In the case of plasma shock behind the insulator, it is more like taking a hammer to the insulator than a heat stress. The impact happens on a time scale much faster than the time to transfer heat into the material. We saw evidence of plasma chemistry on the inside surface of the insulator. The alumina discolors slightly from pristine white to a stained yellow. It remains a hypothesis but the data at the 0.25 MA scale seems to support it.