DeGassing of TWT Tubes

Cause Of Tube Gassing : A TWT tube consists of many internal components that start their life in a normal air atmosphere. In addition, there are multiple welds performed on a tube to hold all of these components together and 'seal' the inside of the tube from the exterior environment. It should be noted that the internal area of the TWT tube is subjected to an extensive air extraction process, during the manufacture of the TWT, which reduces the air content internal to the TWT tube to a very very low level. Although this is not a perfect vacuum, it is very close to it. Also, during the manufacture of the TWT tube, the design includes an internal device, called a "GETTER", whose function is to 'adsorb' (bond any air molecules to the surface of the GETTER when the Filament/Cathode circuitry is activated) any remaining gases inside the TWT tube. It is important to note that the GETTER will only perform its intended function when the tube is operating in High Voltage. It will do nothing when the tube is turned off.

Finally, the internal components have remaining particles of air inside them that will slowly seep into the interior of the TWT tube and, quite often, the tube welds will have microscopic leaks that allows exterior air molecules to enter the interior vacuum. When the tube is operating, the GETTER will remove these random air molecules, but not when the tube is turned off.

Gassy Tubes : When enough air builds up inside a TWT tube that is inactive, they have the ability to interact with the electron beam inside the tube when it is operational. The interaction of the air with the electron beam will scatter some of the energy into the Helix structure, raising the Helix Current to unacceptable levels. This will be indicated by an inability to turn the tube on (High Voltage application) or the presence of frequent Helix Current alarm faults.

The presence of Helix Current alarms after application of high voltage indicates that there may be the possibility of recovery from gassing while the TWT tube is still in the amplifier chassis. If the amplifier can be reset and high voltage can remain applied for a reasonable period of time between Helix Current faults, the internal GETTER may be able to recover an acceptable internal tube vacuum. Note that this type of vacuum recovery can take many days, if it is successful at all.

The more serious condition occurs when a TWT tube is activated after an extended storage period and will not achieve high voltage. This is indicative of excessive internal air and requires that the TWT tube be sent back to the amplifier manufacturer for analysis and return to the TWT tube manufacturer for factory de-gassing. Even with factory de-gassing, the successful recovery of the TWT tube vacuum is not guaranteed. Note that this condition is considered to be outside the warranty coverage of the TWT tube and, if unsuccessful, will require that the customer purchase a new TWT tube.

Avoiding Gassy Tubes : This is the easy part. All a customer has to do to avoid a gassy TWT tube is to operate the amplifier, in High Voltage On, for a period of 24 hours every three months. At the end of this operational cycle, verify that the quiescent Helix Current has returned to a very low value (typically 1-3mA) and deactivate the amplifier. If this is done at a minimum of every three months, the customer should not have any problems with Gassy TWT tubes.

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