Abstract: A hollow cathode ion source for in a vacuum chamber wherein it comprises a cylindrical cathode through one end of which a gaseous medium of at least a discharge maintaining gas or said discharge maintaining gas and a metal vapor is or are introduced, and an anode provided on the other end of said cylindrical cathode and having at least one ion extraction opening, said gaseous medium being ionized by a discharge means between said cylindrical cathode and said anode to produce ions which are extracted through said ion extraction opening in the axial direction of said cylindrical cathode.The cylindrical cathode in the ion source has a large diameter at least about half and preferably about equal to its axial length and may be directly cooled.