Plasma recombination is a process by which positive ions of a plasma capture a free (energetic)

electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...

and combine with electrons or negative ions to form new neutral

atom Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...

s ( gas). The process of recombination can be described as the reverse of ionization, whereby conditions allow the plasma to evert to a gas. Recombination is an exothermic process, meaning that the plasma releases some of its

internal energy The internal energy of a thermodynamic system is the energy of the system as a state function, measured as the quantity of energy necessary to bring the system from its standard internal state to its present internal state of interest, accoun ...

, usually in the form of

heat In thermodynamics, heat is energy in transfer between a thermodynamic system and its surroundings by such mechanisms as thermal conduction, electromagnetic radiation, and friction, which are microscopic in nature, involving sub-atomic, ato ...

. Except for plasma composed of pure

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

(or its

isotopes Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), but ...

), there may also be multiply charged ions. Therefore, a single electron capture results in decrease of the ion charge, but not necessarily in a neutral atom or molecule. Recombination usually takes place in the whole volume of a plasma (volume recombination), although in some cases it is confined to some region of the volume. Each kind of reaction is called a recombining mode and their individual rates are strongly affected by the properties of the plasma such as its energy (heat), density of each species, pressure and temperature of the surrounding environment.

Examples

An everyday example of rapid plasma recombination occurs when a fluorescent lamp is switched off. The low-density plasma in the lamp (which generates the light by bombardment of the fluorescent coating on the inside of the glass wall) recombines in a fraction of a second after the plasma-generating electric field is removed by switching off the electric power source. Hydrogen recombination modes are of vital importance in the development of

divertor In magnetic confinement fusion, a divertor is a magnetic field configuration which diverts the heat and particles escaped from the magnetically confined plasma to dedicated plasma-facing components, thus spatially separating the region plasma ...

regions for

tokamak A tokamak (; ) is a device which uses a powerful magnetic field generated by external magnets to confine plasma (physics), plasma in the shape of an axially symmetrical torus. The tokamak is one of several types of magnetic confinement fusi ...

reactors. In fact they will provide a good way for extracting the energy produced in the core of the plasma. At the present time, it is believed that the most likely plasma losses observed in the recombining region are due to two different modes: electron ion recombination (EIR) and molecular activated recombination (MAR).

Table

References

Recombination, plasma Phase transitions {{plasma-stub