Plasma

The fourth state of matter — an ionised gas of free electrons and ions that responds to magnetic fields.

A plasma is a gas heated until its atoms ionise, separating into free electrons and positively charged ions. Because its charges respond collectively to electric and magnetic fields, a plasma can be shaped, confined and heated magnetically — the basis of every magnetic-confinement fusion machine.

Because its charges respond collectively to fields, a plasma can be shaped, heated and confined magnetically — the entire basis of the tokamak and the mirror. It also supports a rich zoo of waves and instabilities, which is why so much of fusion engineering is really the engineering of plasma behaviour.