Download PDF by A. M. Howatson (Auth.): An Introduction to Gas Discharges

By A. M. Howatson (Auth.)

ISBN-10: 0080205747

ISBN-13: 9780080205748

ISBN-10: 0080205755

ISBN-13: 9780080205755

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Because of their much greater mass, the acceleration of ions and the resulting drift velocity are much smaller than for electrons. The average gain in energy between collisions is readily shown to depend on the ratio E/p, where p is the gas pressure, and to vary inversely as m. So long as it is smaller than the thermal energy, the drift velocity is found to be proportional to E; for ions, linearity holds up to Ejp of order 10 V/cm torr, but for electrons it fails at much lower values. 18) μ = —~m — — - .

For most metals, the threshold lies in the ultraviolet, but prepared surfaces using the alkali metals, such as caesium, can have a threshold well into the infrared. As in other atomic processes, there is a certain probability that a photon with the necessary energy will release an electron; for a particular material this probability varies with frequency above the threshold, as in the response curve of any photoelectric device. It is expressed as a photoelectric yield, or efficiency, in electrons per quantum, has a maximum approaching unity and falls off towards the threshold and at high frequencies.

ELECTRODE EFFECTS The two most important functions of electrodes in a gas discharge are their influence on the initiation of the discharge and their provision of charged particles for its maintenance; the two are related, and depend upon the emission of electrons from a solid or liquid surface. Some metallic materials will emit positive ions under certain conditions, but the emission is usually low and the process rarely important in a discharge. ) There are several means by which electrons can be emitted from a surface, and all can be important in discharges.

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An Introduction to Gas Discharges by A. M. Howatson (Auth.)


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