The purpose of injecting cryogenic pellets into the plasma of a tokamak is to increase its density (large ice cubes in the core) or to act on certain dissipative modes that dangerously heat the walls of the machine (small cryogenic pellets at the periphery). The pellets are propelled by a high-pressure gas, which must of course not penetrate the plasma, and guided in a pipe over 10 to 20 m.