Electric Discharge in GasesA discharge in gas means flow of current through the gaseous medium, when affected by an electrical field. This is accompanied by changes in the condition of such gas (ionization, redistribution of carriers, etc.). Under standard conditions most gases, as well as the air are insulators, because they contain very few ions. That's why, if the applied voltage is not too high, gases do not conduct electrical current. So, in order for a gas to become conductive, we need to increase the number of uncombined charge carriers - ions in it. This can be achieved through one of the two ways: either such charged particles are generated through exposure to some external factor and brought in from the outside, or they could be generated inside the gas under the influence of the electric field that exists between electrodes. The first type of discharge is referred to as non-self sustained, and the second one - as self-sustained. There are two ways to generate a non-sustained discharge - high temperature and various kinds of radiation, such as UV-radiation, gamma-ray quantum, etc. In order to detach an electron from the atom (ionize the atom), a certain energy needs to be applied. It is called ionization energy and depends on the structure of an atom. That's why such energy would be different for various substances. At the same time a reverse process of ions and electrons recombining takes place. This is accompanied by generation of energy, which is partially released as light. This so-called recombination glow is one of the reasons why many of the gas discharge forms produce light. When the ionizer stops working the recombination of ions causes termination of current flow in the gas, which is why such discharge is called non-sustained. And now let's look at how a self-sustained discharge can be generated. According to the current-voltage curve of a gas discharge, when U is low the graph is a straight line, so Ohm's law still approximately applies; but with the increase in U the line becomes curved, and at a certain voltage turns into a vertical line. This means that starting from a certain value of voltage the value of current remains unchanged despite the increasing voltage. This constant value that does not depend upon the value of voltage is called the saturation current. As we see, at the very beginning an increase in voltage leads to an increase in the number of ions passing through the discharge section, i.e. current I increases, and the ions in a stronger field move at a higher speed. However the number of ions passing through the section per time unit can not exceed the number of ions generated per such time unit by the outer ionizing factors within the discharge. But, if we continue to increase the voltage applied to gas after the saturation current value has been reached, the voltage-current curve at some point abruptly changes its direction. When the voltage is high enough, the current experiences a rapid increase as well. Such increase shows that the number of ions has increased dramatically. This is explained by the electrical field in itself: it imparts such high speeds, meaning energy, to electrons, that when the ions collide with the neutral molecules the latter break down into ions. The total number of ions is now defined not by the ionizing factor, but rather by the electric field itself, which can sustain the required ionization: the non-self-sustained conductivity becomes self-sustained. The described notion of sudden generation of self-sustained conductivity that carries the nature of gas gap breakdown is not the only form of self-sustained discharge generation. But it is rather important. Depending on which processes of ion generation prevail there exist spark, arc, glow, corona and other self-sustained discharges.
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Cathode Ray Tubeby TutorVista102,393 views
"uchen horosho"- nice video-data is given in a proper sequence and all your words pertain to the title. I wish your diagrams were in my language.
captkirkconnell 1 year ago