Everyone knows that the game of billiards involves balls careening off the sides of a pool table—but few people may know that the same principle applies to fusion reactions. How charged particles like electrons and atomic nuclei that make up plasma interact with the walls of doughnut-shaped devices known as tokamaks helps determine how efficiently fusion reactions occur. Specifically, in a phenomenon known as secondary electron emission (SEE), electrons strike the surface of the wall, causing other electrons to be emitted. Those secondary electrons cool the plasma’s edge and dampen the plasma’s overall performance.