JPH09148121A - Superfluid pressurized cryostat - Google Patents

Abstract

PROBLEM TO BE SOLVED: To cut down the thermal flux due to super fluid helium passing through a fine gap made on a seat surface in a valve closing time simultaneously capable of opening and closing action having stable reproducibility by taking the true spherical shape of a valve body and a valve seat of a poppet valve in the same radius. SOLUTION: A poppet type safety valve 5 is composed of a valve main body 8 airtightly held by a ceiling wall 6 of a superfluid pressurized helium vessel 1 and a bottom wall 7 of a constantly fluid helium vessel 2, a valve seat 9 fitted to this valve main body 8 and a valve body 10 disconnectably provided from this valve seat 9. The valve body 10 takes a true spherical inner surface and cylindrical shape while both valve seat and valve body are in the same radius. At this time, the valve body 10 made of relatively hard material such as stainless steel, etc., makes the inside thereof hollow. On the other hand, the valve seat 9 is made of a little bit softer material e.g. a copper alloy such as brass, etc., to be machined for finishing the inner surface taking a true spherical shape.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pressurized superfluid cryostat for cooling / cooling a superconducting magnet.

[0002]

Cryostat for cooling the Related Art superconducting magnet is pooled liquid helium with atmospheric saturation temperature of usually 4.2K therein, this immersed superconducting magnet that is the winding, for example NbTi or Nb ₃ Sn superconducting wire Then, the power is supplied in the superconducting state to obtain a strong magnetic field. At this time, the lower the temperature at which the superconducting wire is used, the higher the critical current (Ic) of the superconducting wire becomes. Therefore, in recent years, liquid helium has been supercooled and the superfluid state (liquid helium temperature <
2.17K). In this case, to make the liquid helium inside the cryostat superfluid,
Although the decompression exhaust pump installed outside the cryostat is connected to the cooling chamber (heat exchanger) installed so that liquid helium can be introduced into the liquid helium inside the cryostat, the cooling chamber is decompressed by the decompression exhaust pump to become a superfluid state. Since the inside of the cryostat (including the superconducting magnet and the current lead) is exposed to the reduced pressure helium atmosphere, the discharge voltage cannot be reduced and the critical heat flux of the superfluid helium cannot be maximized. In some cases, discharge may occur or cooling may be insufficient.

Invented in consideration of these points,
A pressurized superfluid cryostat, for example, Japanese Patent Publication Sho
There are those proposed in Japanese Patent No. 62-1275 and US Pat. No. 5,220,800. Hereinafter, a conventional technique of a pressurized superfluid cryostat will be described with reference to FIG. 3 which is an explanatory view of US Pat. No. 5,220,800.

Shown in FIG. 3 is a pressurized superfluid cryostat according to the prior art. In the figure, a 4.2K normal-flow helium tank 2 is located above the adiabatic vacuum tank 4 of the cryostat, and a pressurized superfluid helium tank 1 is located below it, in which a superconducting magnet 3 is incorporated. 2.17
Cooler for producing pressurized superfluid helium below K 11
Is a heat exchanger, and the normal-flow helium is flushed under a reduced pressure atmosphere through a valve (Joule-Thomson valve) 12 provided immediately before the heat exchanger 11 to
11 Saturated superfluid helium (for example, 1.8K) is generated inside, and the liquid helium in the pressurized superfluid helium tank 1 exchanges heat with this to obtain pressurized superfluid helium that is supercooled helium. Has become.

Now, the superconducting magnet 3 in the pressurized superfluid helium tank 1 is supplied with a rated current value for normal use, but in the superconducting magnet 3, a phenomenon in which the superconducting state is suddenly broken due to some physical factor ( Quench)
There is. If a quench occurs, the superconducting magnet 3
The energy stored in is converted into heat energy and is consumed by the temperature rise of the superconducting magnet 3. Then, as the temperature of the superconducting magnet 3 rises, the liquid helium in the pressurized superfluid helium tank 1 evaporates rapidly, and a large amount of evaporated helium gas is released to the outside of the cryostat 4. At this time, the evaporated helium gas is released by operating a safety valve 13 installed between the normal flow helium tank 2 and the pressurized superfluid helium tank 1.

A detailed sectional view of the safety valve 13 is shown in FIG. The safety valve 13 is composed of a valve body 14 and a valve seat 15. This safety valve
The purpose of 13 is to release the vaporized gas when the superconducting magnet 3 is quenched, but the safety valve 13 is normally in a sealed state, and the normal flow helium tank 2 and the pressurized superfluid helium tank 1 are
Is separated and the fluid flow (in particular, heat transport by the superfluid component) is hindered, thereby having the function of reducing heat intrusion from the normal flow helium tank 2 to the pressurized superfluid helium tank 1. Therefore, the valve body 14 has a conical shape, and the valve body 14 and the valve seat 15 are fluid-sealed with a narrow gap. The heat flow rate Q through this gap is expressed by the following equation. Q ＝ [Z (T) 2πaδr] ・ [2.4 (1-b / a)] / {L [(b / a) ^-2.4 -1]
} Where a is the radius of the large diameter part of the valve body b is the radius of the small diameter part of the valve body Z (T): Superfluid helium heat flux coefficient in a narrow passage by Bon Mardion, et al δr: The gap between the valve body and the valve seat L: longitudinal length of valve body

As can be understood from the above equation, it is effective to keep δr narrow or to lengthen L in order to reduce the heat flow rate through the valve gap. The safety valve actually used in the superfluid cryostat is devised so that the above two parameters are optimal. For example, in order to close the gap δr between the valve body and the valve seat, the seat surface is finished by lapping them, or the length L of the seat surface in the longitudinal direction is set larger than that of a normal safety valve used at room temperature. Things are done.

