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RICHARD GERMAIN / SCIENCE PHOTO LIBRARY RICHARD GERMAIN / SCIENCE PHOTO LIBRARY
Plasma is the state of matter obtained after a gas has been ionized. The external boundary is a glass sphere containing a low pressure mixture of inert gases. The small central sphere is a high voltage, high frequency electrode that excites the electrons in the gas. When the electrons go back to a lower energy state, they release their excess energy by emitting a photon. This is therefore light emission by fluorescence. The colour of the light produced by the streamers depends on the energy of the emitted photon and therefore, on the nature of the gas in the sphere. Certain variations in gas temperature generate warmer regions. The warm gas gets ionised more easily, it has better conductivity and so produces the light streamers. The presence of ions in these streamers maintains the current flow which allows the relative persistence of the streamers.
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