James, so many things come to mind. These are solids and air and placed in a variety of circumstances. But at bottom are details of the field strength in the air around it, I think. That doesn't help me suggest a reason for that specific shape. What did pop to mind is light -- particularly UV light from the sun. 295nm and 325nm for Argon may be the worst worry. But I'm just offering a random thought. I'm sure the design takes into account many factors I'm simply ignorant of. Were it me, I'd find a plasma physics professor and just write to them. They do respond (to me in the past, anyway.)
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