[0008]

The safety valve 13 of the pressurized superfluid cryostat described in the above-mentioned prior art is a poppet type (fly-up type) safety valve 13, and its operation is linear in the longitudinal direction of the valve body without rotation. It has a structure that moves dynamically.
In the prior art, since the safety valve 13 that seals the flow path connecting the normal flow helium tank 2 and the pressurized superfluid helium tank 1 has a conical shape, there are many factors that hinder the valve operation as described below. There is. : In particular, since the opening / closing operation of the safety valve 13 is performed at an extremely low temperature, the coefficient of friction of the seat surface between the valve body 14 and the valve seat 15 increases, and the reliability of the initial operation decreases. : When the shaft center of the valve body 14 and the valve seat 15 are tilted, the contact part between them (which is in line contact at the diagonal part of the valve body) causes "galling", which makes it difficult to open and close the safety valve 13. . (FIG. 5
(Refer to): When "bite" of impurities (for example, solidified material such as metal pieces or water) occurs between the valve body 14 and the valve seat 15, the contact between the valve body 14 and the valve seat 15 bites the impurities. The contact surface on the side opposite to the side (in a line contact state in the longitudinal direction of the valve) makes it difficult to operate the valve due to the increased friction coefficient at the cryogenic temperature as described above.
(See Figure 6)

When the above situation occurs, the valve body
Since there is a gap between 14 and the sheet surface, the heat flux due to the superfluid helium in this portion becomes excessive from the above-described formula for the heat flow rate Q in the narrow path portion, and the superconducting magnet 3 is insufficiently cooled.

The present invention has been made to solve the problems of the prior art as described above, and its object is to ensure that the valve operation is performed at an extremely low temperature and to provide a mechanical valve body and valve seat space. Even if the position of the valve changes or "bite" of impurities occurs, it is possible to open and close stably and with good reproducibility, and the opening and closing characteristics (sealability when the valve is closed) are good. At the same time, the seat surface when the valve is closed The present invention provides a pressurized superfluid cryostat equipped with a safety valve that reduces the heat flux due to superfluid helium passing through a minute gap.

[0011]

In order to achieve the above object, a pressurized superfluid cryostat according to the present invention is provided with a normal flow helium tank and a pressurized superfluid helium tank in an adiabatic container, and In a pressurized superfluid cryostat having a poppet valve in a passage connecting the tanks, the poppet valve has a valve body and a valve seat formed in a spherical shape having the same radius.

According to the present invention, since the valve body and the valve seat of the poppet type safety valve are formed in a spherical shape having the same radius, when the valve body separates from the valve seat, the seat surface becomes like a conical shape. At the same time, it is not separated so as to form a gap, but is separated from the lower side of the valve body (pressurized superfluid helium tank side) so that the gap becomes larger. It has good release characteristics, does not easily cause defective valve operation due to an increase in friction coefficient at cryogenic temperatures, and also causes mechanical valve discs, positional changes between valve seats, and "bite" of impurities. However, since the valve sealing action is performed on the spherical surface, the opening / closing operation can be stably performed with good reproducibility, and the opening / closing characteristics (sealing property when the valve is closed) are excellent. Furthermore, since the valve sealing action is performed on the surface (spherical surface) and the effective distance between the pressurized superfluid helium and the normal fluid helium (corresponding to L in the above equation) is maintained at the same level as in the prior art, a small sheet surface is used. There is also no increase in heat penetration by superfluid helium through the gap.

[0013]

FIG. 1 is an enlarged cross-sectional view of a safety valve surrounding a pressurized superfluid cryostat according to the present invention,
The entire structure is a conventionally well-known structure as shown in FIG. 3, for example, and in the following description, the same parts as those of the conventional art are indicated by the same reference numerals.

In the figure, 1 is a pressurized superfluid helium tank,
Reference numeral 2 denotes a normal-flow helium tank. These tanks 1 and 2 are arranged in a predetermined adiabatic vacuum tank 4 of a cryostat (not shown) with a normal-flow helium tank 2 above and a pressurized super-fluid helium tank 1 below. It is provided at intervals. Further, in the pressurized superfluid helium tank 1, a superconducting magnet 3 not shown is shown.
Is built-in.

