A processing system is disclosed, having a power transmission element with an interior cavity that propagates electromagnetic energy proximate to a continuous slit in the interior cavity. The continuous slit forms an opening between the interior cavity and a substrate processing chamber. The electromagnetic energy may generate an alternating charge in the continuous slit that enables the generation of an electric field that may propagate into the processing chamber. The electromagnetic energy may be conditioned prior to entering the interior cavity to improve uniformity or stability of the electric field. The conditioning may include, but is not limited to, phase angle, field angle, and number of feeds into the interior cavity.