JPS60232459A - Method of controlling superfluid helium generator - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a method for controlling a cryogenic device, and in particular, a device for generating unsaturated superfluid helium as a coolant for a superconducting magnet device.

[Technical background of the invention and its problems]

As is well known, superconducting magnet devices use liquid helium as a coolant, and some of these devices require the following superfluid helium in 2.17.

The so-called superfluid helium generator that generates the above-mentioned superfluid helium is generally constructed as shown in Figure 1. That is, the helium tank 2 containing the liquid helium 1 shown in 4.2 and the unsaturated superfluid helium tank 3 are connected via the communication channel 4. A saturated superfluid helium tank 5 for cooling the inside of the unsaturated superfluid helium tank 3 is placed inside the unsaturated superfluid helium tank 3, and one end side of the helium tank 5 is connected to a Joule-Thomson valve (
Hereinafter, it will be abbreviated as JT valve. )6. It is connected to the helium tank 2 via the primary side of the JT heat exchanger 7, and the other end is connected to the suction port of the vacuum pump 8 via the secondary side of the JT heat exchanger 7. . A discharge port of the vacuum pump 8 is connected to a refrigerator IO via a compressor 9. The refrigerator 10 is used to cool one helium tank.

Therefore, this device makes the helium in the unsaturated superfluid helium tank 3 superfluid in the following manner. That is, the inside of the unsaturated superfluid helium tank 3 is maintained at one pressure, and the vacuum pump 8, compressor 9, and refrigerator 1 are
Operate 0. Due to the operation of the vacuum pump 8, part of the helium in the helium tank 2 is transferred to the JT heat exchanger 7.
It flows through the primary side of the JT valve 6, the saturated superfluid helium tank 5, and the secondary side of the JT heat exchanger 57. Assuming that the inside of the saturated superfluid helium tank 5 is evacuated by the vacuum pump 8 to a saturated vapor pressure corresponding to a temperature lower than the temperature Tb inside the unsaturated superfluid helium tank 3, the The helium is precooled to, for example, 2.2 K while passing through the primary side of the JT heat exchanger 7, and then JT-expanded in the JT valve 6 to become a gas and a liquid at a temperature Ts. This liquid evaporates while passing through the saturated superfluid helium tank 5, thereby cooling the helium in the unsaturated superfluid helium tank 3.

By the way, in such a device, there is a particular problem in how to control the JT valve 6 during initial freezing. That is, until now there has been no reliable guideline for adjusting the JT valve 6. For this reason, the reality is that adjustments are made based on so-called intuition.For example, if the JT valve 6 is throttled too much, the refrigerating capacity will be reduced, or if the JT valve 6 is opened too much, there will be a problem in the JT heat exchanger 7. There is a problem in that mist-like helium enters and this causes an unstable phenomenon in which the temperature on the upstream side of the JT valve 6 changes drastically.

[Purpose of the invention]

The present invention was made in view of the above circumstances, and its purpose is to efficiently and stably initialize the temperature of an unsaturated superfluid helium tank to a target superfluid helium temperature. The object of the present invention is to provide a method for controlling a superfluid helium generator that can perform the following steps.

[Summary of the invention]

In the present invention, a saturated superfluid helium tank is disposed within a non-saturated superfluid helium tank and is evacuated by a vacuum pump for cooling the tank, and a saturated superfluid helium tank is connected to the saturated superfluid helium tank via a Joule-Thomson valve. In a superfluid helium generator in which helium for cooling is passed through, the inside of the unsaturated superfluid helium tank is initially frozen from 4.2K to the following superfluid helium temperature of 2.17K. The Joule-Thompson valve is controlled so that the flow rate passing through the Joule-Thompson valve is maximized within a range in which the decay time of the autocorrelation function of the time change of the temperature Ts of the helium bath does not fall below a certain value. , when the temperature Tb of the unsaturated superfluid helium tank reaches the target temperature, the above Ts is controlled by a Joule-Thomson valve to obtain an equilibrium state where the amount of heat input and the refrigeration capacity are equal. It's a characteristic that it's dead.

