JPH0122578B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION SUMMARY OF THE DESCRIPTION An improved cryostat for a superconducting magnet in an NMR spectrometer employs a multiple structure of isothermal shells surrounding a virtually spherical thermally conductive central vessel containing liquid helium. The superconducting solenoid within the central container remains in operation when only partially immersed in liquid. The radiation shield surrounding the central container is cooled by boiling evaporation of the escaping helium vapor. This radiation shield was maintained at the temperature of liquid nitrogen by thermal contact with a liquid nitrogen container located above the region of the central container and shielded from the central container by the walls of the isothermal shell. It is sealed inside an isothermal shell. An outer radiation shield surrounds the liquid nitrogen vessel and its associated isothermal shell, and the outer radiation shield is maintained at a temperature on the order of 180° by external refrigeration equipment. A hermetically sealed envelope forms the outer wall of the cryostat and provides ports for evacuating all interior spaces between adjacent stacked structures to a very low common pressure. . The spacing between adjacent surfaces of the superimposed structure is maintained by a novel mechanical bracing system of low thermal conductivity consisting of polyester strings placed under tension between adjacent shells. .

The present invention relates to a cryostat containing a cryogenic liquid such as liquid helium, and in particular to a cryostat containing a cryogenic liquid such as liquid helium.
It relates to cryogenic chambers that house superconducting devices such as magnets for NMR spectrometers.

Conventional cryostats containing superconducting devices, such as superconducting magnets, are shaped to exhibit a relatively small cylindrical volume surrounding the superconducting magnet in significant communication with a relatively large volume located directly above the solenoid. A helium container was used. In such a geometry, the solenoid is kept completely submerged in a liquid helium bath. The source of liquid helium in its large volume provides sufficient retention time for the liquid helium. This form of helium container exhibits a surface-to-volume ratio that is significantly larger than the minimum achievable, thus introducing additional radiation losses resulting in higher helium boiling evaporation rates.

Conventional cryostats take the form of multi-layered chambers that are internally reinforced, e.g. with stainless steel spokes, to withstand mechanical shock and maintain minimal gaps between adjacent multi-walls. I came to get it. Stainless steel is a common material of choice due to its relatively low thermal conductivity and high strength. However, the thermal conductivity of such a reserve is
This places a limit on the thermal insulation that can be achieved between adjacent surfaces of multiple structures.

Conventional cryostats used a secondary temperature bath to shield the coldest coolant from ambient temperature. Typically, the secondary coolant container itself was insulated from ambient temperature, such as by using multiple layers of insulating material. For superconducting magnets that can be moved in and out from room temperature, conventional cryostat structures require relatively large magnet holes to provide sufficient space for this insulation. As a result, the inner diameter of such conventional solenoids is forced to be enlarged to a relatively large diameter to accommodate the additional insulation, thereby requiring larger lengths of superconducting wire for the manufacture of the solenoid. It was needed.

It is an object of the present invention to provide an improvement in a cryostat containing liquefied gas so that loss of liquefied gas from the cryostat due to boiling of the liquefied gas is minimized.

Another object of the invention is to minimize the amount of liquid helium required for operation of the superconducting magnet and to maximize the time interval between liquid helium replenishments.

Yet another object is to provide a strong and stable internal bracing for such multi-chamber cryostat structures while improving thermal insulation between the multiple chambers of the multi-chamber configuration.

According to one feature of the invention, a substantially spherical central container is provided for containing the liquefied gas forming the primary coolant, so that the surface area to volume ratio of this container is minimized.

According to another feature of the invention, the central container is provided such that the temperature gradient introduced by the necessary filling and venting pipes leaving the central container is still only partially immersed in the liquefied gas. It is constructed of aluminum with a thickness that makes it small enough to allow operation of the superconducting device.

According to yet another feature of the invention, the central coolant container is surrounded by a radiation shield, the shield being spaced from the central container and preventing the liquefied gas contained within the central container. The first intermediate temperature is maintained by the vapor cooling effect provided by the boiling evaporation of.

According to a further feature of the invention, the central container and the surrounding radiation shield are surrounded by an isothermal surface defined by a second enclosure chamber, the second container being surrounded by a second surrounding chamber. A secondary coolant is provided above the enclosure and in thermal contact with the outer surface of the second enclosure, such that the second enclosure and the second vessel form a secondary coolant filling the second enclosure. 2 to form an isothermal body maintained at the temperature of the liquefied gas.

