[Sneap] Turbo pumps and cesium

[Klaus Bahner]

Dear Sneapers,

The vacuum in many (most?) sputter sources is maintained by turbopumps,
quite often large ones, i.e. a significant investment. Since we are in the
process of installing such a system, I wonder whether you take any
precautions in order to limit the possible negative effect of the cesium
vapour on the turbo and corresponding backing pump?
Of course under normal conditions the amount of pumped cesium is only in the
order of milligrams per hour, which still may accumulate to larger amounts
after a long time - or in case of a malfunction, a runaway cesium reservoir
may generate even higher loads . All turbo impellers, I have seen so far,
are made from aluminum, which is easily attacked by cesium, at least once it
is vented.
The pump vendors I asked, claim that they have no or not enough experience
with this kind of setup, in order to give any recommendations, followed by
the statement that their pumps are not suitable for this application and any
use of their pumps under these conditions is at our own risk, which is
neither unexpected nor unreasonable. 
Hence I would appreciate your input on this subject. Do you just ignore the
problem (Does it pose a problem at all??) or do you try to avoid
condensation of the cesium, either by heating the turbo or mechanical
"baffles" in the vacuum chamber.

Thanks in advance
Klaus Bahner

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