CERN Studies on NiobiumCoated 1 5 GHz Copper

CERN Studies on Niobium-Coated 1. 5 GHz Copper Cavities Sergio Calatroni C. Benvenuti M. Hakovirta H. Neupert M. Prada A. -M. Valente

CERN Studies on Niobium-Coated 1. 5 GHz Copper Cavities Outline of the talk • • • Motivation for the study: the high-field residual resistance Thermal effects Surface defects and roughness Role of the grain size and purity Hydrogen studies: – RF measurement – Thermodynamics • Conclusions Sergio Calatroni - CERN SRF 2001 - 10 Sept. 2001

CERN Studies on Niobium-Coated 1. 5 GHz Copper Cavities Motivation: the high-field residual resistance Coatings performed using krypton on electropolished spun copper cavities (Santa Fe 1999) Limited by RF power Sergio Calatroni - CERN SRF 2001 - 10 Sept. 2001

CERN Studies on Niobium-Coated 1. 5 GHz Copper Cavities Testing: higher field operation The performance can be extended straightforwardly at higher fields Limited by RF power (250 W) (Thermal instability of the standard small cryostat is not a limiting issue) Sergio Calatroni - CERN SRF 2001 - 10 Sept. 2001

CERN Studies on Niobium-Coated 1. 5 GHz Copper Cavities Testing: thermal effects In Nb/Cu cavities we must cope with several thermal impedances: Cu/He (~6000 W/m 2 K), across Cu (negligible), Nb/Cu interface (? ), across Nb film (negligible) If the Rs increase would only be due to some heating of the film -> RBCS It is then possible to estimate the temperature increase that corresponds to the measured Rs increase. The dissipated power is also easily calculated A Kapitza-like mechanism would give a straight line in the plot Sergio Calatroni - CERN SRF 2001 - 10 Sept. 2001

CERN Studies on Niobium-Coated 1. 5 GHz Copper Cavities Surface defects & roughness 2. 0 µm • • Chemically polished copper average roughness: 0. 2 mm pinholes of 0. 3 mm Sergio Calatroni - CERN 2. 0 µm • • Electropolished copper average roughness: 0. 02 mm nearly no defects SRF 2001 - 10 Sept. 2001

CERN Studies on Niobium-Coated 1. 5 GHz Copper Cavities Surface defects: measurement of pinholes Irregularities on the substrate surface shadowing effect film inhomogeneities He leak rate experiment ‹ inc› fraction of leaky film surface Substrate disk Machining Film and deposition cleaning p 1 p 2 Substrate removal CP - equator 9° - (~iris 50° EP - equator 9° 4. 4 ppm 25 ppm) 0. 1 ppm film Sergio Calatroni - CERN SRF 2001 - 10 Sept. 2001

CERN Studies on Niobium-Coated 1. 5 GHz Copper Cavities Copper roughness I: electropolishing The window of optimum polishing parameters is rather narrow. Reproducing these parameters over the entire surface of the cavity cell needs further R&D Production of O 2 bubbles! Production of Cu(OH)2 on the surface! Sergio Calatroni - CERN SRF 2001 - 10 Sept. 2001

CERN Studies on Niobium-Coated 1. 5 GHz Copper Cavities Copper roughness II: electropolishing Electropolishing parameters: • 55% vol. H 3 PO 4 45% vol. butanol • Current density: 200 A/m 2 • HP water rinsing Electropolishing with a quasi-homotetic cathode has replaced chemical polishing (LEP standard) for surface preparation. Sergio Calatroni - CERN SRF 2001 - 10 Sept. 2001

CERN Studies on Niobium-Coated 1. 5 GHz Copper Cavities Copper roughness III: electropolishing current density is 5 times larger at iris current density is identical The electropolishing cathode is being optimised by means of a sophisticated computer simulation code. The current density can be made uniform Sergio Calatroni - CERN SRF 2001 - 10 Sept. 2001

CERN Studies on Niobium-Coated 1. 5 GHz Copper Cavities 80 70 60 50 40 30 20 10 20 30 40 50 50 mm mm 0 mm degrees Intrinsic film roughness & incidence angle of the niobium atoms Sergio Calatroni - CERN SRF 2001 - 10 Sept. 2001

CERN Studies on Niobium-Coated 1. 5 GHz Copper Cavities Intrinsic film roughness: incidence angle and residual resistance An issue which is of great interest also for low-ß cavities, is the correlation between the incidence angle of the film and the residual resistance Sergio Calatroni - CERN SRF 2001 - 10 Sept. 2001

CERN Studies on Niobium-Coated 1. 5 GHz Copper Cavities Digression: new ideas for low-beta cavities coating Nb ring Cu half-cell Problem: coating incidence angle in low-beta cavities Solution: coat at favourable angle before welding Sergio Calatroni - CERN SRF 2001 - 10 Sept. 2001

