an ion source for an intense ion beam from a solid source is formed with a cathode around a central anode . a source of magnetic field with closely spaced poles is formed around a central region of the cathode so that the most intense region of the magnetic field is a torus on the inside of the cathode and the field at the anode is weak . a torus of plasma can be formed near the inside surface of the cathode which can be coated with solid source material . an ion beam can be extracted through an aperture in the cathode .