PRINCIPLES OF GAS FILLED TUBES

in a vacuum diode electrons are emitted by the cathode and are attracted by the positively charged anode the number of electrons in the stream moving from cathode to anode depends on the number of electrons emitted by the cathode the potential difference between anode and cathode and the negative space charge due to the electrons in the stream since these electrons carry the same kind of charge the electrons in this part of the tube repel the electrons which are closer to the cathode thus the negative space charge limits the flow of electrons from cathode to anode to reduce this limiting effect of the negative space charge a tube may contain a small quantity of mercury vapor or of an inert gas such as argon or neon such a tube is called a gas-filled tube when the cathode of a gas-filled tube is heated electrons are emitted as in vacuum tubes if there is no positive potential in the anode practically all these electrons return to the cathode if a small positive potential is applied to the anode some electrons will be drawn across to the anode this action is similar to the action in a vacuum diode within the gas-filled tube there are electrons moving from cathode to anode and atoms of gas moving about it random if the anode potential is increased sufficiently a value is reached where the tube begins to glow and the current increases greatly at this point the tube is said to fire or discharge this action is different from that in a vacuum diode in a gas-filled tube many of the fast moving electrons collide with the atoms of the gas as a result electrons in the gas atoms may have their orbits momentarily disturbed in most cases these electrons return almost immediately to their normal orbits the energy received from the collisions is given up in the form of light some of the electrons emitted by the cathode are moving so fast that collisions between them and the gas atoms may force electrons completely out of the atoms atoms which lose one or more electrons are positively charged such atoms are called positive ions for simplicity ions will be represented from now on in this manner the electrons remove from the neutral atoms by these collisions may collide with other atoms to produce other ions the process in which ions are formed is called ionization the positive ions are attracted to the cathode on striking the cathode these ions receive electrons and become neutral atoms again the negative space charge of the electrons near the cathode tends to limit the electron flow from cathode to anode but the presence of positive ions has the opposite effect the positive space charge of the ions neutralizes most of the negative space charge of the electrons at any given moment each ion shown here neutralizes the space charge of one electron over a period of time each ion counteracts the space charge of many rapidly moving electrons one after the other the result is that the influence of the negative space charge is almost completely neutralized thus the gas diode acts quite differently from a vacuum diode when the proper anode potential is applied the gas is ionized and the current across the tube is greatly increased because of these actions two regions develop within the tube one region called the cathode sheave is located close to the cathode the other region called the plasma extends from the cathode sheave to the anode in the cathode sheath electrons emitted by the cathode are greatly accelerated toward the anode by the electric field at the same time the electric field accelerates the positive ions toward the cathode because of their greater mass the ions move more slowly than the electrons electrons entering the plasma from the cathode sheath move rapidly about colliding with atoms and ions there are also positive ions and neutral atoms moving slowly about in the plasma electrons are leaving the plasma by way of the anode as rapidly as they are arriving from the cathode sheath because of the almost complete balancing of the positive and negative space charges in the plasma the voltage drop across the plasma is very small therefore the voltage drop across the cathode she may be considered almost the total voltage drop across the tube in the cathode sheave electrons and ions are greatly accelerated in the plasma electrons drift toward the anode and ions drift toward the cathode if the anode voltage of a gas-filled diode is made high enough to start ionization of the gas the neutralization of the negative space charge which results permits a sudden and very large increase in the current for this reason it is usually necessary to include a current limiting device in the external circuit to prevent destruction of the tube elements changing the resistance of the external circuit changes the current the greater the resistance the smaller the current increasing the current within certain limits does not materially affect the voltage drop across the tube the voltage drop across the tube is merely that necessary to maintain ionization to stop the flow of current the anode potential must be reduced nearly 20 or made negative if alternating voltage is applied to the anode circuit the anode will be alternately positive and negative during the positive half cycle the tube conducts during the negative half cycle no current flows the tube thus serves as a rectifier of alternating current since the voltage drop across the tube is small the gas diode is an efficient rectifier adding a grid to this tube changes it to a gas filled triode or thyratron if the grid is given a high- bias the tube will not conduct when the anode potential is applied the presence of the negative space charge of the grid prevents the flow of electrons across the tube now if the grid is made less negative electrons begin to flow across the tube with further reductions of the negative potential on the grid the electron flow across the tube increases as the reduction of the grid voltage continues a value is reached at which the tube fires the gas within the tube is ionized light is produced and the current increases sharply this action is accompanied by the formation of a cathode sheath and the plasma which extends beyond the grid to the anode because the grid is negative it attracts positive ions this sheath of positive ions around the grid neutralizes the negative space charge of the grid making the grid more negative merely draws more positive ions to it the sheave thickens and the space charge of the grid remains neutralized thus once the tube fires the grid loses its ability to alter or stop the current the amount of current that flows through the anode depends on the resistance of the external circuit just as in the case of the gas diode before the grid can again act as a control the anode current must be stopped either by reducing the anode voltage to zero or by making it negative if alternating voltage is applied to the anode circuit the thyratron may be employed as a rectifier capable of controlling the amount of current passing through the load since the voltage applied to the anode is alternating the tube will conduct for only part of the time here the tube is conducting for almost the entire positive half cycle making the grid more negative will cause the tube to conduct for a smaller part of the positive half cycle in a ferret Ron therefore the grid provides a means of controlling the average current in the anode circuit of the tube the grid in a gas-filled tube serves only to start the current its action is like that of a trigger once the tube begins to conduct the grid has no further control in gas filled tubes as in vacuum tubes electrons are emitted by the cathode they are attracted by the positively charged anode ionization of the gas in the tube causes the tube to conduct current freely with the addition of a grid the gas field to provides a rapidly acting control of the current in the circuit of which the tube is apart you