CERN Studies on Niobium-Coated 1. 5 GHz Copper Cavities Recall: standard and oxide-free coatings Sergio Calatroni - CERN SRF 2001 - 10 Sept. 2001

CERN Studies on Niobium-Coated 1. 5 GHz Copper Cavities Properties of the coatings I : grain size with FIB micrographs Standard films Oxide-free films 0. 5 µm Courtesy: P. Jacob - EMPA Sergio Calatroni - CERN SRF 2001 - 10 Sept. 2001

CERN Studies on Niobium-Coated 1. 5 GHz Copper Cavities Properties of the coatings II: purity, Hc 1, texture, etc. . . RRR TC Ar cont. Texture Hc 1 Hc 2 a 0 Da /a Stress Grain size Sergio Calatroni - CERN Standard Oxide-free 11. 5 ± 0. 1 9. 51 ± 0. 01 K 435 ± 70 ppm (110) 85 ± 3 m. T 1. 150 ± 0. 1 T 3. 3240(10)Å 0. 636 ± 0. 096 % -706 ± 56 MPa 110 ± 20 nm 28. 9 ± 0. 9 9. 36 ± 0. 04 K 286 ± 43 ppm (110), (211), (200) 31 ± 5 m. T 0. 73 ± 0. 05 T 3. 3184(6) Å 0. 466 ± 0. 093 % -565 ± 78 MPa > 1 µm SRF 2001 - 10 Sept. 2001

CERN Studies on Niobium-Coated 1. 5 GHz Copper Cavities Properties of the coatings III: effect on residual resistance OX: oxidised copper substrate OF: oxide-free copper substrate CP: chemical polishing EP: electropolishing • Oxide-free films (larger grains, better purity, less stress, etc. . . ) have on average a larger residual resistance (measured at low field) • In contrast with models linking the residual resistance with Josephson dissipation at grain boundaries Sergio Calatroni - CERN SRF 2001 - 10 Sept. 2001

CERN Studies on Niobium-Coated 1. 5 GHz Copper Cavities Hydrogen studies I: film loading Spun cavities coated using argon as sputter gas loaded with 1. 4 at. % of hydrogen Hydrogen loading induces a much larger degradation for oxide-free films Sergio Calatroni - CERN SRF 2001 - 10 Sept. 2001

CERN Studies on Niobium-Coated 1. 5 GHz Copper Cavities Hydrogen studies II: outgassings Question: why were coatings on oxide -free hydroformed cavities consistently worse than for spun cavities? (Legnaro 1997) Sergio Calatroni - CERN Possible answer: a larger quantity of hydrogen was migrating into the film from the hydroformed cavity SRF 2001 - 10 Sept. 2001

CERN Studies on Niobium-Coated 1. 5 GHz Copper Cavities Hydrogen studies III: outgassings Bare Cu The hydrogen content and has been measured also by thermal outgassing to 350 ºC, for passivated and non-passivated Nb films (i. e. Nbx. Oy at the surface) Oxidised copper: 2200± 200 ppm Oxide free: 2000± 200 ppm Sergio Calatroni - CERN 800± 200 ppm 980± 200 ppm SRF 2001 - 10 Sept. 2001

CERN Studies on Niobium-Coated 1. 5 GHz Copper Cavities Hydrogen studies IV: isosteres Oxidised copper p. H 2 (c. H)2 exp(-EH/k. T) The hydrogen content and its binding energy with the lattice can be measured with isosteric cycles and successive injections of known quantities of hydrogen. Oxidised copper: 400± 200 ppm Oxide free: 1000± 200 ppm Sergio Calatroni - CERN 340± 10 me. V/at 320± 10 me. V/at SRF 2001 - 10 Sept. 2001

CERN Studies on Niobium-Coated 1. 5 GHz Copper Cavities Digression: HPWR I Nb/Cu film on CP copper HPWR effectively reduces Rres 0 (1999 result) Sergio Calatroni - CERN SRF 2001 - 10 Sept. 2001

CERN Studies on Niobium-Coated 1. 5 GHz Copper Cavities Digression: HPWR II 10 µm Before HPWR After HPWR Bulk Nb - EP with KEK bath HPWR smoothens the grain boundaries (2001 results) Sergio Calatroni - CERN SRF 2001 - 10 Sept. 2001

CERN Studies on Niobium-Coated 1. 5 GHz Copper Cavities Conclusions • The maximum field of Nb/Cu cavities is not limited by any intrinsic phenomena. • However, the residual resistance suffers from a steep increase at high field (threshold ~15 MV/m). • Several possible causes have been investigated. The most probable sources are: surface defects, hydrogen content. • Possible cures are: better electropolishing (under way), reducing hydrogen content (difficult). • Coating for low-beta cavities are being addressed with a suitable modification of the sputtering technique. Sergio Calatroni - CERN SRF 2001 - 10 Sept. 2001