Embodiments herein are directed to a resonator for an ion implanter. In some embodiments, a resonator may include a housing, and a first coil and a second coil partially disposed within the housing. Each of the first and second coils may include a first end including an opening for receiving an ion beam, and a central section extending helically about a central axis, wherein the central axis is parallel to a beamline of the ion beam, and wherein an inner side of the central section has a flattened surface.