Patent ID: 7365339

Claim:
An ion source having a structure where a cathode is heated by a filament and thermal electrons are emitted from the cathode into a plasma generating chamber also serving as an anode, the ion source comprising: an opening for a cathode provided in the wall surface of said plasma generating chamber; a tubular cathode holder for holding the cathode, the tip of which is inserted into the opening for the cathode from outside said plasma generating chamber so as to leave a gap between the tip of the cathode holder and the plasma generating chamber, the tip of the cathode holder positioned on the same plane with an inner wall surface around the opening for the cathode of the plasma generating chamber or further outward from the plasma generating chamber; the cathode held in the cathode holder, a front surface of the cathode positioned on the same plane with the inner wall surface around the opening for the cathode of the plasma generating chamber or further outward from the plasma generating chamber; a tubular first heat shield arranged to enclose a side surface of the cathode by at least one layer with a gap provided between the first heat shield and the side surface of the cathode, the tip of the first heat shield positioned on the same plane with the inner wall surface around the opening for the cathode of the plasma generating chamber or further outward from the plasma generating chamber; a filament provided in the cathode holder for heating the cathode from a rear surface of the cathode; and an electrical insulating material provided in the opening for the cathode, the electrical insulating material filling the gap between the cathode holder and the plasma generating chamber.