[Sneap] Turbo pumps and cesium

[Doug Will]

Klaus,

We've run a locally modified 860i sputter source for over twenty years 
here at the University of Washington.  It is pumped by a Leybold model 
360 turbo turning at high speed.  The rotor, stator and housing of this 
pump are all aluminum.  We've never had a problem caused by Cs or Rb vapor.

You should note that our ion source has a large, aluminum body cooled to 
  about 50 F = 10 C by chilled, circulating liquid.  For Cs vapor this 
chilled body serves as a huge pump, turning the vapor to solid.  Further 
protection is provided by mounting the pump in a configuration similar 
to Karl's, described below. When we open the source here in our moist 
Seattle climate, we use inert gas if we hope to reclose it without 
cleaning.  Else, we quickly disassemble and clean the source.

When you run a system like ours, you can readily tell how far the Cs is 
spreading.  Cs deposits quickly deliquesce on exposure to air. (They 
pull moisture from the air and turn into a film of Cs hydroxide 
solution.  Alkali metal hydroxide solutions all attack aluminum.)  One 
can look for the glistening of the moist film.  And one can take 
moistened strips of indicator paper and touch them to various locations 
inside the source and the pumping tee to see how far the Cs has spread. 
  We see alkali in decreasing amounts from the source body into the 
cross arm of the pump tee (which serves as our beamline at this point). 
  But a foot off the beamline at the pumping gate valve we see no 
detectable Cs.

So I think this is a non-issue if your pump does not look directly at 
your Cs source and you have chilled surfaces to condense the Cs vapor.

Doug Will
Research Engineer
University of Washington
Center for Experimental Nuclear Physics and Astrophysics
Seattle

Karl von Reden wrote:
> The SNEAP 2005 Meeting was held at Brookhaven National Laboratory.  Please visit the meeting Homepage at: https://www.bnl.gov/sneap2005/default.asp
> 
> The ATF-SNEAP 2006 web site is now available:
> http://www.ansto.gov.au/nugeo/conference/private/ATF_SNEAP.htm
> 
> ++++++++++++++++++++++++++++++++++++
> 
> 
> ------------------------------------------------------------------------
> 
> 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:
>> The SNEAP 2005 Meeting was held at Brookhaven National Laboratory.  
>> Please visit the meeting Homepage at: 
>> https://www.bnl.gov/sneap2005/default.asp
>>
>> The ATF-SNEAP 2006 web site is now available:
>> http://www.ansto.gov.au/nugeo/conference/private/ATF_SNEAP.htm
>>
>> ++++++++++++++++++++++++++++++++++++
>> 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
>> --------------------------------------------
>>  
>> _______________________________________________
>> Sneap mailing list
>> Sneap at mail.tunl.duke.edu
>> http://mail.tunl.duke.edu/mailman/listinfo/sneap
>>
>>   
> 
> _______________________________________________
> Sneap mailing list
> Sneap at mail.tunl.duke.edu
> http://mail.tunl.duke.edu/mailman/listinfo/sneap