How thin do the electron emitters in the onion have to be?
I can imagine a heavy metal layer deposited or plated super thin on a substrate.
Will the electrons go only in the direction of the x-rays, or will both sides of the emitter film need to be exposed?
A substrate below a fragile super thin film could be etched away in a pattern leaving a mostly open but thick mesh for strength.
I don’t think the electron collectors have to be that thin also? Instead of a film the collectors could be a layer of carbon tubes with coolant in them?
Would a metal like gold or tungsten be used in all the layers or could they vaporize in the inner most intense layers of the onion, instead requiring medium z metals that would not absorb as many x rays per layer?