|
Electric discharge in gases occurs when electric current flows through a gaseous medium due to ionisation of the gas. Depending on several factors, the discharge may radiate visible light. The properties of electric discharges in gases are studied in connection with design of lighting sources and in the design of high voltage electrical equipment. ==Discharge types== In cold cathode tubes, the electric discharge in gas has three regions, with distinct current-voltage characteristics:〔(Reference Data for Engineers: Radio, Electronics, Computers and Communications ) By Wendy Middleton, Mac E. Van Valkenburg, p. 16-42, Newnes, 2002 ISBN 0-7506-7291-9〕 *I: Townsend discharge, below the breakdown voltage. At low voltages, the only current is that due to the generation of charge carriers in the gas by cosmic rays or other sources of ionizing radiation. As the applied voltage is increased, the free electrons carrying the current gain enough energy to cause further ionization, causing an electron avalanche. In this regime, the current increases from femtoamperes to microamperes, i.e. by nine orders of magnitude, for very little further increase in voltage. The voltage-current characteristics begins tapering off near the breakdown voltage and the glow becomes visible. *II: glow discharge, which occurs once the breakdown voltage is reached. The voltage across the electrodes suddenly drops and the current increases to milliampere range. At lower currents, the voltage across the tube is almost current-independent; this is used in glow discharge voltage stabilizers. At lower currents, the area of the electrodes covered by the glow discharge is proportional to the current. At higher currents the normal glow turns into abnormal glow, the voltage across the tube gradually increases, and the glow discharge covers more and more of the surface of the electrodes. Low-power switching (glow-discharge thyratrons), voltage stabilization, and lighting applications (e.g. Nixie tubes, decatrons, neon lamps) operate in this region. *III: arc discharge, which occurs in the ampere range of the current; the voltage across the tube drops with increasing current. High-current switching tubes, e.g. triggered spark gap, ignitron, thyratron and krytron (and its vacuum tube derivate, sprytron, using vacuum arc), high-power mercury-arc valves and high-power light sources, e.g. mercury-vapor lamps and metal halide lamps, operate in this range. Glow discharge is facilitated by electrons striking the gas atoms and ionizing them. For formation of glow discharge, the mean free path of the electrons has to be reasonably long but shorter than the distance between the electrodes; glow discharges therefore do not readily occur at both too low and too high gas pressures. The breakdown voltage for the glow discharge depends nonlinearly on the product of gas pressure and electrode distance according to Paschen's law. For a certain pressure × distance value, there is a lowest breakdown voltage. The increase of strike voltage for shorter electrode distances is related to too long mean free path of the electrons in comparison with the electrode distance. A small amount of a radioactive element may be added into the tube, either as a separate piece of material (e.g. nickel-63 in krytrons) or as addition to the alloy of the electrodes (e.g. thorium), to preionize the gas and increase the reliability of electrical breakdown and glow or arc discharge ignition. A gaseous radioactive isotope, e.g. krypton-85, can also be used. Ignition electrodes and keepalive discharge electrodes can also be employed.〔(Handbook of optoelectronics, Volume 1 ) by John Dakin, Robert G. W. Brown, p. 52, CRC Press, 2006 ISBN 0-7503-0646-7〕 The E/N ratio between the electric field E and the concentration of neutral particles N is often used, because the mean energy of electrons (and therefore many other properties of discharge) is a function of E/N. Increasing the electric intensity E by some factor q has the same consequences as lowering gas density N by factor q. Its SI unit is V·cm2, but the Townsend unit (Td) is frequently used. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Electric discharge in gases」の詳細全文を読む スポンサード リンク
|