Field emission X-ray tube and method of focusing electron beam using the same

A method of focusing electron beams using a field emission X-ray tube comprising a cathode electrode provided on one end of a vacuum container, comprising a field emission emitter;

• a gate electrode provided inside the vacuum container to be adjacent to the cathode electrode, comprising a first opening;
• a focusing electrode electrically connected to the gate electrode and provided on one surface of the gate electrode to be farther from the cathode electrode than the gate electrode while comprising a second opening with a greater width than that of the first opening;
• and an anode electrode provided inside the vacuum container on another end thereof in a direction where the vacuum container is extended, wherein a height of the focusing electrode is identical to the width of the second opening, and wherein the width of the first opening is ⅓ or less of the second opening, the method comprising:
• one of controlling the height of the focusing electrode and changing the width of the first opening.