Directly heated high temperature cathodes of refractory metals such as tungsten run electric current of more than several tens of amperes. The electric current makes magnetic field around the cathode wire, and the magnetic field causes inhomogeneous emission of electrons from the cathode. To solve this problem we have designed the cathode having a co‐axial heater current flow structure, and mounted it in a Bernas‐type ion source. A plasma produced by co‐axial hot cathode showed a clearer column along the external magnetic field and less displacement in the direction perpendicular to the field than that produced by a hair‐pin filament. Stable discharge current as high as 5000 mA was obtained for Ar and BF3 gases with the co‐axial cathode. Boron and phosphorus ion beams were extracted from the source on an actual ion implanter. The ion beam currents were 1.5 times as large as those obtained with a hair‐pin filament.

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