CERN Studies on Niobium-Coated 1.5 GHz Copper Cavities Sergio Calatroni C. Benvenuti M. Hakovirta H. Neupert M. Prada A.-M. Valente

Motivation for the study: the high-field residual resistance 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

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

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

Testing: thermal effects In Nb/Cu cavities we must cope with several thermal impedances: Cu/He (~6000 W/m2K), 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

Surface defects & roughness 2.0 µm 2.0 µm Chemically polished copper average roughness: 0.2 mm pinholes of 0.3 mm Electropolished copper average roughness: 0.02 mm nearly no defects Sergio Calatroni - CERN

Surface defects: measurement of pinholes Irregularities on the substrate surface shadowing effect film inhomogeneities He leak rate experiment ‹inc› fraction of leaky film surface - equator 9° 4.4 ppm - (~iris 50° 25 ppm) - equator 9° 0.1 ppm CP EP Substrate disk Machining and cleaning Film deposition Substrate removal p1 p2 film Sergio Calatroni - CERN

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 O2 bubbles! Production of Cu(OH)2 on the surface! Sergio Calatroni - CERN

Copper roughness II: electropolishing Electropolishing parameters: 55% vol. H3PO4 45% vol. butanol Current density: 200A/m2 HP water rinsing Electropolishing with a quasi-homotetic cathode has replaced chemical polishing (LEP standard) for surface preparation. Sergio Calatroni - CERN

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

Intrinsic film roughness & incidence angle of the niobium atoms 10 20 30 40 50 60 70 80 degrees mm 50 mm 0 mm Sergio Calatroni - CERN

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

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

Recall: standard and oxide-free coatings Sergio Calatroni - CERN

Properties of the coatings I : grain size with FIB micrographs Standard films Oxide-free films 0.5 µm 0.5 µm Courtesy: P. Jacob - EMPA Sergio Calatroni - CERN

Properties of the coatings II: purity, Hc1, texture, etc... Standard Oxide-free RRR 11.5 ± 0.1 28.9 ± 0.9 TC 9.51 ± 0.01 K 9.36 ± 0.04 K Ar cont. 435 ± 70 ppm 286 ± 43 ppm Texture (110) (110), (211), (200) Hc1 85 ± 3 mT 31 ± 5 mT Hc2 1.150 ± 0.1 T 0.73 ± 0.05 T a0 3.3240(10)Å 3.3184(6) Å Da/a 0.636 ± 0.096 % 0.466 ± 0.093 % Stress -706 ± 56 MPa -565 ± 78 MPa Grain size 110 ± 20 nm > 1 µm Sergio Calatroni - CERN

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

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

Hydrogen studies II: outgassings Question: why were coatings on oxide-free hydroformed cavities consistently worse than for spun cavities? (Legnaro 1997) Possible answer: a larger quantity of hydrogen was migrating into the film from the hydroformed cavity Sergio Calatroni - CERN

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. NbxOy at the surface) Oxidised copper: 2200±200 ppm 800±200 ppm Oxide free: 2000±200 ppm 980±200 ppm Sergio Calatroni - CERN

Hydrogen studies IV: isosteres Oxidised copper pH2(cH)2exp(-EH/kT) 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 340±10 meV/at Oxide free: 1000±200 ppm 320±10 meV/at Sergio Calatroni - CERN

HPWR effectively reduces Rres0 (1999 result) Digression: HPWR I Nb/Cu film on CP copper HPWR effectively reduces Rres0 (1999 result) Sergio Calatroni - CERN

Digression: HPWR II Before HPWR After HPWR 10 µm 10 µm Bulk Nb - EP with KEK bath HPWR smoothens the grain boundaries (2001 results) Sergio Calatroni - CERN

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