connected to the container. Although the vacuum pump 16 and the pumping line 12 for the preliminary cooler may be the same ones which are used for exhausting the superfluid cooler 10, since the heat which has to be removed by the preliminary cooler 8 corresponds to the heat leak entering from surroundings, and thus is considered to be very few, a compact vacuum pump may be used. When a mechanical booster pump or a turbo-molecular pump is used for depressurizing the superfluid cooler 10, the rotary pump in a subsequent stage may be used for depressurizing the preliminary cooler 8. The preliminary cooler 8 may be supported by the pumping line 12 for the preliminary cooler from the room temperature part, or by the pumping line 12 for the preliminary cooler from the 4K thermal shield 17.
The roles of the first flow rate control valve 7 installed between the preliminary cooler 8 and the liquid helium inlet 6 are to control the quantity of liquid helium introduced from the 4.2K liquid helium bath 2 and to subject the introduced liquid helium to the J-T expansion. Similar to the J-T valve 9, the first flow rate control valve 7 preferably uses a needle valve by which flow rate is readily controlled.
An operation method of a pressurized superfluid helium cooled cryostat will be described.
In order to operate the pressurized superfluid helium cooled cryostat, a superfluid helium bath 3 and a 4.2K liquid helium bath 2 are preliminary cooled by liquid nitrogen, and thereafter liquid helium is stored in the superfluid helium bath 3 and the 4.2K liquid helium bath 2.
After the liquid helium is accumulated, the first flow rate control valve 7 is fully opened and a valve 21 attached to the inlet of the vacuum pump 16 is fully closed, and while watching a liquid helium level meter and a thermometer installed in the superfluid cooler 10, the J-T valve 9 and a valve 22 attached to the inlet of the vacuum pump 15 are controlled so that the liquid level is present in the superfluid cooler 10, and the temperature of the superfluid cooler becomes a desired temperature. At that time, the temperature of the superfluid helium bath 3 is also cooled to a cooling temperature. Subsequently, while watching a liquid helium level meter 20 and a thermometer 19 installed in the preliminary cooler 7, the first flow rate control valve 7 and the valve 21 attached to the inlet of the vacuum pump 16 are controlled so that the liquid level is present in the preliminary cooler 7, and the temperature of the preliminary cooler becomes a desired temperature.
At that time, it is necessary to make the temperature of the preliminary cooler 8 higher slightly than the temperature of the superfluid cooler 10. The reason for this is to cause liquid helium to flow from the 4.2K liquid helium bath 2 into the superfluid cooler 10. Moreover, there is also an object to prevent the liquid level of the superfluid cooler 10 from reaching the inside of the pumping line 11 for the superfluid cooler. This is because if the liquid level of the superfluid cooler 10 reaches the inside of the pumping line 11 for the superfluid cooler, the pumping capacity is lost by film flow. Although temperature control can be performed by installing a heater in the superfluid helium bath 3, since the vapor pressure is high, it is considered to be more advantageous to perform the temperature control by controlling the opening degree of the valve, for example, by using an electromagnetic valve as the valve 21 installed in a discharge outlet. Moreover, it is necessary to take note so that the liquid helium level does not become lower than the liquid helium inlet 6. This is because if the liquid helium level becomes lower than the liquid helium inlet 6, cooling is stopped, resulting in tendency of abrupt temperature rise inside the superfluid helium bath 3 and occurrence of quenching of the superconducting magnet 4.
Since if the temperature of the liquid helium fed from the preliminary cooler 7 to the superfluid cooler 10 is changed, the quantity of the liquid helium passing through the J-T valve 9 is also changed, the J-T valve 9 and the valve 22 installed in the inlet of the vacuum pump 15 are re-adjusted so that the liquid level is present in the superfluid cooler 10 to obtain a desired temperature. After that, a steady state is obtained. The opening degree of the valve in the steady state changes depending on the heat leak into the superfluid helium bath 3.
When the heat leak into the superfluid helium bath 3 is large, in order to allow the liquid helium level to be present in the superfluid cooler 10 and to increase the cooling power, the opening degrees of all valves are set to be larger. On the contrary, when the heat leak is small, in order to prevent the liquid helium level of the superfluid cooler 10 from reaching the pumping line 11 for the superfluid cooler, the opening degrees of the J-T valve 9 and the first flow rate control 7 are made smaller. At that time, the valves 21 and 22 installed at the inlets of the vacuum pumps 15 and 16 are adjusted while watching the liquid helium level meter.
As shown in the present example, the cooling efficiency of the superfluid cooler 10 can be enhanced by using the preliminary cooler 8 of the type of achieving cooling by depressurization by the vacuum pump 16, and preferably by further subjecting the liquid helium to the J-T expansion before and after the preliminary cooler.
It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.

Claims

1. A pressurized superfluid helium cooled cryostat, comprising:

a first liquid helium bath which stores liquid helium;

a second liquid helium bath which stores superfluid liquid helium, the temperature of which is lower than that of the liquid helium in the first liquid helium bath;

a vacuum chamber which contains the first liquid helium bath and the second liquid helium bath, and insulates heat from room temperature; and

a superfluid cooler provided in the second liquid helium bath, which communicates with the first liquid helium bath via a passage and generates the superfluid helium to cool the second liquid helium bath, wherein

the pressurized superfluid helium cooled cryostat comprises a preliminary cooler provided between the first liquid helium bath and the superfluid cooler for lowering the temperature of the liquid helium by lowering vapor pressure, and the first liquid helium bath and the superfluid cooler communicate with each other via the preliminary cooler.

2. The pressurized superfluid helium cooled cryostat according to claim 1, further comprising:

a first flow rate control valve which controls the flow rate of the liquid helium flowing through the passage communicating the first liquid helium bath and the superfluid cooler; and

a second flow rate control valve which controls the flow rate of the liquid helium flowing through a passage communicating the preliminary cooler and the superfluid cooler.

3. The pressurized superfluid helium cooled cryostat according to claim 2, wherein the first flow rate control valve is composed of a valve which causes J-T expansion of the liquid helium.

4. The pressurized superfluid helium cooled cryostat according to claim 2, wherein the second flow rate control valve is composed of a valve which causes J-T expansion of the liquid helium.

5. The pressurized superfluid helium cooled cryostat according to claim 1, further comprising a pumping line for the preliminary cooler for depressurizing the preliminary cooler by a vacuum pump provided outside the vacuum chamber.

6. The pressurized superfluid helium cooled cryostat according to claim 1, further comprising a pumping line for the superfluid cooler for depressurizing the superfluid cooler by a vacuum pump provided outside the vacuum chamber.

7. The pressurized superfluid helium cooled cryostat according to claim 1, further comprising a safety valve between the first liquid helium bath and the second liquid helium bath.