[Sneap] Turbo pumps and caesium

[David Weisser]

Klaus,

At ANU, we have had a turbo pump close coupled to the vacuum housing of our
General Ionex 832 Cs sputter source for more than 10 years without any
problem associated with Cs.  There was a line-of-sight baffle between the
source and the right angle valve to the pump which blocked Cs from getting
beyond the source.

We have had a similar arrangement for the last 2 years on our Gas Cathode
SNICSII also with no problems.

I would not hesitate to use an ordinary turbo and rotary pump in such an
application as long a line-of-sight baffles were used.

David

********************************************
D.C. Weisser
Nuclear Physics Department
Research School of Physical Sciences and Engineering
Australian National University
Canberra A.C.T. 0200
AUSTRALIA

Tel:    + 61 (0)2 6125 2080
Fax:    + 61 (0)2 6125 0748
Mobile: + 61 (0)414 249 209

-----Original Message-----
From: sneap-bounces at tunl.duke.edu [mailto:sneap-bounces at tunl.duke.edu]On
Behalf Of Klaus Bahner
Sent: Wednesday, 24 January 2007 12:13 AM
To: 'Symposium of Northeastern Accelerator Personnel'
Subject: [Sneap] Turbo pumps and cesium


The SNEAP 2005 Meeting was held at Brookhaven National Laboratory.  Please
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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

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