According to a further feature of the invention, an outer radiation shield is provided in sealed contact with the isothermal body comprising the second container and the second enclosed chamber, such outer radiation shield The temperature is maintained between the second coolant and the ambient temperature.

According to yet another feature of the invention, the outer radiation shield is maintained at the desired temperature by auxiliary refrigeration equipment.

According to another feature of the invention, the hermetic
a sealed envelope including an outer radiation shield;
The multilayer structures formed by the coolant-cooled isothermal surfaces, the inner radiation shield, and the central vessel are then sealed, and the spaces between these multilayer structures are in communication, thereby closing these spaces. They can be commonly exhausted.

According to yet another feature of the invention, adjacent ones of the laminates are braced by taut polyester cords to reduce heat transfer between adjacent surfaces of the several laminates. while maintaining an air gap between those surfaces.

According to the invention, the primary coolant (hereinafter referred to as liquid helium) is contained within a substantially spherical central container. The central container is formed of aluminum and has a hole in its center defined by an aluminum cylindrical wall welded to the quasi-spherical container. A superconducting solenoid is located inside the central container coaxially with the hole. In accordance with the present invention, the surface-to-volume ratio is minimized to reduce the area of the central vessel for radiant heat absorption and to reduce the area of the solenoid when the level of liquid helium drops well below the top of the solenoid. Enable continued operation.

A radiation shield surrounding the helium container is provided to establish a first isothermal surface intermediate the central container and a second isothermal surface surrounding it, which second isothermal surface is provided with a secondary coolant (hereinafter referred to as (referred to as liquid nitrogen). This radiation shield is maintained at about 50° by vapor cooling, which is carried out by boiling evaporation of helium flowing through the filling and venting tubes in thermal contact with it. This feature constitutes the gist of the invention related to Patent Application No. 18559 (Japanese Patent Publication No. 1983-57733) filed in 1972 by the applicant of this patent on the same date as this application.

A secondary isothermal shell surrounding this radiation shield and partially surrounding the filling and venting tubes is cooled in thermal contact with a liquid nitrogen container located outside the shell above the area of the central container. is provided. In this geometry, the filling and venting tubes from the central vessel (which is partially surrounded by the cylindrical part of the secondary isothermal shell) are connected to the secondary coolant vessel compared to conventional cryostats. It penetrates a greater length, resulting in improved thermal insulation for the liquid helium container.

Thermal isolation of the secondary coolant vessel and associated secondary isothermal shell from ambient temperature is, according to the invention,
A further improvement is provided by providing an outer radiation shield surrounding the secondary coolant vessel and associated isothermal shell. This outer radiation shield is maintained at a temperature intermediate between the secondary coolant temperature and ambient temperature using auxiliary refrigeration equipment.

An outer hermetically sealed container seals the outer radiation shield and the interior of the cryostat, allowing the space between adjacent multiplex structures to be evacuated to pressures on the order of 16 ^-6 torr. In this way, gas heat conduction and convection effects that transfer heat to the central vessel are minimized.

By replacing conventional stay spokes with stays formed of polyester string, the present invention virtually eliminates direct heat conduction losses between multiple structures due to mechanical supports and internal stays.

With the exception of the filling and venting tubes for both the liquid nitrogen and liquid helium vessels, all of the walls forming the multiple structure of this cryostat are constructed of aluminum. As a result, heat transfer by radiation is further reduced between adjacent surfaces. This feature is disclosed in Japanese Patent Application No. 54-18562 (Japanese Unexamined Patent Publication No. 54-128821) assigned to the present patent applicant.
(a) constitutes the gist of the invention pertaining to item (a).

Other features and advantages of the invention will become more apparent upon reading the following detailed description taken in conjunction with the accompanying drawings.

The superconducting NMR spectrometer device uses a cryostat 1 that can be moved in and out of room temperature to a magnetic field created inside the cryostat 1 in a manner that will be explained in more detail below. A probe 5 containing a sample 7 is introduced into the hole 3 for analysis. transmitter 9, receiver 11, control unit 13,
The data processing unit 15 and the display mechanism 17 constitute a complete spectrometer (excluding the power supply system for activating the continuous current to the magnet).

FIG. 2 is a top view of an embodiment of the cryostat of the present invention. The hole 3 is a cryostat 1 as described below.
forming a room-temperature entrance/exit to the magnetic field created by the internal device.

