Some embodiments include a structure, comprising: an insulator forming at least a part of a wall of a vacuum chamber, the insulator having a first end and a second end wider than the first end; a first conductive structure disposed at the first end of the insulator; and a second conductive structure disposed at the second end of the insulator, contacting the insulator, and including at least a portion surrounded by the insulator; wherein: a portion of an outer surface of the insulator extends radially outward from a triple junction between the insulator, the second conductive structure, and a medium contacting the outer surface of the insulator.