Application Note #104: Long Term Care of TWT Amplifiers
This application note outlines and describes "Long Term Care of TWT Amplifiers".
A major cause of failure of TWT Amplifiers is overheating due to load mismatch or gas build-up in the tube with long periods of disuse. Since the need for proper load matching, cooling, and turn-on sequence is reviewed in the Operations Manual, this discussion will concentrate on what happens in the tube during long periods of disuse and during initial turn-on.
All high vacuum systems are subject to slow build-up of gases when shut down for extended periods. During initial processing, the tube is pumped continuously while being baked at as high a temperature as possible. The baking helps to cause adsorbed gases to cook off all internal parts of the tube.
In operation, the tube is protected from overheating by the amplifier manufacturer by proper cooling, as well as various overload monitors, such as VSWR monitors, helix current and, of course, gross overload cutouts. However, if the tube has been inoperative for an extended period, any gas that has built up is immediately ionized by the electron beam and goes to the nearest most negative element, where it is absorbed. An excessive amount of gas can cause sputtering of material from this negative element, which contaminates everything if it occurs frequently. In the extreme case, an arc may destroy the tube. The power supply shutdown tends to catch this before serious damage occurs.
Beam de-focusing is usually not a problem in modern power TWTs, but the presence of gas will cause this phenomena....