Referring now to FIG. 3, the cryostat 1 includes a superconducting solenoid assembly 50 inside a central container 110. Vessel 110 contains a primary coolant, preferably liquid helium, to maintain the superconducting state of the windings comprising solenoid assembly 50. Leads from the solenoid winding, generally designated by the numeral 50, terminate in a connector 54 for a connection to an external current source introduced as described below. Additional circuitry that constitutes a connectivity switch to control the transition between normal and superconducting states for selected windings is not shown. These circuits and advantageous persistent switches are described in Japanese Patent Publication No. 1983-1980 by the applicant of this patent.
No. 61277 (Patent No. 1389997) and Special Publication No. 1989-
44684 (Patent No. 1433872).
Solenoid assembly 50 is also not within the scope of the present invention and is separately described in commonly assigned co-pending US Pat. Nos. 4,213,092 and 4,180,769.

The central coolant vessel 110 is a 3.175 mm (0.125″) thick aluminum plate and is formed into a substantially spherical shape as shown by well known potter's wheel techniques. In one convenient embodiment, the vessel 110 is It has a coolant capacity of approximately 25 liters. Vessel 110 is further characterized by an aperture formed by a cylindrical wall 111 welded to vessel 110. On the other hand, the holes are used to take in and out from room temperature.The container 110 has cylindrical tubes 113, 115,
It is insulated from the ambient temperature by means of a plurality of surrounding chambers 112, 114, 116 and 118, one on top of the other, with coaxial holes defined by 117 and 119. The wall thickness of each of these separate cylindrical coaxial tubes is determined by the thermal load on each and ranges from 0.508 mm (0.02″) to 1.245 mm.
(0.049″). Chambers 112, 114, 1
The spaces between 16 and 118 are in communication with each other, as described below, to minimize heat transfer ^between adjacent multistructure surfaces by gas heat conduction or convection. The outer chamber 118 is evacuated through an exhaust port 120 to achieve a low pressure.

A secondary coolant vessel 114' is positioned above the central vessel 110 and in thermal contact with the chamber 114, thereby forming the chamber 114, which is preferably formed of aluminum having a nominal dimension of 0.190'' thickness. teeth,
A shell is formed which is maintained at the temperature of the secondary coolant, preferably liquid nitrogen.

Returning to FIG. 2, two fill vent tubes 130 and 130' are required to enter and exit the central container. They are constructed of 15.875 mm (5/8") internal diameter stainless steel with a wall thickness of 0.127 mm (0.005"). Two such filling vent pipes 130
and 130' are depicted in FIG. 2, and one such structure is shown in more detail in FIG. These fill vent tubes are connected to electrical connectors 54
The only difference is that is only required for tube 130. Tubes 130 and 130' are preferably made of stainless steel to minimize heat transfer from the liquid helium container to the exterior of the cryostat. The tube 130 includes coaxial tubes 132, 134,
136 and 138, each of which tubes leads to a respective separate chamber 112, 114, 1 of the multiple structure.
16 and 118. It is preferably made of aluminum. Heat transfer collar 133 (and 133', not shown) connects tubes 130 and 1
Heat is transferred to the boiling, vaporizing helium vapor passing through 30', thereby serving to maintain the isothermal shell 112 at a constant temperature.

Radiation shield 112 is preferably made from aluminum by conventional wheel turning techniques;
The temperature of the secondary coolant (77.4〓 for liquid nitrogen) and 1
Next, set up an isothermal shell of temperature between the coolant temperature (4.2〓 for liquid helium). For the "liquid nitrogen-liquid helium" combination, the temperature of the radiation shield 112 is optimally about 50°. Heat is transferred primarily by radiation (and by conduction through mechanical restraints as described below) from the surrounding inner surface of the shell 114 to the radiation shield, and from the radiation shield 112 to the fill and vent tubes 130 and The helium vapor in 130' is transmitted through aluminum contact collars 133 and 133' welded to fill and vent tubes 130 and 130' and radiation shield 112, respectively. Thermal contact between the tubes 130 and 130' and the respective collars 133 and 133' is such that approximately 10 mW of thermal power is removed from the escaping helium vapor by the radiation shield 1.
12.

The radiation shield 112 is surrounded by an isothermal shell 1.
14 and its isothermal shell 11
4 is maintained at liquid nitrogen temperature by welded contact with liquid nitrogen container 114'. Isothermal body 114-1
The outer surface of 14' is itself shielded by an outer radiation shield, which is maintained at a temperature intermediate between the temperature of liquid nitrogen and room temperature in a manner described in detail below.

