<p>A hollow cathode ion source comprising a cylindrical cathode (1) through one end of which at least a discharge maintaining gas or said discharge maintaining gas and a metal vapor is or are introduced, and an anode (2) provided on the other end of said cylindrical cathode (1) and having an ion extraction opening (2a), said gas or said gas and metal vapor being ionized by a discharge between said cylindrical cathode (1) and said anode (2) to produce ions which are extracted through said ion extraction opening (2a) in the axial direction of said cylindrical cathodes (1). The cylindrical cathode in the ion source has a large diameter and may be directly cooled.</p>
