CAVITY TREATMENT (BCP, HEAT TREATMENT & HPR) Sergio Calatroni with many contributions from: Rama Calaga, Leonel Ferreira, Antonio Mongelluzzo LHC CCEM, Sergio Calatroni1

Outline Caveat: several personal remarks What do we want to do, and why – Cortical layer, defects etc.. – H2 degassing – Cleanliness How we are going to do it – Surface treatments – Vacuum furnace – HPWR CERN hardware LHC CCEM, Sergio Calatroni2

Surface treatment goals Removal of defects – Due to welding (irregularities, beads, etc) – Due to machining (EDM oxidation & cracks, machining chips, etc) Removal of “cortical layer” – Rule of thumb: 5% sheet thickness either side Obtaining a smooth surface – Allows highest RF performance (suppression of B-field enhancement at steps, etc.) LHC CCEM, Sergio Calatroni3

Options Buffered Chemical Polishing, Electropolishing, Centrifugal Barrel Polishing Need to specialize material removal & finishing treatment? No Efficient and economical treatment is more important than ultimate performance My conclusion: BCP LHC CCEM, Sergio Calatroni4 TreatmentMaterial removalSurface finish BCP******* EP******* CBP*****

Is it feasible? LHC CCEM, Sergio Calatroni5 4 rods ¼ wave RF dipole All three designs have features which allow correct acid flow and evacuation

BCP LHC CCEM, Sergio Calatroni6 Niowave CERN BCP 1:1:2 done 8/2012! 150 µm removed All hardware ready for BCP (but we need to invest for “production”)

Degassing goal Removal of hydrogen in solid solution – Prevent “hydrogen disease” LHC CCEM, Sergio Calatroni7 B. Bonin SRF1991 (DESY) M Hakovirta SRF2001 (Tsukuba) (S. Isagawa J. Appl. Phys. 51 (1980), 4460)

H 2 degassing Done at >600 °C for >24h – H 2 partial pressure / furnace total pressure have a peak then have to decrease Final equilibrium is reached (Sievert’s law for exothermic material) for 1 ppm H: < °C < °C Note: mbar !!! (but slow kinetics for uptake) LHC CCEM, Sergio Calatroni8

CERN furnace LHC CCEM, Sergio Calatroni9 P=3.5x °C P=1x10 -7 RT Venting with Air

Oxygen contamination It is well known that Nb acts as a GETTER – When “activated”, ie surface oxide is dissolved in the bulk (> 380 °C), its surface is extremely reactive, and impurities steadily diffuse in the bulk Surface layer gets contaminated – Diffusion progresses in the bulk with time and temperature Solution: “light” (chemical) polishing, typically around 20 µm – More material removal may increase H 2 uptake from the acid bath LHC CCEM, Sergio Calatroni10

Cleanliness Removal of dust particles – Prevent electron field emission – (Improves Q) Peak of E-field: LHC CCEM, Sergio Calatroni11 4 rods ¼ wave RF dipole (**) (****)

HPWR CERN Removal of dust particles – HPWR – Line-of-sight access to all high E-field regions LHC CCEM, Sergio Calatroni12 Bulk Nb 1.3 GHz cavity, Ep + CERN, circa year 2000 Recently refurbished with new ultrapure H 2 O production plant & filters, new controls

New HPWR and clean room LHC CCEM, Sergio Calatroni13 From Janic Chambrillon

Conclusions BCP favoured Minimal sequence: – Heavy BCP – H 2 degassing – Light BCP – HPWR All crab designs compatible with surface treatments – 4-rods seems not optimal for HPWR At CERN – HPWR and H 2 degassing OK – BCP OK, dedicated cabinet needed for series “production” LHC CCEM, Sergio Calatroni14