A hermetically sealed envelope 118 hermetically seals the cryostat structure to provide mechanical vacuum integrity to the exterior.

As shown, the rounded opening 13
5 and 137 are provided on the radiation shields 112 and 116. Although not visible in the cross-sectional view of FIG. 2, similar butted openings in shell 114 provide communication between all interior spaces of this multiple structure.
These interior spaces are thereby maintained at a common pressure by evacuation through the exhaust port 120.

The liquid nitrogen container 114' and its associated shell 114 are effectively insulated by cooling the outer radiation shield 116 to a temperature intermediate between the temperature of the liquid nitrogen and ambient temperature. Radiation shield 116 is preferably 173-183 by providing a heat exchanger in tube 145 as described below to provide heat exchange between outer radiation shield 116 and auxiliary refrigeration equipment 140. It is necessary to maintain 〓・. Neslab Instruments
External mechanical refrigerators, such as the Cryocool model CC-100 manufactured by J.D. Instruments, Inc., have proven convenient for this purpose. No doubt similar devices would be useful as well. Referring now to FIG. 5, the mechanism by which the external refrigeration equipment is coupled to the cryostat is shown in more detail.

Entrance/exit 142 (second
(FIG.) is hermetically sealed by tube 143, which supports a longitudinal hole assembly for thermal access to and from outer radiation shield 116. Outer hole wall 144 and inner conduction tube 14 for the longitudinal hole
4' is formed by a tube of low thermal conductivity, such as stainless steel, and the inner contact tube 14
5 is welded along the periphery of the shield 116.
It consists of a thermally conductive tube joined to the outer shield 116 in the form of a coil of turns. The inner and outer walls 144' and 144 are connected to the end plug 1
44". Thermal coupling of refrigerator 140 to radiation shield 116 is achieved by inserting cold head 147 into cooling tube 145, as shown in more detail in FIG. It is done by doing.

Cold head 147 has mounting flange 1
49 and a coaxial thin tube 150.
It consists of a flexible bellows 148 closed at one end of a plated brass or copper tube. Cold refrigerant from refrigerator 140 flows through capillary tube 150 and
along the outside of 0 and back through the hole in flange 149. The pleats of the bellows create a large cooling area, and the space between the pleats of the bellows and the inner wall of the cooling tube 146 provides a
Filled with a 90% methanol/10% water mixture to facilitate heat transfer between the tubes.

FIG. 4 shows liquid nitrogen filling and venting tube assembly 152.
FIG. A non-thermally conductive central fill tube 153, preferably made of stainless steel tubing and having a wall thickness of 0.127 mm (0.005"), extends from the liquid nitrogen container 114' over a distance of approximately 4 1/4" (107.95 mm). A temperature gradient is created between the temperature of 77〓 and the ambient temperature. This tube is shielded by concentric tubes 154 and 155, each of which has an outer radiation shield 116.
and serves as a liquid nitrogen filling tube shielding portion of the outer container 118. An aluminum end contact tube 156 brazed to the central fill tube 153 provides strength and also provides a surface for welding to the container 114' and outer shield tube 155. Thermally conductive collar 157 ensures that the transfer of heat from outer radiation shield 116 to the liquid nitrogen flowing up into central fill tube 153 brings outer radiation shield 116 to a temperature intermediate between the temperature of the liquid nitrogen and the ambient temperature. Contact is made with the central nitrogen fill tube 153 at a point along the temperature gradient that is of sufficient magnitude to maintain the desired temperature. Similarly, helium fill and vent tube 130 (see FIG. 3) is thermally joined to liquid nitrogen container 114' through a heat transfer collar 158 and another heat transfer collar at points along the temperature gradient of tube 130. 159 constitutes a heat transfer path from the outer radiation shield 116 to the steam outlet tube 130. The temperature of the thermal contact point of collar 159 is chosen to be substantially equal to the temperature of the thermal contact point of collar 157 above nitrogen fill and vent tube 153. A second helium fill and vent tube 130', not shown in the figures, establishes another thermal contact point, the details of which are the same as shown in the figures and described above. In this way, in addition to the cooling provided by the refrigerator, as previously described and described in the above-mentioned patent application No. 18559 of 1973 (Japanese Patent Publication No. 63-57733), In much the same way as the outer radiation shield 112 is cooled, the outer radiation shield 116 is vapor cooled.

