Surface Preparation of superconductingcavities and industrial ProductionExperience
Dietrich Bloess
International Workshop on Thin Films applied to superconducting RF, and new ideaspushing the Limits of RF Superconductivity
Legnaro, Italy 9. - 12. October 2006
For the LEP storage rings the accelerating Cavities
had to be superconducting:
●Power consumption for normal conducting
cavities would have been 300 MW
power consumption of superconducting
cavities: 60 MW
●The large number of copper cavities and their
special form would have made the stored beam
unstable by their transvers impedance
therefore:
Superconducting cavities
but which technology?
● Bulk niobium cavities was a technology already
done elsewhere, however, 50 tons of high purity
niobium would have been needed.
The price of niobium would have increased the
cavity cost by 30%
● Niobium sputtered on copper was an unknown
technology
therefore:
CERN developed and produced in industry 32 bulk
niobium cavities
and in parallel
developed Nb/Cu technology and
had 256 Nb/Cu-cavities produced by three
different firms
Nb/Cu-cavities are of course cheaper, however,
more difficult to produce:
● EB-welding is delicate because of the high thermal
conductivity more power than for bulk Nb is
needed:
problems with thermal deformation,
holes in the weld,
porosities due to very fast cooldown of vortices
in the liquid copper, and due to impurities on
the surface
● Great care has to be taken to produce a smooth
and clean surface without holes and inclusions:
➔ Electrochemical polishing(EP) 60 m
➔ Inspection and carefull grinding
➔ EP 60 m
➔ Inspection and if necessary another correction
➔ EB-welding
➔ Inspection and grinding of welding beads
➔ Chemical polishing (CP) 20-25 m
➔ Rinsing with high purity water in clean room
➔ Drying with reagent grade ethanol in clean room
After sputtering:
very thorough cleaning by spraying with high purity
demineralised water, preferably at high pressure,
in front of a laminar flow unit of class 100
assembly in a clean room of class 100 with protective
clothing. The operators have to be trained in clean
room work!
How was this technology transfered to industry?
●A working relationship of mutual trust is most important
Nothing should be hidden. (not easy to achieve)
●The firms had full access to everybody in the lab working
on the project.
●One senior technician was responsible for the relation with
one firm. He had free access to everybody in the firm
working on the project
●Regular meetings were held at CERN or at the Firms
at least once per month
continued:
●Extensive log keeping (full traceability of materials and
all operations).
●Good access to diagnostic tools ( SEM, particle counters,
telescopes for in situ surface evaluation, chemical analysis,
fast SRF measurements)
●To have had more than one firm did help
●And, of course, we had to know each step of the
production process at CERN
Results:
●Overall production efficiency was about 80%
●Non of the cavities were lost during all years of operation
●No magnetic shielding was needed for Nb/Cu-cavities
●The Nb/Cu-cavities did not quench
●All cavities operated between 9 and 10 MV/m
The Q0 was better than 3*10 9