Superconducting electromagnet

1. A superconducting electromagnet comprising:

an inner vessel containing a superconducting coil and liquid coolant to cool the superconducting coil;

an outer vessel thermally insulated from the inner vessel and holding the inner vessel in the outer vessel;

a discharge pipe to discharge, to an outside of the outer vessel, coolant gas generated by evaporation of the liquid coolant in the inner vessel; and

a first spring check valve, a second spring check valve, and a third spring check valve that are disposed in the discharge pipe, wherein

the discharge pipe includes a first split-flow pipe and a second split-flow pipe that are connected in parallel to each other, wherein the second split-flow pipe is larger in diameter than the first split-flow pipe,

a part of the second split-flow pipe is branched into at least a first branch pipe and a second branch pipe,

the first spring check valve is disposed in the first split-flow pipe to open when a pressure difference between an upstream side and a downstream side of the first spring check valve in the first split-flow pipe becomes more than or equal to a first set pressure higher than atmospheric pressure,

the second spring check valve is disposed in the first branch pipe to open when a pressure difference between an upstream side and a downstream side of the second spring check valve in the first branch pipe becomes more than or equal to a second set pressure higher than the first set pressure,

the third spring check valve is disposed in the second branch pipe to open when a pressure difference between an upstream side and a downstream side of the third spring check valve in the second branch pipe becomes more than or equal to a third set pressure higher than the first set pressure, and

the second branch pipe is different from the first branch pipe in terms of at least one of diameter, length, and inner volume.

2. The superconducting electromagnet according to claim 1 , wherein

the first set pressure is less than or equal to 1.1 times as high as atmospheric pressure, and

the second set pressure is more than or equal to 1.25 times as high as atmospheric pressure.

3. The superconducting electromagnet according to claim 1 , wherein the third set pressure is equal to the second set pressure.

4. The superconducting electromagnet according to claim 1 , wherein the third set pressure is higher than the second set pressure.

5. The superconducting electromagnet according to claim 3 , wherein

the part of the second split-flow pipe is branched into the first branch pipe, the second branch pipe, and a third branch pipe,

the superconducting electromagnet further comprises a fourth spring check valve disposed in the third branch pipe,

the fourth spring check valve opens when a pressure difference between an upstream side and a downstream side of the fourth spring check valve in the third branch pipe becomes more than or equal to a fourth set pressure equal to the second set pressure, and

the third branch pipe is different from the first branch pipe in terms of at least one of diameter, length, and inner volume.

6. The superconducting electromagnet according to claim 4 , wherein

the part of the second split-flow pipe is branched into the first branch pipe, the second branch pipe, and a third branch pipe,

the superconducting electromagnet further comprises a fourth spring check valve disposed in the third branch pipe,

the fourth spring check valve opens when a pressure difference between an upstream side and a downstream side of the fourth spring check valve in the third branch pipe becomes more than or equal to a fourth set pressure higher than the second set pressure, and

the third branch pipe is different from the first branch pipe in terms of at least one of diameter, length, and inner volume.

7. The superconducting electromagnet according to claim 3 , further comprising a throttle located downstream of the third spring check valve in the second branch pipe, to reduce a flow channel area for the coolant gas in the part of the second branch pipe.

8. The superconducting electromagnet according to claim 2 , wherein the third set pressure is equal to the second set pressure.

9. The superconducting electromagnet according to claim 2 , wherein the third set pressure is higher than the second set pressure.

10. The superconducting electromagnet according to claim 8 , wherein

the part of the second split-flow pipe is branched into the first branch pipe, the second branch pipe, and a third branch pipe,

the superconducting electromagnet further comprises a fourth spring check valve disposed in the third branch pipe,

the fourth spring check valve opens when a pressure difference between an upstream side and a downstream side of the fourth spring check valve in the third branch pipe becomes more than or equal to a fourth set pressure equal to the second set pressure, and

the third branch pipe is different from the first branch pipe in terms of at least one of diameter, length, and inner volume.

11. The superconducting electromagnet according to claim 9 , wherein

the part of the second split-flow pipe is branched into the first branch pipe, the second branch pipe, and a third branch pipe,

the superconducting electromagnet further comprises a fourth spring check valve disposed in the third branch pipe,

the fourth spring check valve opens when a pressure difference between an upstream side and a downstream side of the fourth spring check valve in the third branch pipe becomes more than or equal to a fourth set pressure higher than the second set pressure, and

the third branch pipe is different from the first branch pipe in terms of at least one of diameter, length, and inner volume.

12. The superconducting electromagnet according to claim 8 , further comprising a throttle located downstream of the third spring check valve in the second branch pipe, to reduce a flow channel area for the coolant gas in the part of the second branch pipe.