In the figure, reference numeral 5 is a poppet type safety valve, and the safety valve 5 is a valve body which is hermetically held by a ceiling wall 6 of a pressurized superfluid helium tank 1 and a bottom wall 7 of a normal flow helium tank 2. 8 and a valve seat 9 attached to the valve body 8 and a valve body 10 which is detachable from the valve seat 9, and the valve body 10 has a spherical shape. 9 also has a cylindrical shape having a spherical inner surface with the same radius as the valve element 10.

The valve body 10 is made of a relatively hard material such as stainless steel and has a hollow interior. As a forming method, two semi-spherical semi-finished products are manufactured, joined by welding, immersed in liquid nitrogen to remove low temperature strain, and further machined by a numerical control (NC) machine tool or the like to obtain a true spherical shape. To process. On the other hand, the valve seat 9 is a slightly softer material than the valve body 10,
For example, a copper alloy such as brass is machined by an NC machine tool or the like to machine the inner surface into a spherical shape. Similarly to the valve body 9, the valve seat 10 is also subjected to a low temperature strain removal treatment with liquid nitrogen or the like before finishing. Further, the valve body 10 and the valve seat 9 which have been individually machine-finished are subjected to lapping work as a final finish before assembling the cryostat. In this case, it is possible to finish the sphericity of the valve body 10 and the valve seat 9 to 0.5 μm or less by using abrasive grains of about # 10,000.

In the present invention, since the safety valve 5 having the above structure is used, when the valve body 10 separates from the valve seat 9, the valve body 10
Since it is separated from the lower side with a large gap, the valve body
No. 10 has a good disengagement property from the valve seat 9 from the initial minute lift, and it is less likely that defective valve operation will occur due to an increase in the friction coefficient at cryogenic temperature, and the valve sealing action is performed on a spherical surface. Even when mechanical position change or "bite" of impurities occurs, the open / close operation can be performed stably and with good reproducibility, and the open / close characteristics (sealability when the valve is closed) are good. While the safety valve 5 is sealed, the normal flow helium tank 2 and the pressurized superfluid helium tank 1 are separated to reduce heat intrusion from the normal flow helium tank 2 into the pressurized superfluid helium tank 1, while the superconducting magnet 3 is used. During the quench, it operates smoothly to release the vaporized gas, and the pressurized superfluid cryostat can operate smoothly.

In the above example, the valve body 10 is hollow, but in this case, it is desirable to vacuum seal the inside. However, since the safety valve 5 is used in superfluid helium, a minute helium leak may occur, so it is better to fill the hollow portion with epoxy resin or the like to make it solid. With such a structure, heat intrusion due to solid heat conduction through the valve body 10 can be more reliably reduced.

Further, in the above example, the case where the valve body 10 is a sphere has been described as an example, but it may be a hemisphere as shown in FIG. It may be sealed in vacuum or filled with an epoxy resin or the like in the hollow portion to form a solid state.

In the above example, the valve body 10 is made of a relatively hard material such as stainless steel.
It may be made of a material having a low thermal conductivity, for example, a fiber reinforced material using an epoxy resin.

[0021]

As described above, according to the pressurized superfluid cryostat of the present invention, the valve body and the valve seat are provided with the poppet type safety valve in which the shapes of the valve seat and the valve seat are true spheres. Since the valve element has a good disengagement property from the valve seat from the initial minute lift, it is less likely that defective valve operation will occur due to an increase in the friction coefficient at extremely low temperatures, and a mechanical valve element,
Even if there is a change in the position between the valve seats or "bite" of impurities, the valve's sealing action is performed by the spherical surface, so stable open / close operation is possible and open / close characteristics (seal at valve closing) Since the safety valve is sealed, the normal flow helium tank and the pressurized superfluid helium tank are separated at all times to prevent heat from entering the pressurized superfluid helium tank from the normal flow helium tank. On the other hand, when the superconducting magnet is quenched, the superconducting magnet operates smoothly to release the vaporized gas, and the pressurized superfluid cryostat can be operated smoothly.

[Brief description of the drawings]

FIG. 1 is an enlarged cross-sectional view around a safety valve of a pressurized superfluid cryostat according to the present invention.

FIG. 2 is a conceptual sectional view of a safety valve according to another embodiment of the present invention.

FIG. 3 is a cross-sectional explanatory view of a conventional pressurized superfluid cryostat.

FIG. 4 is an enlarged cross-sectional view of the safety valve of FIG.

FIG. 5 is an explanatory diagram of a usage state of a conventional safety valve.

FIG. 6 is an explanatory diagram of a usage state of a conventional safety valve.

[Explanation of symbols]

1: Pressurized superfluid helium tank 2: Normal fluid helium tank 3: Superconducting magnet 4: Cryostat
5: Safety valve 6: Ceiling wall 7: Bottom wall
8: Valve body 9: Valve seat 10: Valve body

Claims (1)

[Claims] 1. A pressurized superfluid cryostat having a normal-flow helium tank and a pressurized superfluid helium tank in an adiabatic container, and a poppet valve in a flow path connecting the respective tanks, wherein a valve element of the poppet valve. A pressurized superfluid cryostat, characterized in that the valve seat and the valve seat are formed in a spherical shape having the same radius.