We have tried two different kinds of helium detectors with mixed success in terms of improving FoFu-1’s vacuum—We found some leaks, but not others.  They were a Dielectric MGD-2002 and a GasCheck G3.

We’d like to take it up a notch with a residual gas analyzer.  RGA’s tend to work up to about a maximum pressure of 10^-4 Torr, so we would most likely want a Partial Pressure Reduction add-on that would allow sampling for helium as the product of future pB11 shots at several Torr.  Another benefit from the PPR would be if our base pressure in FoFu-1 was >10^-4, which is often the case, even though much higher vacuums should be standard operating procedure.  A “sniffer” attachment for overpressure leak hunting (filling the chamber with >1atm helium) could be another plus.

Best of all we have a pledge towards the purchase of such a system, but first we want to make sure it will have both the functionality and ease-of-use we’d need without a dedicated vacuum technician.  So large bonus to anything we could try before buying, rental or otherwise.

Some helpful background links:

http://www.angelfire.com/va2/TomLigon/RGA.htm

http://www.thinksrs.com/downloads/PDFs/ApplicationNotes/cis.pdf

http://www.inficon.com/download/en/RGA for Leak Detection.pdf

http://www.lesker.com/LeskerTech/Archives/0g11m3h/LeskerTech_Issue4.pdf

Any experts out there wanna chime in with their recommendations?

I’ve worked with SRS RGA’s in the past.  You can do gas sampling or leak checking down to the 1E-9 Torr range with ease.  The key problems we’ve encountered (not on PF devices) are sensitivity to plasma environment as one might expect, electronics problems with pulse plasmas (200A, 100 ms), disruption of the comm link to the computer and the variable maintenance cycle (depends on what deposits where and when). 

I think most of the problems are universal and could be dealt with using proper electronic and vacuum isolation.  I don’t know if you could run the RGA with the PF on-line but some sort of He monitoring on-line will probably be required down the road so might as well find the problems now.  SRS software is straightforward to use.  I learned it in about 20 minutes so I could do leak checking and base vacuum analysis.  SRS is pretty good about maintenance but it does cost.  I recommend their 100 AMU model for FoFu-1.  Most of the key gases are less than 50 AMU.

Thanks, a_i, I’ll definitely be getting a quote from them.

Dumb idea: what about filling the chamber with cold gas and using infrared cameras to spot the leak?  Is that too much risk to the system?

ikanreed - 08 January 2013 03:10 PM

Dumb idea: what about filling the chamber with cold gas and using infrared cameras to spot the leak?  Is that too much risk to the system?

why use cold gas? why not gas at the normal operating temperature?

Using cold gas makes the chamber cold so it will be hard to see the gas leak which tends to be very small quantities. 

When vendors qualify components, it is common to place a detector on the vacuum side of the system and puff small amounts of helium on the air side.  Those of us that are users tend to use the same protocol because you can localize (~4”) the leak as helium moves quickly, easily penetrates the leaks and does not occur in natural air at a reasonable quantity.  It is a bit of an art to get started but the key is very low flow rates or small puffs.  Don’t start spraying helium everywhere.  If you take your time and do it well, a vacuum system like FoFu-1 can be leak checked in ~1 hr.  With some practice you can localize a leak to less than 1”.  Normally, you can find the leaking spot if you localize the leak to 4”.  The common problems are damage to the sealing surface, debris on an o-ring or damage to a knife edge.  Debris can be as small as a human hair or as large as finger nail clipping (yeah, I saw it once).  Good vacuum practice will eliminate the debris.  Damage can be accidental or due to manufacturing problems.  Usually you will feel them before you see them.  I hope someone has fingernails.  Run them over sealing surfaces. You can’t hurt the metal but you will generally feel some damage before you see it. 

A trick we use is to pump the system in pieces by using blank flanges to test the chamber separately from the electrode assembly region.  I can’t speak for anyone else but I find solving a few little problems better than solving one huge problem.

Can you not do it in reverse, pressurise the system and use leak detection fluid?

Probably not the best method, but an idea.

Generally, vacuum systems are not designed to be pressurized.  Also, vacuum is to create a clean environment.  Adding a fluid means you are adding something to the system.  You can take it out and wipe all the surfaces down but you still need to pump out any residue.  Also, the leak rate is likely so small that it would be hard to see with a liquid.  How would you localize it without taking the system back apart. 

Helium leak detection by puffing helium outside the chamber under vacuum allows you to localize the leak without contaminating the chamber.  It’s a tried and true technique used across the vacuum community.

I know in our systems we have problems with out gassing rather than leaking. But 10^-4torr it shouldn’t that bad I assume. We where going down to 10-8 and outgassing even after long bake periods where constantly a problem.

Has LPPX received their leak detector yet?
Here are some links to rental units:
http://www.heliumleakdetection.net/Helium-Leak-Detectors/helium-mass-spectrometer-leak-detectors.html
http://www.lacotech.com/services/leakdetectorrental.html
http://www.atecorp.com/products/varian-vacuum/vspr021.aspx

Hi, all—We were able to finish setting up our new SRS RGA last week, and the initial results were very helpful.  It showed our earlier leak hunting had left a trapped reservoir of isopropanol, which enabled the leak to keep coming back after firing, but now that we understand what was happening we should be able to avoid this in the ongoing re-assembly.  We’ll also be extending the functionality of the RGA with a pressure reducing sampling path.  Onward to more shots!  ;-D