An ablation apparatus places electrodes at the perimeter of a cavity. In an embodiment, the alternating electric field is used to expose the cavity to enough energy to ablate the cavity. In an embodiment, two modes are used to expose different regions of the cavity to different amounts of power for so that the thermal effect is more uniform. In an embodiment, the electrodes have a relatively large surface area so as to avoid charring the cavity, but are shaped so as to fit within a body orifice. For example, the diameter of the sheathed housing the electrodes during penetration may be only 5.5 mm.