DK2041_08 - 8 FUNDAMENTAL ASPECTS OF GASEOUS BREAKDOWN-I E...

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8 FUNDAMENTAL ASPECTS OF GASEOUS BREAKDOWN-I E lectrical breakdown in gases has been studied extensively for over one hundred years and the delineation of the various manifestations of discharges has advanced in parallel with a better understanding of the fundamental processes. This vast research area is covered by several excellent books 1 ' 2 . Fig. 8.1 shows different types of discharge one encounters in practice depending upon the combinations of parameters. The type of discharge is determined by primary factors such as gas pressure, gas density, electrode shape and distance, polarity of the voltage, type of voltage meaning dc, normal frequency ac, high frequency ac, impulse voltage, etc. Secondary factors are the electrode materials, type and distance to the enclosure, duration of the application of voltage, previous history of the electrodes, etc. Obviously it is not intended to explain all of these phenomena even in a condensed fashion but the fundamental processes that occur are similar, though the intensity of each process and its contribution to the overall process varies over a wide range. In the interest of clarity we limit ourselves to fundamental processes concentrating on the progress that has been achieved during the past twenty five years. However, to provide continuity we recapitulate some fundamental definitions and equations that are relevant to all discharge processes. 8.1 COLLISION PHENOMENA 8.1.1 ELASTIC An electron acquires energy in an electric field and during its acceleration elastic collision occurs between an electron and a molecule. During an elastic collision there is very little exchange of energy with the electron, losing an energy that is proportional to m/M where m and M are masses of the electron and molecule, respectively. The internal
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384 energy levels of the molecule do not change during an elastic collision. The main consequence of an elastic collision is that the direction of travel of an electron changes depending upon the angle at which the electron strikes the molecule. A more accurate term for elastic collisions is the momentum transfer collision which is an average value that takes into account the angle of approach of the electron. DISCHARGES 50 or 60 HZ Impulse High frequency superposed Fig. 8.1 Various manifestations of electrical discharges. The range of electron energy and the electron density encountered in a wide range of plasmas are shown in fig. 8.2 3 . The parameters chosen to characterize the plasmas are the electron temperature expressed in units of electronvolts and the electron density. One electronvolt (eV) is equal to 11600 degree Kelvin in accordance with T = (e/k) s where s = in eV e = electronic charge k = Boltzmann constant at 300 K is correspondingly 0.026 eV. The electron energy shown in Fig.
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This note was uploaded on 03/03/2010 for the course POWER 332 taught by Professor Dr during the Spring '10 term at Ain Shams University.

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DK2041_08 - 8 FUNDAMENTAL ASPECTS OF GASEOUS BREAKDOWN-I E...

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