Surface Preparation of superconducting cavities and industrial Production

Experience

Dietrich Bloess

International Workshop on Thin Films applied to superconducting RF, and new ideas pushing 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