A gas distribution assembly for the ceiling of a plasma reactor includes a center fed hub and an equal path length distribution gas manifold underlying the center fed hub. The rail portion extends upwardly on a first side of the mattress to extend beyond the sleeping surface of the mattress and prevent a person from rolling out of bed. At least one of object-side and image-side surfaces of at least one of the second to fourth optical lens elements is aspheric. The marginal edge of the protrusion 81 merges into a sloping face, on which an auxiliary electrode 84B is formed.