Plasma
A gas in which electrons have been stripped from their atoms, so that electrons and nuclei move around separately. Often called the fourth state of matter, after solid, liquid and gas.
Heat a gas hard enough and its atoms collide so violently that electrons are knocked loose. Negative electrons and positive nuclei (ions) then move independently of one another. That is a plasma. Lightning, the inside of a fluorescent tube and the Sun itself are all plasmas.
Fusion fuel has to be heated beyond 100 million °C, and at that temperature it is necessarily a plasma. Because a plasma is made of charged particles, a magnetic field can bend their paths. That is the starting point of magnetic confinement: holding the plasma away from the walls.
The name was introduced by Irving Langmuir in a 1928 paper on the behaviour of ionised gas in discharge tubes[1].
Plasmas also tend to tear themselves apart through their own currents and pressure — so-called instabilities. Much of the history of fusion research is the accumulated effort to keep that behaviour in check.
Related terms
- Magnetic confinementHolding a hot plasma away from the vessel walls in a cage of magnetic field, using the fact that charged particles spiral along magnetic field lines.
- TokamakA doughnut-shaped machine that confines plasma by combining the magnetic field of external coils with the field produced by a current driven through the plasma itself. The dominant approach to magnetic confinement.
- Lawson criterionThe condition that a plasma's density, confinement time and temperature must meet for a fusion reactor to produce net energy, derived by John Lawson in Britain in 1955.