Abstract: An outer tubular electrode is adapted to be connected to a first terminal of a high voltage AC source. A dielectric tube is positioned within the outer tubular electrode and is spaced therefrom to form a discharge gap. The dielectric tube has a closed first end and an open second end. An electrically conductive layer, for example of metal, covers the major portion of the inner surface of the dielectric tube. An electrically conductive cap, for example of metal, includes a partial spherical portion, a conical portion and a cylindrical portion. The cylindrical portion is in contact with the electrically conductive layer adjacent the open second end of the dielectric tube. The partial spherical portion of the cap has a concave surface which faces the interior of the dielectric tube. The partial spherical portion is connected to a second terminal of the high voltage AC source, such that the cap and the layer form an inner electrode.