Supercritical Helium

Helium will remain liquid in a bath at 1 atmosphere pressure provided the temperature does not rise above 4.2K. If the ITER coils are placed in such a coolant bath and a high pulse of heat ensues in their operation, most of the helium must be vented to avoid large overpressures. To avoid this, the coils of ITER operate with pumped supercritical helium, just above the critical temperature, which retains a large measure of the heat transfer properties of liquid helium without the risk of overpressure.