JPS6352444B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to a superconducting device that can reduce the amount of evaporation of cryogenic refrigerant during demagnetization. FIG. 1 is a block diagram showing a conventional superconducting device. In the figure, 1 is a superconducting coil, 2 is a persistent current switch (the most common thermal persistent current switch is shown here), and 3 is for setting superconducting coil 1 and persistent current switch 2 to a superconducting state. 4 is a cryogenic container filled with cryogenic refrigerant and houses the superconducting coil 1 and the persistent current switch 2; 5 is a heater for opening and closing the persistent current switch 2; 6 is a power source for the heater 5; 7 is a cryogenic container filled with the cryogenic refrigerant; The switch 8 is a current lead for supplying current to the superconducting coil 1 from the outside, and is housed in the sheath tube 9 and is cooled by passing the evaporated gas of the cryogenic refrigerant through the inside of the sheath tube 9 as a cooling gas. It has a structure. 10 is an auxiliary heater installed in the cryogenic refrigerant 3 to forcibly secure the flow rate of cooling gas in the current lead 8; 11 is an auxiliary heater 10;
12 is a switch, 13 is a superconducting coil 1
A protective resistor that consumes most of the generated energy outside the cryogenic container 4 in the event of an abnormality such as superconductor breakdown during operation of the superconducting coil 1, self-inductance, and permanence of the superconducting coil 1. An appropriate value is selected from the normal conduction resistance value of the current switch 2, the allowable generated voltage, etc. 14 is an external power source for supplying current to the superconducting coil 1 through the current lead 8, and 15 is a switch. A conventional degaussing system will be explained with reference to FIG. Now, the superconducting coil 1 and the persistent current switch 2 each constitute a closed circuit in a superconducting state, and the persistent current I ₀
Let us consider the case of demagnetizing the device while it is being operated. First, the switch 15 is closed and the external power source 14 is connected to the current lead 8. Next, the switch 12 is closed, the auxiliary heater 10 is energized, the cryogenic refrigerant 3 is forcibly evaporated, and the current lead 8 is precooled.
When the current lead 8 is cooled to a predetermined temperature or lower, the external power supply 14 increases the current flowing through the current lead 8 until the current value reaches the same current value as the persistent current I ₀ . At this time, the current in superconducting coil 1 is I ₀
However, the current of persistent current switch 2 does not change from
I decreases from ₀ and eventually becomes zero. When this state is reached, the switch 7 is closed, the heater 5 is energized, and the persistent current switch 2 is changed to the normal conduction state, thereby opening it. When the persistent current switch 2 is opened and the voltage of the external power supply 14 is gradually lowered, the current in the superconducting coil 1 gradually decreases from I ₀ and finally reaches zero, completing the degaussing operation. Such conventional degaussing systems have the following drawbacks. That is, during persistent current operation of the superconducting coil 1,
In other words, the cross-sectional area of the conductor of the current lead 8 cannot be made very large because it is necessary to suppress the amount of heat flowing from the normal temperature section through the current lead as much as possible when the current lead 8 is not energized, and to suppress the evaporation of the cryogenic refrigerant 3 as much as possible. Therefore, it is necessary to prevent thermal runaway of the current lead 8 due to resistance heat generation when the current lead 8 is energized. Therefore, as mentioned above, the current lead 8 is not only gas-cooled during energization, but also pre-cooled before energization, so the auxiliary heater 10 actively evaporates the cryogenic refrigerant 3 during this time. Become. Therefore, since it is necessary to secure the amount of cryogenic refrigerant necessary for the