[Sneap] Turbo pumps and cesium

[Karl von Reden]

Hello Klaus,
We have run an Alcatel turbo and fore pump on our AMS Cs sputter source 
for over 10 years without major issues, other than regular maintenance 
on the fore pump. The pump was located ~0.5 m downstream from the 
ionizer at right angle to the beam line. In my opinion, the Cs vapor has 
little chance to make it all the way to the pump past plenty of cold 
surface and we never had to worry about the stators/rotors. It is 
different in the case Powell describes, where the pump sits near a 
charge exchange canal. We had a similar experience as the Floridians 
with our Mg charge exchange canal, actually trashing a turbo after a 
short time of operation.
Cheers,
Karl

Klaus Bahner wrote:
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>
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> ++++++++++++++++++++++++++++++++++++
> 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
>
> --------------------------------------------
> AMS 14C Dating Laboratory
> University of Aarhus
> Ny Munkegade, Byg. 520
> DK - 8000 Aarhus C
> Phone: +45 8942 3736
> Fax:   +45 8612 0740
> --------------------------------------------
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