An electron gun is an electrode structure that produces and may control, focus, and deflect a beam of electrons, as in a television picture tube, where the beam produces a visual pattern on the tube’s screen. The source of the electron beam is the cathode, flat metal support covered with oxides of barium and strontium. When heated by a coil behind the support, these oxides emit electron sources, which are drawn toward a positively charged sleeve (first anode) that is contoured to allow the electron beam to flow within the inside diameter. The beam is then electrostatically constricted and collimated by a metal disk with a hole (the control electrode) before it is directed to strike a phosphor-coated screen.
Emission of Electrons
The electrons are emitted through the indirectly heated cathode. Indirectly heated cathode means the cathode electrodes are surrounded by the filament, and the electrodes emit electrons when the power applies across it.
For getting the high emission of electrons at the moderate temperature, the layer of barium and strontium oxide is applied at the end of the cathode. The current and voltage required by the indirectly heated cathode are approximately equal to the 600 mA and 6.3V.
Working of Electron Gun
After exiting from the cathode, the electron passes through the control grid. The control grid is made up of nickel material. It is centrally hole and coaxial with the CRT axis. The intensity of the control beams depends on the number of electrons emitted from the cathode. The grid has negative biasing which controls the flow of electrons.
The electron which passes from the control grid is accelerated by the high positive potential which is applied across the pre-accelerating and accelerating grids. The electron beam is focused by the focusing anode. The beam after passing through the focusing anode passes through the deflection plates and goes to the fluorescent screen.
Construction of Electron Gun
The main function of the electron gun is to produce and accelerate the beam of an electron inside the vacuum tube of the CRT. For generating and accelerating the gun requires the heater, cathode electrodes, grid, and different types of anodes. The detailed explanation of their parts is shown below.
- Heater – The heater converts the electric energy in the form of heat. It has a resistor which obstructs the flow of current and converts it into the thermal energy. The heater heats the cathode electrodes and emits the electrons.
- Control Grid – The control grid is a nickel cylinder. It is the metallic cup which has lower permeability steel. It is about 15 mm long and has a diameter of 15 mm. The hole of about 0.25 mm is drilled in the cap of the grid for the flow of the electron. The intensity of the electron beam passing through the grid depends upon the emission of electrons. The control grid is negative biasing due to which it controls the flow of electrons.
- Pre-Accelerating & Accelerating Anode – The pre-accelerating and accelerating anode accelerated the beams passing through the gun. These anodes are connected to the high potential for accelerating the electrons.
- Focussing Anode – After passing through the pre-accelerating and accelerating electrodes, the electrons are passing through the focusing beam. The focusing anode produces the beams of sharply focus electrons.
The electron guns are placed inside the glass tube so that the electron beam did not interact with the air molecules.