demagnetization work before starting the demagnetization work, there is a serious drawback that the operating time of the device is limited. This invention was made in view of the above, and when the superconducting coil is energized, it has a resistance value as a protective resistance, and when demagnetized, the persistent current switch is set to the optimum resistance value to reduce the amount of evaporation of the cryogenic refrigerant. To provide a superconducting device that can significantly reduce the amount of evaporation of a cryogenic refrigerant during demagnetization by performing forced demagnetization. An embodiment of the present invention will be described below with reference to FIG. 2. In the figure, 16 is a degaussing resistor,
As will be described later, it is set to an optimal value for reducing the amount of evaporation of the cryogenic refrigerant during demagnetization work. 17
is means for switching between the protection resistor 13 and the degaussing resistor 16. Now, the demagnetization procedure of the present invention is as follows. First, the switching means 17 is switched to the demagnetizing resistor 16. Next, the switch 7 is closed, the heater 5 of the persistent current switch 2 is energized, and the persistent current switch 2 is opened. When the persistent current switch 2 is opened, the current in the superconducting coil 1 is (1) with a time constant determined by the combined resistance value R of the generated normal conductive resistance and the demagnetizing resistance 16 and the self-inductance L of the superconducting coil 1.
The superconducting coil 1 is demagnetized by attenuation according to the formula. Here, I(t); Current _I0 at time t; Current at time t=0 At this time, there are the following three factors that cause the cryogenic refrigerant to evaporate. That is, the heat generated by the heater 5 to open the persistent current switch 2. When the current in the superconducting coil 1 attenuates according to equation (1), the current shunted to the persistent current switch 2 and the shunt loss due to the normal conduction resistance of the persistent current switch. The heat generated by the current lead due to the current flowing through the current lead 8 to the demagnetizing resistor 16 enters the cryogenic refrigerant from the low temperature end of the current lead 8 by thermal conduction. Among these amounts of heat generation, the loss W _H of the heater 5 of the persistent current switch 2 is a fixed value determined from the switching characteristics required for the persistent current switch 2 used, and the amount of heat generated Q _H
is determined by W _H × (demagnetization time). shunt loss of
W _pcs is the normal conduction resistance value of persistent current switch 2.
If R _pcs and the resistance value of the demagnetizing resistor 16 are R _L , then the equation (1) can be expressed as follows. However, R _L ≪ R _pcs . Therefore, the amount of heat generated due to shunt loss during demagnetization
Q _pcs is Q _pcs = ∫ ₀ ^∞ W _pcs dt = R _L LI ₀ ² /2R _pcs ……(3). In addition, the amount of heat entering from the current lead 8 of
Q _pL is the material of the conductor of current lead 8, the cross-sectional area,
It is determined by specifications such as length, current flow value, current flow time, etc., and it is difficult to express it mathematically, but by calculating the temperature distribution of the current lead by simulation etc., equations (4) and (5) can be obtained. It is possible to obtain it quantitatively from the formula. Q _pL =∫ ₀ ^∞ qdt...(4) However, q=kSdT/dx...(5) Here, k: thermal conductivity, S: conductor cross-sectional area,
dT/dx: Temperature gradient at low temperature end The inventor describes an example of the superconducting coil 1, persistent current switch 2, and current lead 8 that are in practical use.
The amount of heat generated during demagnetization was calculated and compared between the case of the demagnetization pattern shown in FIG. 3 in the conventional degaussing system and the case of the demagnetization pattern shown in FIG. 4 in the demagnetization system according to the present invention. The trial calculation conditions are shown in Table 1, and the results are shown in Table 2 and Figure 5.