Central container 110, radiation shield 112, liquid nitrogen container 114', shell 114, outer radiation shield 1
16 and the envelope 118 are preferably 1100-0.
Made from aluminum alloy, which is an alloy. This alloy is well known and can be obtained commercially from several manufacturers. After the parts described above have been formed by turning, the adjacent inner surfaces of each part are polished and then subjected to a marking treatment that reduces the emissivity of the surface by 35%. In this way, the transfer of heat to the liquid helium central container by radiation is significantly reduced. The technology for changing this emissivity characteristic is disclosed in Japanese Patent Application No. 54-18562 (Japanese Unexamined Patent Publication No. 54-18562) assigned to the applicant of this patent.
-128821) constitutes the gist of the invention.

The multiple structure of the cryostat as disclosed by the present invention requires mechanical internal supports to maintain the alignment of the various shells and the coaxial arrangement and narrow width tolerances between the shells. I need.
It is important that the coaxial tubes 111, 113, 115, 117 and 119, which form the holes for entry and exit from room temperature, are accurately positioned. Because of the mechanical fragility criteria that result from the thermodynamic specifications of certain components, it is equally important to restrain multiple structures during shipping of equipment. It is clear that the mechanical restraints that bind adjacent structures inevitably result in conductive paths for heat transfer, and as a result it is essential that they have extremely low thermal conductivities. Moreover, high strength is essential to provide the necessary mechanical restraint. Braided polyester string has been found to be an ideal material for this purpose, despite the precision required in the alignment of the cryostat components.

Referring again to FIG. 3, multiple structures 110,
It will be appreciated that adjacent members within 112, 114, 114', 116 and 118 are constrained by centering spokes of polyester string. For clarity of illustration, only representative spokes 160 are shown in detail. The spokes themselves are formed from polyester laces, preferably braided Dacron. The strength and thermal conductivity parameters of this material are well known and exhibit the highest ratio of strength to thermal conductivity known to date. The polyester material used in one preferred embodiment was manufactured by Rocky Mount Cord Company, Rocky Mount, North Carolina, USA.
Mount Cord Company as #2 Coreair DB. An aluminum sleeve 162 is secured over the cord to form loops at each end. One of the loops thus formed is attached to the eye bolt 1 on one of the pairs of adjacent shells.
64 and the other loop engages a post 166 welded to the other adjacent shell.
These polyester spokes 160
They are arranged at equal intervals, for example 120°, around the axis of .

Tube bodies 111-11 with adjacent coaxial holes
3, 113-115, 115-117 and 117
-119 is 4.52 mm (0.178") to 4.06 mm for the widest and narrowest spacing of the tube pairs described above. (0.16″) and the tubular body of these holes with an accuracy better than 0.76mm (0.03″),
It is desirable to remain coaxial with each other and with respect to the solenoid assembly 50. This was accomplished with the polyester string described above, but also resulted in an additional improvement in the shipping properties of the device at room temperature. Stainless steel spokes suitably dimensioned for operation in the liquid helium-liquid nitrogen temperature range are under considerable tensile stress at room temperature. Such rigid spokes exhibiting thermal conductivity comparable to the spokes of the present invention are known to be extremely susceptible to damage due to shock and vibration. In contrast, the polyester tensile loaded spokes of the present invention exhibit some elongation at room temperature during shipping. The bore tubes are thereby allowed to contact when subjected to lateral shocks or vibrations. For shipping, a mandrel fitted into the central bore prevents permanent deformation of the tube of several coaxial bores in the event of a collision.

The properties of the coefficient of expansion of the spoke materials of the present invention in the range from liquid helium temperatures to ambient temperatures facilitate accurate positioning of the components. As a result of the present invention, the coefficient of expansion of this material, which behaves normally up to about -25 DEG C., abnormally changes sign and the material expands as the temperature is further lowered. This results in a very low net thermal expansion for this material.

The cryostat according to the embodiment described above provides a very significant improvement over the conventional one with respect to coolant consumption. For example, the boiling evaporation rate of liquid helium measured for some conventional cryostat reaches 30 c.c./hr, whereas the above-described embodiment of the present invention has a boiling evaporation rate of about 6 c.c./hr.
Measured boiling evaporation rate in cc/hr. This low boiling evaporation rate, combined with the geometry of the central vessel 110 of the present invention, provides an extended average time between replenishments of liquid helium of approximately 120 days, within which time approximately 20.5 liters of liquid helium are consumed. A superconducting NMR spectrometer with a magnet of comparable characteristics would require replenishment of liquid helium every eight days, and would consume about 86.4 liters of liquid helium over the same 120 days.

