Welding method and superconducting accelerator

1. A welding method for welding a cylindrical stiffening member to an outer circumference of a superconducting accelerator tube body using a laser beam in a process of manufacturing a superconducting accelerator tube, the welding method comprising:

butting the cylindrical stiffening member and the superconducting accelerator tube body to form a butting portion as an irradiated face;

irradiating a defocusing beam, as the laser beam, to the butting portion; and

welding the butting portion between the cylindrical stiffening member and the superconducting accelerator tube body by melting the butting portion with the defocusing beam, wherein

the laser beam is configured such that a distribution profile of energy density on the irradiated face to which the laser beam is irradiated is a Gaussian distribution profile having a peak section and an outer edge section, and the energy density of the peak section is 5.8×10 5 W/cm 2 or more,

the energy density of an outer circumference of a region in which 50% of the total energy in the distribution profile of the energy density is included centered on the peak section is less than or equal to 75% of the energy density of the peak section, and

the defocusing beam includes:

a first region which evaporates and melts metal of the butting portion, the first region being configured of the peak section; and

a second region which melts the metal of the butting portion without evaporation, the second region being configured of the outer edge section.

2. The welding method according to claim 1 , wherein the metal of the superconducting accelerator tube body and the stiffening member is formed of niobium.

3. The welding method according to claim 1 , further comprising supplying the irradiated face, front and rear of the irradiated face in a welding direction, and a reverse face of the irradiated face inside the superconducting accelerator tube body with an inert gas.

4. The welding method according to claim 3 , wherein the supplying comprises supplying the inert gas from a center nozzle installed so as to surround the laser beam, a front nozzle installed in front of the center nozzle in the welding direction, a rear nozzle installed rearward of the center nozzle in the welding direction, and a reverse face-side nozzle installed toward the reverse face of the irradiated face inside the superconducting accelerator tube body.

5. The welding method according to claim 1 , further comprising supplying an inert gas between the cylindrical stiffening member and the superconducting accelerator tube body.

6. The welding method according to claim 5 , wherein, between the cylindrical stiffening member and the superconducting accelerator tube body, a partition plate is installed to partition a space in a circumferential direction, and the cylindrical stiffening member includes a supply port through which the inert gas is supplied to an inside thereof on one side of the partition plate in the circumferential direction, and a discharge port that discharges the inert gas from the inside thereof on another side of the partition plate in the circumferential direction.

7. The welding method according to claim 1 , wherein

the superconducting accelerator tube body and the cylindrical stiffening member are installed so that central axes thereof are in a horizontal direction, and

the welding comprises:

irradiating the laser beam to an upper side above the central axes of the superconducting accelerator tube body and the cylindrical stiffening member; and

rotating the superconducting accelerator tube body and the cylindrical stiffening member about the central axes in a direction opposite to a direction directed toward the laser beam from an upper end of the superconducting accelerator tube.

8. A superconducting accelerator having the superconducting accelerator tube manufactured by the welding method according to claim 1 .

9. A superconducting accelerator having the superconducting accelerator tube manufactured by the welding method according to claim 2 .

10. A superconducting accelerator having the superconducting accelerator tube manufactured by the welding method according to claim 3 .

11. A superconducting accelerator having the superconducting accelerator tube manufactured by the welding method according to claim 4 .

12. A superconducting accelerator having the superconducting accelerator tube manufactured by the welding method according to claim 5 .

13. A superconducting accelerator having the superconducting accelerator tube manufactured by the welding method according to claim 6 .

14. A superconducting accelerator having the superconducting accelerator tube manufactured by the welding method according to claim 7 .

15. The welding method according to claim 3 , wherein the supplying of the inert gas occurs during the welding.

16. The welding method according to claim 5 , wherein the supplying of the inert gas occurs during the welding.