【table】

【table】

[Table] From Figure 5, in the case of the degaussing system of the present invention,
There is a resistance value for degaussing that minimizes the amount of evaporation of the cryogenic refrigerant. It can be seen that when liquid helium, for example, is used as the cryogenic refrigerant, the amount of evaporation is reduced to about 1/3, compared to 7.47 for the conventional refrigerant, to 2.49 for the refrigerant of the present invention. Now, as mentioned above, in FIG. 5, there is a resistance value at which the total amount of heat generated is the minimum with respect to the magnitude of the resistance value for demagnetization. This is because the heater loss Q _H of the persistent current switch 2 and the amount of heat Q _pL penetrating from the low-temperature end of the current lead 8 increase as the time required for demagnetization increases.
In the range where the resistance value for demagnetization is large, the shunt loss Q _pcs of the persistent current switch 3 is dominant, and in the range where the resistance value is small, the sum of the heater loss Q _H and the amount of heat intruded from the current lead Q _pL becomes dominant. It is from. The reason why the demagnetization time becomes longer as the demagnetization resistance value becomes smaller is that, as can be seen from equation (1), the current in the superconducting coil 1 decreases exponentially, and the decay time constant is τ=L/R. This is because the smaller _{the L} and demagnetizing resistance values, the longer the length. Therefore, if the rate of current attenuation in the high current area is not excessively increased and the rate of current attenuation in the low current area is not decreased, the shunt loss of persistent current switch 2 can be reduced.
It is possible to shorten the demagnetization time without significantly increasing Q _pcs , reduce the heater loss Q _H and the amount of heat Q _pL entering from the current lead 9, and further reduce the amount of evaporation of the cryogenic refrigerant during demagnetization. . The invention made to realize this is another embodiment of the present invention shown in FIGS. 6a and 6b, in which a switching means is used such as a thyristor 18 whose voltage drop does not depend much on the current flow value. Configure. In other words, the forward voltage drop of the thyristor 18 has little dependence on the current and is a voltage drop that is almost constant with respect to the current value. It is possible to give the superconducting coil 1 a characteristic of high resistance, thereby preventing the initial reduction rate of the current in the superconducting coil 1 from becoming excessive and shortening the time required for the current to decay to zero. Now, considering the configuration shown in Fig. 6b, the demagnetizing resistor 16 is R _L and the thyristor forward voltage drop is E _Th.
Then, the decay current I(t) of superconducting coil 1 is
It is given by equation (6). However, R _L ≪ _{R Pcs} . Therefore, the time t ₁ until I(t)=0 is
It becomes as shown in equation (7). t ₁ =-L/R _L ln (E _Th /R _L I ₀ +E _Th ) ...(7) The shunt loss of the persistent current switch 2 is obtained by equation (8). Equation (6) was calculated using R _L = 0.02 (Ω), E _Th = 2 (V), and other conditions were as shown in Table 1, and Equation (1) was calculated using R _L = 0.02 (Ω), 0.05 Ω. Figure 7 shows a comparison of the results. As can be seen from Figure 7, in the case of R _L + E _Th , compared to the case of R _L only, the rate of current attenuation in a range of small currents does not become excessive, and the rate of current attenuation in a range of small currents does not become excessive. The demagnetization time can be shortened. Similarly, R _L =0.02
(Ω) and E _Th =2 (V), the results of trial calculations of Q _Pcs Q _H and Q _PL Q are plotted in Figure 5 with the symbols 〇□△×. As can be seen from FIG. 5, in the case of other embodiments of the present invention, the amount of heat generated during demagnetization Q
It is clear that this method is more effective in reducing the amount of evaporation of the cryogenic refrigerant during demagnetization, as it can be reduced to about 65% and to about 22% compared to the conventional degaussing system. Additionally, the above calculations were made under the condition that no gas cooling of the current lead is performed at all. The heat intrusion from the low temperature end of the current lead in Table 2 and FIG. 5 can be reduced to almost zero, and the amount of evaporation of the cryogenic refrigerant during demagnetization can be reduced. As described above, according to the present invention, the amount of evaporation of the cryogenic refrigerant during demagnetization can be significantly reduced without impairing the protection function during excitation.

[Brief explanation of drawings]

Fig. 1 is a block diagram of a conventional superconducting device, Fig. 2 is a block diagram showing an embodiment of the present invention, and Figs.
The figures are explanatory diagrams showing the conventional demagnetization patterns and those of the present invention, respectively. Fig. 5 is an explanatory diagram showing the result of trial calculation of the amount of heat that causes evaporation of the cryogenic refrigerant during demagnetization. Fig. 6 7 is a configuration diagram showing the main part of another embodiment of the present invention, and FIG. 7 is an explanatory diagram showing the state of the decay current of the superconducting coil. In the figure, 1 is a superconducting coil, 2 is a persistent current switch, 3 is a cryogenic refrigerant, 4 is a cryogenic container, 5 is a heater for opening and closing the persistent current switch, 8 is a current lead, 13 is a protective resistor, and 16 is a It is demagnetizing resistance. Note that the same reference numerals in each figure indicate the same or equivalent parts.

Claims (1)

[Claims] 1. A parallel circuit of a superconducting coil and a persistent current switch is arranged in a cryogenic container filled with a cryogenic refrigerant, and current leads connected to both ends of the parallel circuit are connected to the outside of the cryogenic container. In this case, when demagnetizing the superconducting coil, a demagnetizing resistor is connected between both current leads, and a protective resistor is connected between both current leads to protect the superconducting coil during abnormality. The amount of heat generated due to shunt loss of the current switch, and the amount of heat generated due to the power consumed to open the persistent current switch,
A superconducting device characterized in that the value of the demagnetizing resistor is selected so that the sum of the amount of heat intruding from both the current leads is a minimum value. 2. The superconducting device according to claim 1, wherein switching between the protective resistor and the demagnetizing resistor is performed by ignition control of a semiconductor element.