The key purpose of a fusion reactor is to create the environment necessary for a Burning Plasma and maintain that state for long enough to extract energy in the form of heat. The consequences of this extreme environment are The Punishing Conditions Inside a Fusion Reactor.
First, the environment must be extremely hot (~10M Degrees Celsius). Heat is a measure of particle energy, if the particles are moving faster then they will collide with greater force.
Next, the environment must create sufficient pressure to make the fuel extremely dense. The closer the particles are together the more likely they are to collide.
Finally, the environment must be well confined so that heat isn't escaping needlessly. The faster heat escapes the more energy inputs needed to maintain a burning plasma.
This environment is measured by the Fusion Triple Product. There are various Fusion Reactor Designs which are proposed to create these conditions most efficiently.
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