Thermal physics and fusion

A letter to the Scotsman published over at SONE gets it a bit wrong.

the temperature required for a deuterium-tritium reaction is about 300 million degrees, not 100. This is nearly 20 times hotter than the temperature at the centre of the Sun. A device containing plasma at such a temperature has a high potential for catastrophic failure.

This is the classic mistake of confusing heat and temperature. Heat is the energy in matter that gives it its temperature. Temperature is the behaviour of the particles of the matter due to that heat. If you have bath tub full of hot water and a lit match, the flame of the match will have a higher temperature, but because there is so little of the gases, which cause the flame, the actual amount of energy is very limited. The bath tub on the other hand, may be relatively cool, but because there is so much water, there is a lot of heat.

The torus of plasma in a fusion reactor actually has a very small mass, less than a gram in the case of ITER. So even though it is at a phenomenally high temperature, the total energy is not as much as you might expect. If confinement fails, the torus rapidly expands and cools in the process. That's adiabatic expansion at work.

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