X-ray tube

1. An X-ray tube comprising:

an anode target configured to radiate X-rays by incidence of an electron beam;

a cathode comprising an elongated electron emission source configured to emit electrons, and a converging electrode including a trench portion accommodating the electron emission source, and configured to converge the electron beam towards the anode target through an opening of the trench portion as the electrons are emitted from the electron emission source, and

a vacuum envelope accommodating the anode target and the cathode,

wherein the trench portion comprises:

a closest inner circumferential wall extending linearly in a depth direction of the trench portion, having dimension shorter than dimension of the electron emission source in the depth direction of the trench portion, and facing the electron emission source with a narrowest gap between the closest inner circumferential wall and the electron emission source over an entire circumference of the electron emission source in width direction of the electron emission source,

an upper inner circumferential wall located on the opening side of the trench portion with respect to the closest inner circumferential wall and having a shape widening in the width direction further from the closest inner circumferential wall,

a lower inner circumferential wall located on an opposite side to the upper inner circumferential wall with respect to the closest inner circumferential wall and having a shape widening in the width direction further from the closest inner circumferential wall,

the electron emission source projects towards the opening of the trench portion from a boundary between the closest inner circumferential wall and the upper inner circumferential wall,

within an area through the electron emission source in the width direction at an imaginary cross section, a first distance between the lower inner circumferential wall and an imaginary line facing each other in the width direction is longer than a second distance between the closest inner circumferential wall and the imaginary line facing each other in the width direction, and

the imaginary line extends through an end of the electron emission source in the width direction, and extends in the depth direction.

2. The X-ray tube of claim 1 , wherein the electron emission source is formed of a material of tungsten as a main component.

3. The X-ray tube of claim 1 , wherein the trench portion further comprises at least one of:

one or more other upper inner circumferential walls located on the opening side of the trench portion than the closest inner circumferential wall and having a shape widening in the width direction from the closest inner circumferential wall; and

one or more other lower inner circumferential walls located on an opposite side to the upper inner circumferential walls with respect to the closest inner circumferential wall and having a shape widening in the width direction from the closest inner circumferential wall.

4. The X-ray tube of claim 1 , wherein the gap between the electron emission source and the closest inner circumferential wall is 0.2 mm or more.

5. The X-ray tube of claim 1 , wherein the upper inner circumferential wall has a curved surface shape.

6. The X-ray tube of claim 1 , wherein a cross section of the lower inner circumferential wall in the width direction has a shape of a circle, an ovally rounded rectangle or a portion thereof.

7. The X-ray tube of claim 1 , wherein within the area, the first distance increases with increasing distance from the opening.