An ion source has a vacuum envelope structure having a cylindrical portion with a lengthwise axis and an inside diameter defining an interior volume, joined at one end to a flange concentric with the axis, the cylindrical portion open by an exit aperture through the flange and open at an end opposite the flange, an RF feedthrough closing the open end of the cylindrical portion opposite the flange, creating a cylindrical interior volume open only through the exit aperture, and a magnet system carrier structure surrounding the cylindrical portion of the vacuum envelope and carrying at least one annular permanent magnet concentric with the lengthwise axis, providing a magnetic field penetrating the interior volume. The ion source is characterized in that the magnet system carrier structure is translatable along the lengthwise axis enabling variable positioning of the magnetic field in the interior volume along the lengthwise axis.