Ion pumps only work on existing vacuum, i.e., you cannot pump down a vacuum
chamber or bottle from atmosphere with an ion pump alone.
When a chamber is evacuated, there is always some residual gas remaining.
Even in outer space there is some gas present, although very little.
In addition, if the chamber or bottle you are pumping on is not 100% clean,
there will always be some outgassing. This means once you turn off your
high-vacuum pump the pressure inside the chamber will slowly rise. One way
of making vacuum chambers "clean" is to bake them for several hours while
being pump on. The high temperatures boil off any residual water or oils. I
presume vacuum tubes are baked-out to some degree when first assembled.
When an ion pump is attached to a chamber under vacuum, some of that
residual gas will find it's way into the pump. A high voltage potental
ionizes the gas molecules, giving them a positive charge. The walls of the
ion pump are negatively charged, and the ions are attraced to and imping on
the wall. As long as there is high voltage present, the residual ionized gas
will remain in the pump. The higher the pressure inside the chamber, the
more current an ion pump will draw. Usually, an ion pump will draw higher
current at first, then slowely drop as the chamber's pressure drops.
I believe a getter material is used inside vacuum tubes to get rid of
residual oxygen. I once overheard someone on the air say that vacuum tubes
have a vacuum of about 70 Torr (mm of mercury), which is not "high" vacuum.
But I don't know how accurate that 70 Torr figure is.
I work with high vacuum systems, not vacuum tubes, so I am not sure of those
details.
Mike WA6FQM
************
John I have wondered how an ion pumps works. What does it do, where
connected and polarity?
I have seen that they are usually rated for only a few milliamperes
typically so apparently there is not a lot of power involved.
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