Ion generator and thermal electron emitter

1. An ion generator, comprising:

an arc chamber;

a cathode that extends outward from the inside of the arc chamber in an axial direction and that emits a thermal electron into the arc chamber;

a thermal reflector with a cylindrical shape provided around the cathode in a radial direction and extending in the axial direction; and

a narrow structure configured to narrow a width in the radial direction of a gap between the cathode and the thermal reflector at a predetermined position in the axial direction.

2. The ion generator of claim 1 , wherein the cathode and the thermal reflector have the same potential.

3. The ion generator of claim 1 , wherein:

the narrow structure includes a protruding part that is provided on at least one of the cathode and the thermal reflector and that protrudes in the radial direction and extends in the axial direction; and

the protruding part narrows the width of the gap in the radial direction at the predetermined position within a section extending in the axial direction.

4. The ion generator of claim 3 , wherein the width of the protrusion in the radial direction of the protruding part is larger than the width of the gap in the radial direction within the section extending in the axial direction.

5. The ion generator of claim 3 , wherein the protruding part is provided so that the length of the section extending in the axial direction is twice the width of the gap in the radial direction within the section extending in the axial direction or more.

6. The ion generator of claim 3 , wherein:

the protruding part includes an inner protruding part provided on the cathode and an outer protruding part provided on the thermal reflector; and

the inner protruding part and the outer protruding part are provided at positions that overlap with each other in the axial direction.

7. The ion generator of claim 3 , wherein:

the protruding part includes an inner protruding part provided on the cathode and an outer protruding part provided on the thermal reflector; and

the inner protruding part and the outer protruding part are provided at positions that do not overlap with each other in the axial direction but overlap with each other in the radial direction.

8. The ion generator of claim 1 , wherein:

the cathode includes a cathode cap that emits a thermal electron into the arc chamber, and a thermal break with a cylindrical shape that extends outward from the inside of the arc chamber in the axial direction and that fixes the cathode cap; and

the narrow structure is configured to narrow the width in the radial direction between the thermal break and the thermal reflector.

9. The ion generator of claim 3 , wherein:

the cathode includes a cathode cap that emits a thermal electron into the arc chamber, and a thermal break with a cylindrical shape that extends outward from the inside of the arc chamber in the axial direction and that fixes the cathode cap; and

the protruding part is not provided on the cathode cap.

10. The ion generator of claim 8 , wherein the thermal break includes a latch end for latching the cathode cap, and an attachment end attached to a member provided outside the arc chamber.

11. The ion generator of claim 10 , wherein:

the thermal reflector includes a connection end that extends inward in the radial direction toward the thermal break near the attachment end and is connected to the thermal break;

the gap includes a bottom region of which a position in the axial direction is closer to the connection end than a middle position between the latch end and the connection end, and a middle region of which a position in the axial direction is closer to the latch end than the bottom region; and

the narrow structure is configured so that the width in the radial direction is larger in the bottom region than in the middle region.

12. The ion generator of claim 11 , further comprising a filament that is provided inside the thermal break and that heats the cathode cap, wherein:

the cathode cap includes a front surface that faces the internal space of the arc chamber, and a rear surface that is latched by the latch end and that faces the filament;

the gap includes a top region of which a position in the axial direction is closer to the front surface than a middle position between the front surface and the rear surface;

the middle region is a region of which a position in the axial direction is closer to the latch end than the top region; and

the narrow structure is configured so that the width in the radial direction is larger in the top region than in the middle region.

13. The ion generator of claim 11 , wherein the bottom region is a space in which the plasma density is lower than that in the internal space of the arc chamber.

14. The ion generator of claim 8 , further comprising a gas introducing port that introduces a source gas used to generate plasma into the arc chamber, wherein:

the source gas contains a halide; and

the narrow structure inhibits degradation of thermal insulation properties of the thermal break caused by deposition of a conductive substance made from the halide.

15. The ion generator of claim 8 , further comprising a gas introducing port that introduces a source gas used to generate plasma into the arc chamber, wherein:

the source gas contains oxygen atom (O); and

the narrow structure inhibits damage to the thermal break caused by a substance made from oxygen.

16. A thermal electron emitter for generating plasma in an arc chamber, comprising:

a cathode that extends outward from the inside of the arc chamber in an axial direction and that emits a thermal electron into the arc chamber;

a thermal reflector with a cylindrical shape provided around the cathode in a radial direction and extending in the axial direction; and

a narrow structure configured to narrow a width in the radial direction of a gap between the cathode and the thermal reflector at a predetermined position in the axial direction.