This will be explained in more detail as follows.

That is, let us now consider a steady state in which the mass flow rate in the JT valve is equal to the exhaust mass flow rate mp by the vacuum pump. Assuming that the temperature at the outlet of the vacuum pump is 300K, as the temperature Ts increases, the refrigeration capacity increases.

However, the temperature [Ts is the temperature Tb of the unsaturated superfluid helium bath
As the temperature approaches , the amount of helium that evaporates in the saturated superfluid helium tank is limited due to the Capitzer resistance between the metal and liquid helium that make up the saturated superfluid helium tank, and the refrigerating capacity Q becomes A=a akAcTb'' (Tb-Ts), where Ac is the heat transfer area of the saturated superfluid helium bath, ak is the Capitzer resistance, hk
= a j (coefficient when TI).

Currently, the exhaust capacity of the vacuum pump is 700017 m1n, Tb
:= 2. When the refrigerating capacity Q is calculated in the case of OK%akAc -200WK, it becomes as shown in Fig. 2. As can be seen from this figure, there are equilibrium points of 2wU at A and B for the amount of heat penetration Q i n. In state B, the flow rate passing through the Joule-Thomson valve is greater than the amount of helium that evaporates in the saturated superfluid helium tank per unit time, and the liquid that cannot be evaporated in the saturated superfluid helium tank flows into the JT heat exchanger. Since the amount of liquid in the saturated superfluid helium tank is increasing, it cannot be a steady state. On the other hand, in state A, all of the helium that expanded and liquefied JT evaporates, and its latent heat balances the amount of heat penetration. Here, the temperature Tm and temperature [
The temperature range between Tb and Tb is an unstable range where liquid may enter the JT heat exchanger. Therefore, in order to perform initial freezing at the maximum refrigerating capacity, the temperature Ts must always be adjusted to [Tb
This means that it is sufficient to set the temperature close to the lower Tm and operate.

If this relationship is expressed in terms of temperature Ts and temperature [Tb, the third
It will look like the figure. In FIG. 3, the shaded area is the area where the effective refrigerating capacity is positive. The refrigerating capacity is maximum at the solid line portion. Moreover, the dotted line portion is a state of stable equilibrium.

The horizontal line area is an unstable area, and when the relationship between Tb and T8 falls into this horizontal line area, as mentioned above, the liquid that has not been evaporated in the saturated superfluid helium bath enters the JT heat exchanger.
Furthermore, liquid begins to accumulate inside the heat exchanger. When this state is actually observed using a temperature sensor, it becomes as shown in Figure 4.

The area where Ts is largely vibrating corresponds to the hatched area in FIG. 3, and the area where Ts has stopped vibrating and is smooth is the area corresponding to the horizontal line in FIG. 3. That is, while the mist-like helium is flowing through the saturated superfluid helium tank, the temperature oscillates violently, but once the liquid begins to accumulate, the thermal uniformity improves, and the temperature oscillation I+ almost disappears. For these two states, the autocorrelation function f (
t), that is, as shown in FIG. 5, and it can be seen that the decay times are extremely different.

That is, Fig. 5 (a) shows the autocorrelation function in the shaded area, and Fig. 5 (b) shows the autocorrelation function in the horizontal line area.

The control method according to the present invention skillfully utilizes such a relationship. First, during initial freezing from 4.2 K, J
Adjust the opening degree of the T valve and increase the flow rate passing through the JT valve within a range where the decay time of the autocorrelation function does not become smaller than a certain value, controlling it so that it reaches a value close to the solid line that provides the maximum refrigerating capacity. -ing

Then, in response to the temperature drop in the unsaturated superfluid helium tank, the JT valve is throttled while monitoring the autocorrelation function of the temperature Ts to prevent it from entering an unstable region. Furthermore, when the target temperature is reached, the JT valve is further throttled to control the temperature Ts so that the refrigerating capacity is commensurate with the amount of heat input, thereby shifting to a stable equilibrium state.

