Runaway and suprathermal particles

Under certain conditions an electron or ion is accelerated continuously when subject to a force greater than the dynamic friction; as a result it is decoupled from its thermal state of motion and is said to be `runaway'. Runaway particles lead to, and form part of, a class of particles which have been referred to elsewhere as `suprathermal'. The study of the way in which runaway particles are produced and accelerated is not only of importance in laboratory gas discharges, but also in a wide variety of applications in astrophysics. The subject of runaway electrons and ions is treated, first in a uniform ionized gas in which the particles have Maxwellian velocity distributions and, secondly, in a current conducting plasma. The conditions for decoupling and accelerating particles from their thermal state are discussed and, in particular, it is shown that in the case of a weak electric field ions are unlikely to achieve speeds very much in excess of the drift speed of the electrons, except in restricted regions where runaway electrons are continually forming and escaping. When the ratio of the thermal and magnetic energy densities, given approximately by β, is small, the runaway electron current grows rapidly at the expense of the conducting current. A number of ways are considered in which the runaway current may grow and become predominant. Finally, the generation of suprathermal ions by Fermi interactions is considered, from which it is evident that when β is the order of unity the ion gas is heated and when β is small relatively few ions are accelerated to high energies.