The length of time between filling the central container 110 is 1
This is achieved in part by the fact that the central container 110 has a substantially spherical shape. In accordance with the present invention, the center container 110 is made of sufficiently thick aluminum so that the temperature gradient from the top to the bottom of the center container (heat flowing by conduction from the fill and vent tubes 130 and 130' and the shield 110) (attributable to radiation from) is reduced such that vessel 110 is isothermal regardless of the level of liquid helium contained therein. It has been found that the level of liquid helium in this vessel can be allowed to fall well below the top of the superconducting solenoid without causing any deleterious effects during operation of the solenoid. Approximately 254mm
(10") long solenoid assembly 50 exposes approximately 178 mm (7") of solenoid assembly 50 such that the level of liquid helium within vessel 110 is approximately 76 mm.
Satisfactory movements were performed in a state where the temperature had decreased.

The consumption rate of liquid nitrogen coolant is also reduced and the average time between refills is increased. The boiling evaporation rate of liquid nitrogen is 173
−183〓． It is measured to be about 20c.c./hr when cooled to . In the case of liquid nitrogen containers insulated from ambient temperature without the use of cooling radiation shields, the boiling evaporation rate of liquid nitrogen increases to 80 c.c./hr and no shielding is used. in case of
Increases to 160c.c./hr. An outer radiation shield cooled to the desired temperature described above reduces heat transfer by radiation to the liquid nitrogen container 114' by approximately 88% compared to an unshielded container. This means that the energy radiated (or absorbed) by a radiating object in a unit time is proportional to the difference between the absolute temperature of the radiating (or absorbing) object and the absolute temperature of its surroundings to the fourth power. This is the result of Stefan Boltzmann's law of radiation.

The cryostat of the present invention was specifically illustrated using a liquid nitrogen shielded, liquid helium cooled superconducting magnet for an NMR spectrometer. The contribution of the present invention to the cryostat design described herein is not limited to any particular application or particular coolant. The contribution of the present invention is
It is performed on a device housed in a cryostat used for various low-temperature phenomena applications and other superconducting device applications.

Since many modifications may be made to the arrangement described above, and numerous embodiments of the invention, which at first glance appear to be quite different, may be practiced without departing from the scope of the invention, it will be appreciated that many modifications may be made to the above-described arrangement, and that many embodiments of the invention, which at first glance may seem quite different, may be practiced without departing from the scope of the invention. All matters set forth above should be construed as illustrative only and not as limiting.

[Brief explanation of drawings]

Fig. 1 is a schematic diagram of an NMR spectrometer apparatus for carrying out the present invention, Fig. 2 is a top view of the cryostat in the embodiment of Fig. 1, Fig. 3 is a vertical cross-sectional view of the cryostat in Fig. 5 is a detailed cross-sectional view of the cryostat taken along line 4-4 of FIG. 2, and FIG. 5 is a detailed cross-sectional view of the cryostat taken along line 5-5 of FIG. FIG. 6 shows a cold head for thermal connection to a refrigerator. 50...Superconducting solenoid assembly, 110...
Central liquid helium container, 112... Inner radiation shield, 114... Isothermal shell, 114'... Liquid nitrogen container, 116... Outer radiation shield, 118...
outer container.

Claims (1)

[Scope of Claims] 1. A central container sealed around a hole constituting a room-temperature inlet/outlet, into which a first liquefied gas is introduced through a filling vent pipe; A cryostat comprising a shielding mechanism maintained at the temperature of another liquefied gas having a high boiling point, and an envelope maintained at ambient temperature having an inner surface spaced from an outer surface of the shielding mechanism, further comprising: a radiation shield disposed between an inner surface of the envelope and an outer surface of the shielding mechanism; and maintaining the radiation shield at a temperature intermediate between the temperature of the shielding mechanism and the ambient temperature. an external refrigerator and a refrigeration mechanism having a heat transfer mechanism communicating therewith, wherein the envelope has a vertical hole through which the heat transfer mechanism maintains thermal contact with the radiation shield. Characteristic low temperature chamber.