GAS-FILLED TUBES

GAS DIODES

An operating gas-filled tube has molecules, ions, and free electrons present within the envelope. In a gas-filled diode, the electron stream from the hot cathode encounters gas molecules on its way to the plate. When an electron collides with a gas molecule, the energy transmitted by the collision may cause the molecule to release an electron. This second electron then may join the original stream of electrons and is capable of freeing other electrons. This process, which is cumulative, is a form of ionization. The free electrons, greatly increased in quantity by ionization, continue to the plate of the diode. The molecule which has lost an electron is called an ion and bears a positive charge. The positive ions drift toward the negative cathode and during their journey attract additional electrons from the cathode.

The velocity of the electrons traveling toward the plate varies directly with the plate voltage. If the plate voltage is very low, the gas-filled diode acts almost like an ordinary diode except that the electron stream is slowed to a certain extent by the gas molecules. These slower-moving electrons do not have enough energy to cause ionization when they hit the gas atoms. After the plate voltage is raised to the proper level of conduction, the electrons have enough energy to cause ionization when they hit the gas molecules. The plate potential at which ionization occurs is known as the IONIZATION POINT, or FIRING POTENTIAL, of a gas tube. If the plate voltage is reduced after ionization, it can be allowed to go several volts below the firing potential before ionization (and hence, high-plate current) win cease. The value of the plate voltage (E_p) at which ionization stops is called the DEIONIZATION POTENTIAL, or EXTINCTION POTENTIAL. The firing point is always at a higher plate potential than the deionization point.