Editing Stellarator (section)

=== Requirements for fusion ===
Heating a gas increases the energy of the particles within it, so by heating a gas into hundreds of millions of degrees, the majority of the particles within it reach the energy required to fuse. 

According to the [[Maxwell–Boltzmann distribution]], some of the particles will reach the required energies at much lower average temperatures. Because the energy released by the fusion reaction is much greater than what it takes to start it, even a small number of reactions can heat surrounding fuel until it fuses as well. In 1944, [[Enrico Fermi]] calculated the D–T reaction would be self-sustaining at about {{val|50000000|u=K}}.{{sfn|Asimov|1972|p=123}}

Materials heated beyond a few tens of thousand degrees ionize into their [[electron]]s and [[atomic nucleus|nuclei]], producing a gas-like [[state of matter]] known as [[plasma (physics)|plasma]]. According to the [[ideal gas law]], like any hot gas, plasma has an internal [[pressure]] and thus wants to expand.{{sfn|Bishop|1958|p=7}} For a fusion reactor, the challenge is to keep the plasma contained. In a magnetic field, the electrons and nuclei orbit around the magnetic field lines, confining them to the area defined by the field.{{sfn|Thomson|1958|p=12}}{{sfn|Bishop|1958|p=17}}