(Copied from www.plasmas.org)
9. "How does a plasma produce light?"
First I should define what a plasma is: it is a collection of free-moving electrons and ions - atoms that have lost electrons. Examples of plasmas include, fluorescent tubes, auroras, the Sun's photosphere; other examples are shown in the figure below. In fact, plasma is by far the most common form of matter, for plasma in the stars and in the tenuous space between them makes up over 99% of the visible universe and perhaps most of that which is not visible.
Plasma temperatures and densities range from relatively cool and tenuous (like aurora) to very hot and dense (like the central core of a star). Ordinary solids, liquids, and gases are both electrically neutral and too cool or dense to be in a plasma state. We are mostly aware of "ordinary" matter; the different states of matter generally found on Earth are solid, liquid, and gas. However, Sir William Crookes, an English physicist, identified a fourth state of matter, now called plasma, in 1879, while he experimented with gas-discharge tubes, the fore-runner of today's fluorescent tubes. The word plasma itself was first applied to ionized gas by Dr. Irving Langmuir, an American chemist and physicist, in 1929.
Energy is needed to strip electrons from atoms to make plasma. The energy can be of various origins: thermal (from nuclear reactions, say), electrical (through the large electric fields in gas discharge tubes), light (by ultraviolet light or intense visible light from a laser). However, without sufficient sustaining power, plasmas recombine into neutral gas. During this recombination process, the electrons become "re-attached" to the ions and can fall from higher to lower energy states emitting photons, i.e., producing light. These photons can be in the visible range, as in a neon-tube and auroras, and in the uv region as in mercury/argon fluorescent tubes in common use today. Obviously, in order to maintain a plasma, energy has to be supplied to re-ionize the atoms.
(As an aside, in the fluorescent tubes we use in our homes and offices, the uv light is absorbed by a phosphor that is coated on the inside of the tube, which, in turn, luminesces in the visible region ... that is the "light" we see. Phosphors are relatively efficient at changing uv light into visible light, a process known as fluorescence. Various mixtures of phosphors are in use today to produce different types of white light - for example, "cool" white, "warm" white, daylight, etc. - and other colors as well.)
Some plasmas are so diffuse that collisions between particles are rare; these "collisionless" plasmas are sometimes far better conductors than metals and they can sustain large currents using very modest electric fields. Because they are comprised of charged particles, plasmas can be accelerated and steered by electric and magnetic fields, which allows them to be controlled. An example of the latter is the "magnetic" bottle that is currently under development for controlled nuclear fusion. The aim is to use magnetic fields to confine a plasma produced from hydrogen and then steadily raise its temperature to 100's of millions of degree until the hydrogen nuclei undergo fusion, releasing large amounts of energy. The best that has been achieved so far is several megawatts of power for somewhat less than a second. Clearly, for commercial use much higher power levels have to be achieved continuously.
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