An apparatus includes an electromagnetic waveguide; an iris structure providing an iris in the waveguide. The iris structure may define an iris hole, a first iris slot at a first side of the iris hole, and a second iris slot at a second side of the iris hole. A plasma torch is disposed within the iris hole. An electric field in the waveguide changes direction from the first iris slot to the second iris slot. The plasma torch generates a plasma which is substantially symmetrical around a longitudinal axis of the plasma torch, such that the plasma may have a substantially toroidal shape. In some embodiments, a dielectric material is disposed in the iris hole, outside of the plasma torch. In some embodiments, the height of at least one of the iris slots is greater at the ends thereof than in the middle.