Plasma is considered to be the fourth state of matter, consisting
of a mixture of electrons, ions and neutral particles, although overall
it is electrically neutral. The degree of ionisation of a plasma is the
proportion of atoms that have lost (or gained) electrons and, in the
case of thermal plasmas of interest for this review, this is controlled
mostly by temperature. Plasma technology involves the creation of
a sustained electrical arc by the passage of electric current through
a gas in a process referred to as electrical breakdown. Because of
the electrical resistivity across the system, significant heat is generated, which strips away electrons from the gas molecules resulting
in an ionised gas stream, or plasma. At 2000 ◦C gas molecules dissociate into the atomic state and when the temperature is raised to
3000 ◦C, gas molecules lose electrons and become ionised. In this
state, gas has a liquid-like viscosity at atmospheric pressure and the
free electric charges confer relatively high electrical conductivities
that can approach those of metals