〔Effect of the invention〕

With this control method, the above-mentioned problems occur due to over-throttling or over-opening of the JT valve.

This can be reliably prevented and initial freezing can be performed efficiently to the desired temperature.

[Embodiments of the invention]

The present invention will be explained in detail below.

FIG. 6 shows an example of a device for implementing the control method according to the present invention, and the same parts as in FIG. 1 are designated by the same reference numerals. Therefore, the explanation of the overlapping parts will be omitted.

In the figure, 21 is the temperature T of the saturated superfluid helium tank 5.
It is a temperature sensor that detects s, and also.

22 is a temperature sensor that detects the temperature Tb of the unsaturated superfluid helium tank 3. The outputs of these temperature sensors 21, 22 are input to the controller 25 via interface circuits 23, 24, respectively. ° On the other hand, JT valve 6
The drive shaft of is connected to a stepping motor 26. Then, the stepping motor 26 is connected to the controller 2.
Controlled by 5.

The controller 25 is composed mainly of a computer, for example, and has an internal storage device that stores in advance the decay time of the autocorrelation function of the temperature [Ts] at a limit point that does not enter the unstable region. ing.

Then, the time series of 8 is read in at a predetermined time interval, the autocorrelation function is calculated, and the stepping motor 26 is operated so that the decay time is within a range larger than the above-mentioned limit point and in a direction that increases the refrigerating capacity. Through this, the opening degree of the JT valve 6 is controlled.

That is, the opening degree of the JT valve 6 is controlled so that @1iiTs gradually decreases as shown by the solid line 2 in FIG.

By repeating such control, when the temperature Tb drops to the target temperature, the JT valve 6 is throttled down to control the temperature Ts so that it reaches exactly the dotted line in FIG. 3. Therefore, it is possible to initially freeze the helium to the target temperature, that is, the target superfluid helium temperature, without causing any instability phenomena, and it is possible to stabilize the helium in its original state. The effect will be obtained.

Of course, the present invention is not limited to the embodiments described above, and can be modified in various ways without departing from the spirit thereof.

[Brief explanation of the drawings] Figure 1 is a schematic diagram of the superfluid helium generator;
3 and 3 are diagrams for explaining the principle of the control method of the present invention, FIG. 4 is a diagram showing an actual observation example of temperature Tb and temperature Ta near the region where the refrigerating capacity is maximum, and FIG. 5 6 is a diagram showing an autocorrelation function of the temperature Ts in each stable/unstable region, and FIG. 6 is a schematic configuration diagram of an apparatus implementing the control method of the present invention. 2... Helium tank, 3... Unsaturated superfluid helium tank, 5... Saturated superfluid helium tank, 6... Gert-Thompson valve (JT valve), 7... JT heat exchanger, 8 ...
Vacuum bong 7", 21.22...Temperature sensor, 25.
...Controller, 26...Stepping motor. Representative Patent Attorney Noriyuki Chika (and 1 other person) Figure 1 Figure 2 Ts (kl Figure 8 t6 L7 1J /, q z, l): 2. f22
Figure 4 Figure 5 (α) tb> Timing Figure 6 L

Claims (1)

[Claims] A saturated superfluid helium tank, which is evacuated by a vacuum pump to cool the tank, is placed inside the unsaturated superfluid helium tank, and helium for cooling is supplied to the saturated superfluid helium tank through a Gert-Thompson valve. In a superfluid helium generator configured to allow current to flow, when initially freezing the inside of the unsaturated superfluid helium tank from 4.2 K to a superfluid helium temperature below 2.17, the temperature of the saturated superfluid helium tank is The decay time of the autocorrelation function of the temporal change in temperature Ts is monitored, and the Gert-Thompson valve is controlled so that the flow rate passing through the Gert-Thompson valve is maximized within a range in which the decay time does not fall below a certain value, and the The above-mentioned Ta is controlled by a Joule-Thomson valve to obtain an equilibrium state so that the amount of heat input and the refrigerating capacity become equal when the temperature Tb of the fluidized helium tank reaches a target temperature. Control method for a fluidized Heliu-5 generator.