A metal conductor in contact with an insulator in air or vacuum forms a dielectric "triple point" where the conductor, insulator, and air (or vacuum) meet. Field lines from high voltage on the conductor are pushed out of the higher dielectric insulator to form a concentrated electrical field in the insulator-conductor gap, and it is here where breakdowns are usually initiated to cause high voltage flashover on the insulator surface. These effects have been extensively studied, particularly with high voltage transmission line insulators, with an aim toward mitigating the high voltage stress. Toroidal "guard rings" are a common method used to reduce the occurrence of flashover breakdown on high voltage insulators.
I needed some small toroidal guard rings for insulators I'm building that will go in vacuum, but the smallest I've found available are 2.5 inch diameter spun aluminum (and cost $50 each!). I devised a method to make 1 inch id stainless steel toroidal guard rings (see photo) for less than $5 each, and I thought it might be useful to others in the group. I simply TIG welded stainless steel fender washers inside stainless steel O-rings (1/4" x 1" id) purchased from West Marine. I "dimpled" the edge of the washers with a center punch to enable a force fit to simplify welding. West Marine has the O-rings up to ½" x 2-1/2" id. As with most boat stuff, they are expensive, but are still cheaper than spun toroids. There are cheaper O-rings available on eBay if you are not in a hurry (they ship from China).
A potential use of such guard rings in fusor work would be on the terminal of high voltage feedthroughs. The second photo below shows guard rings on a laboratory fusor that operates at over 100 kV. Use of such guard rings could enable pushing a feedthrough to use at higher voltage without flashover on the atmosphere side.
For posts specifically relating to fusor design, construction, and operation.
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- Real name: Dan Knapp