Comment by topspin

I learned recently that another limit to plasma duration is contamination. As fusion occurs and high energy particles that escape magnetic confinement blast the toroid wall, ions of metal get mixed into the plasma and degrade performance.

I've seen photos of what the inside of experimental tokamaks look like after many cycles. Metal is eroded away and deposited around the chamber in interesting patterns. Unfortunately a image search isn't surfacing the images I have in mind.