There's also the fact that the plasma has extremely low density, so there is very little fuel in the core at a time: "Less than 1g of fusion fuel is present in

There's also the fact that the plasma has extremely low density, so there is very little fuel in the core at a time: "Less than 1g of fusion fuel is present in the Vacuum Vessel at any one moment." http://www.iter.org/mach/fuelcycle http://www.iter.org/mach/fuelcycle