Solids, liquids and gases make up the world around us, but they account for less than 0.01% of all matter in the universe. The remaining 99.9% is made up almost entirely of plasma, the fourth state of matter. The dynamics of a plasma are similar to that of a gas, however they have some interesting additional properties.
What is a Plasma?
Adding heat to a system results in phase changes. Solids melt to form liquids and liquids evaporate to form gases. Eventually, when enough heat has been transferred, a gas will become a plasma. Electrons are ripped off atoms within the gas in a process known as ionisation, leaving behind a sea of positive ions and delocalized electrons. The kinetic energies of particles in the plasma are so large that the electron-nucleus attraction is insignificant, allowing both to flow freely in space.
Plasmas in Space
Stars are mainly composed of elements such as hydrogen and helium. Due to the very high temperatures found within stars, these elements form a plasma. Temperatures within the Sun’s outer atmosphere, known as the corona, are so high that charged particles are ejected from the surface. This stream of particles move through space as a plasma at around 400km/s and is known as the solar wind.
Conduction in Plasmas
Unlike gases, plasmas are excellent conductors of electricity. The overall charge of a gaseous atom is neutral; atoms possess an equal number of protons and electrons which are prevented from forming currents by their mutual electrostatic attraction. The ionisation of atoms within the plasma means there are lots of free positive and negative charges available to form an electric current.
It should be noted that, whilst on local scales charges in the plasma separate to form positive and negative regions, overall the charge of the plasma is neutral. This is known as quasi-neutrality.
Waves in Plasmas
Electron plasma oscillations occur when electrons are subject to a restoring force due to positive ions within the plasma. Such oscillations are often self-sustained and can continue for long periods of time. The frequency of oscillation depends only on the density of electrons in the plasma. Such collective behaviour distinguishes a plasma from a gas, where a particle’s influence on those nearby is negligible.
Plasmas on Earth
The uppermost region of the Earth’s atmosphere, known as the ionosphere receives a constant bombardment of solar radiation. This radiation is so energetic it can ionise gas atoms within the ionosphere and form a plasma. The ionosphere is composed of several layers, namely the F, E and D layers. At night, the intensity of solar radiation decreases and only the F layer remains partially ionised. During the day, however, all three layers of the ionosphere contain plasma.
The movement of electric charges within the plasma induces a magnetic field. The plasma magnetic field can vary significantly in space and time due to local behaviour. The dynamics of the field are governed by the equations of magnetohydrodynamics which involve modelling the plasma as an electrically conducting fluid. This description is the intersection of Maxwell’s equations of electromagnetism and the equations of fluid flow.
Plasmas for Fusion
Nuclear fusion is the process by which stars produce their energy and is far more efficient than methods of energy production currently used on Earth. Billions of pounds are being invested into researching and developing fusion reactors, with working reactors expected to be put to use in the next 60 years. Current efforts involve containing a plasma within a tokamak, a large toroidal vessel, using a strong magnetic field. The plasma acts as a medium to allow fusion fuel to reach exotic temperatures, collide and fuse.