Getter

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 absorption. 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 absorbed 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.

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. Different types of getter use different ways of doing this:

Flashed getters are prepared by arranging a reservoir of volatile and reactive material inside the vacuum system. Once the system is evacuated and sealed, the material is heated (usually by radio frequency induction heating). After evaporating, it is deposited as a coating on the interior surfaces of the system. Flashed getters (typically made with barium) are commonly used in vacuum tubes. The getter will usually be seen as a silvery metallic spot on the inside of the tube's glass envelope. Large transmission tubes and specialty systems often use more exotic getters, including aluminium, magnesium, calcium, sodium, strontium, caesium and phosphorus.

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