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Concept
Glow discharge
Summary
A glow discharge is a plasma formed by the passage of electric current through a gas. It is often created by applying a voltage between two electrodes in a glass tube containing a low-pressure gas. When the voltage exceeds a value called the striking voltage, the gas ionization becomes self-sustaining, and the tube glows with a colored light. The color depends on the gas used.
Glow discharges are used as a source of light in devices such as neon lights, cold cathode fluorescent lamps and plasma-screen televisions. Analyzing the light produced with spectroscopy can reveal information about the atomic interactions in the gas, so glow discharges are used in plasma physics and analytical chemistry. They are also used in the surface treatment technique called sputtering.
Conduction in a gas requires charge carriers, which can be either electrons or ions. Charge carriers come from ionizing some of the gas molecules. In terms of current flow, glow discharge falls between dark discharge and arc discharge.
In a dark discharge, the gas is ionized (the carriers are generated) by a radiation source such as ultraviolet light or Cosmic rays. At higher voltages across the anode and cathode, the freed carriers can gain enough energy so that additional carriers are freed during collisions; the process is a Townsend avalanche or multiplication.
In a glow discharge, the carrier generation process reaches a point where the average electron leaving the cathode allows another electron to leave the cathode. For example, the average electron may cause dozens of ionizing collisions via the Townsend avalanche; the resulting positive ions head toward the cathode, and a fraction of those that cause collisions with the cathode will dislodge an electron by secondary emission.
In an arc discharge, electrons leave the cathode by thermionic emission and field emission, and the gas is ionized by thermal means.
Below the breakdown voltage there is little to no glow and the electric field is uniform. When the electric field increases enough to cause ionization, the Townsend discharge starts.
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