![]() Thus equipotential surface acts as a “double concave lens”. Due to this difference in potential between focusing anode and accelerating anode a non-uniform field exists on each end of focusing anode. The pre-accelerating anode and accelerating anode are connected to a high positive potential and focusing anode is connected to a lower potential as we have discussed earlier. Now, let’s have a look at the functional diagram of the electrostatic focusing arrangement A force is experienced by this electron in the normal direction to the surface S and thus it gets accelerated. Consider an electron is moving from A to B enters on the left side of S. The potential on the left side of the surface is –V and on the right side is +V. Let us now consider the region of two sides of equipotential surface S as represented in the figure given below. The force is in opposite direction to that of the field and an equipotential surface is perpendicular to the field thus the force on the electron is in the normal direction to the equipotential surface. This is the reason why field intensity is less at the ends. The lateral repulsion of electric field lines causes spreading of spaces between the field lines which results in curved lines at the ends. It is necessary that the field is of uniform intensity where the electron is placed. The negative sign indicates that the acting force is opposite to that of the field. In the figure shown below, we can see an electron placed at rest in an electric field produced two parallel plates. Let us now discuss electrostatic focusing in detail Electrostatic focusing The highly focused beam moves towards vertical and horizontal deflection plates after which it reaches to fluorescent screen.įocusing of an electron beam is accomplished by two different methods- Electrostatic focusing and Electromagnetic focusing. Focusing anode helps in focusing the electron beam and is connected with a lower adjustable voltage of about 500 V. This high positive potential is about 1500 V. ![]() The number of electrons emitted from the cathode is controlled by the grid.Ī positive high potential is applied at the pre-accelerating anode by which the beam gets accelerated. The intensity by which beam of electron moves depends entirely on the electron emitted from the cathode. The control grid is basically a nickel cylinder of aperture 0.5. The emitted electrons further pass through a small hole present in the control grid. In the case of a highly efficient system the value is 300 mA at 6.3 V. To obtain high emission of electrons at a moderate temperature a layer of barium and strontium oxide is deposited at the end of the cathode.Īn indirectly heated cathode requires current and voltage value 600 mA at 6.3 V. The cathode is heated indirectly from which electrons are emitted. It consists of a heater, a cathode, pre-accelerating anode, accelerating anode which emits electrons and forms them into a beam. The electron gun is the originator of focused accelerated electron beam. The main parts of CRT are responsible for the working of it that is explained below- Electron gun The two movements are not dependent on each other that causes the beam to be arranged anywhere on the screen.ĬRT parts are confined in a glass envelope in order to permit free movement of the electron from an end to the other. This applied voltage enable the beam to move vertically up – down and horizontally from an end to other. The two electrostatic deflection plates deflect the accelerated beam by the application of voltage. This high-velocity beam strikes the fluorescent screen thus causing a luminous spot on the screen. The diagram given below shows the internal structure of the CRTĪ sharply focused beam is produced by the electron gun assembly.
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