Flashover occurs when simulating conditions during nuclear explosions.
Adam Rifkin stashed this in Science!
Imgur comments: http://imgur.com/gallery/pJu5ljm
It is. Description from the Reddit comments:
They're essentially pumping a large amount of current through wire arrays and other targets, which are then compressed into plasma by the resulting force. This (very short-lived) plasma can be studied for a whole bunch of things in science and defense research. It's called a z-pinch generator if you'd like to look up more about them.
Pictures of z-pinch experiments are taken using a long-exposure, so that you can catch the arcing action that goes on in the equipment. The picture shown here is from Sandia National Laboratories.