1. DESY Experiences on Use of Gas overlay during Assemblies
Choice of gas
Benefit and Drawback of gas overlay
Conclusion for application at DESY
A.Matheisen DESY
TTC Topical Workshop on SRF Cryomodule Clean Room Assembly
CEA Saclay, 12th november 2014

2. DESY Experiences on Use of Gas overlay during Assemblies Choice of gas
Benfit:
low cost
Available in high cleanliness
standard particle counters can be applied
Low Hydro carbon when extracted from pure N2 liquid
Draw back:
At higher temperatures reaction with Nb and N2 possible
(Problem during helium vessel welding)

3. DESY Experiences on Use of Gas overlay during Assemblies
DESY Experiences on Use of Gas overlay during Assemblies Choice of gas (cont.)
Ar gas
Draw back:
Cost
Special calibration of gas particle counters needed
Special equipment for slow pumping slow venting vacuum stands
Benefit
Available in high cleanliness
Low Hydrocarbon when extracted from pure liquid.
At higher temperatures no reaction with Nb and Ar
(during helium vessel welding)
Easy to detect with RGA in case of leaking valves in venting / flushing line

4. DESY Experiences on Use of Gas overlay during Assemblies
DESY Experiences on Use of Gas overlay during Assemblies Choice of gas (cont.)
At DESY Cleanroom Argon is in use
for cavity preparation
preparation of CAV for tank welding
and module assembly
During tank welding locally higher temperatures can not be exclude
DESY Cleanroom is equipped with Argon for venting and flushing
To prevent errors during cavity processing and for economical reasons only one ultra pure gas line is in use  Argon
In General both gases work well for venting (Fast and Slow venting) Gas overlay
Only when Nb is exposed to higher temperatures Argon is beneficial

5. DESY Experiences on Use of Gas overlay during Assemblies Benefit and drawback of gas overlay
Why we should use gas overlay (flushing) during module assembly?
Costs money
needs more infrastructure
We are in ISO 4 cleanroom 
But Film 1 2 3

6. DESY Experiences on Use of Gas overlay during Assemblies Benefit and drawback of gas overlay Conclusion
Benefit
Stable and defined condition at ports in resp. to laminar flow conditions
Over pressure inside cavity blows off remaining residues
during start of dismounting of flanges
Beneficial if particle are remaining on the flanges or bore holes
Very flexible and adjustable air flow to different geometries
Draw back
Needs extreme clean piping (contradiction to Slow pumping slow venting)
Connecting valves have to be clean before connecting to flushing line
QC needs to be done with great care before connecting to flushing line
High gas consumption
Point of use filters are mandatory in the flushing line
In case of particle in the lines “heavier” once may rest at areas with min flow speed
CAV Iris and beam tube 78 mm ID Equator 110 mm ID

7. DESY Experiences on Use of Gas overlay during Assemblies
Conclusion for application at DESY
At DESY ISO 4 cleanroom Argon overlay gas is requested due
Tank welding procedure on CAV in Cavity preparation cycle
For module assemblies N2 gas is adequate
At DESY cleanroom vacuum lines und cavity accessories
underlay a strong quality control, even if slow pumping and slow venting is in use
Investigation by flow visualization shows a benefit on gas
At DESY Argon gas overlay is in use sine years for
power coupler assembly
during string assembly
and “special” repairs