It's a planar magnetron.
A small stack of neodymium ring magnets are placed around a top-hat-shaped steel core, which also has an O-ring groove and the two cooling water tubes (1/8 inch refrigeration tubing). A copper pipe cap (w/ thinned end and soldered on tapped ears) is slipped over and screwed down, compressing the O-ring. A cathode plate is then bolted to the top. Copper is currently used, but I intend to try silver since it's cheap.
Cooling water is supplied by a large 30ml syringe and a yogurt tub full of water; the low power of this system means that very little water is actually needed but it can't be allowed to go dry.
Gas is air, as you could probably tell. I intend to use argon, but don't have a cylinder or regulator yet.