The mechanical design features and operating characteristics of a hot‐cathode arc‐discharge type ion source are described. Proton currents of 500 milliamperes have been obtained under dc test conditions. A maximum current of 3 milliamperes of 22‐Mev protons has been obtained with the source installed in the ORNL 86‐inch cyclotron. Distinguishing features of the ion source are its arc chamber of carbon, its heavy tantalum filament heated with dc, and its use of a specially shaped accelerating electrode.