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A getter is a deposit of reactive material that is placed inside a vacuum system, for the purpose of completing and maintaining the vacuum. When gas molecules strike the getter material, they combine with it chemically or by adsorption. Thus the getter removes small amounts of gas from the evacuated space. The getter is usually a coating applied to a surface within the evacuated chamber. A vacuum is initially created by connecting a closed container to a vacuum pump. After achieving a vacuum, the container can be sealed, or the vacuum pump can be left running. Getters are especially important in sealed systems, such as vacuum tubes, including cathode ray tubes (CRTs), and vacuum insulated panels, which must maintain a vacuum for a long time. This is because the inner surfaces of the container release adsorbed gases for a long time after the vacuum is established. The getter continually removes this residual gas as it is produced. Even in systems which are continually evacuated by a vacuum pump, getters are also used to remove residual gas, often to achieve a higher vacuum than the pump could achieve alone. Although it weighs almost nothing and has no moving parts, a getter is itself a vacuum pump. Getters cannot react permanently with inert gases, though some getters will adsorb them in a reversible fashion. Also, hydrogen is usually handled by adsorption rather than reaction. Small amounts of gas within a vacuum tube will ionize, causing undesired conduction leading to major malfunction. Small amounts of gas within a vacuum insulated panel can greatly compromise its insulation value. Getters help to maintain the vacuum. ==Types== To avoid being contaminated by the atmosphere, the getter must be introduced into the vacuum system in an inactive form during assembly, and activated after evacuation. This is usually done by heat.〔 on Pete Miller's (Tubebooks ) website〕 Different types of getter use different ways of doing this: *''Flashed getter'' – The getter material is held inactive in a reservoir during assembly, then heated and evaporated after initial evacuation, usually by induction heating. The vaporized getter, usually a volatile metal, instantly reacts with any residual gas, then condenses on the cool walls of the tube in a thin coating, the ''getter spot'' or ''getter mirror'', which continues to absorb gas. This is the most common type, used in low power vacuum tubes. *''Non-evaporable getter'' (NEG) The getter remains in solid form. * *''Coating getter'' – a coating applied to metal parts of the vacuum system that will be heated during use. Usually a nonvolatile metal powder sintered in a porous coating to the surface of the electrodes of power vacuum tubes, maintained at temperatures of 200° to 1200 °C during operation. * *''Bulk getter'' – sheets, strips, wires or sintered pellets of gas absorbing metals which are heated, either by mounting them on hot components or by a separate heating element. These can often be renewed or replaced * *''Getter pump'' or ''sorption pump'' – In laboratory vacuum systems the bulk NEG getter is often held in a separate vessel with its own heater, attached to the vacuum system by a valve, so that it can be replaced or renewed when saturated.〔 * * *''Ion getter pump'' – Uses a high voltage electrode to ionize the gas molecules and drive them into the getter surface. These can achieve very low pressures and are important in ultrahigh vacuum (UHV) systems.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「getter」の詳細全文を読む スポンサード リンク
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