You could try putting samples parallel to the plasma flow between two cathode rods near the top of the cathode rods. You could mount 4 samples and try the three proposed insulators and alumina. It might not be the fairest test but it would be apples-to-apples. If you test it with the ion beam or electron beam you need to use samples at least 0.25″ thick. I put ceramics in the base of the anode for some experiments and thin sheets will crack easily under the stress of either beam. Alumina fairs very well in the tests I’ve completed.
