Superconducting RF Materials University Collaboration STATUS Lance Cooley June 07, 2010

Status of the Collaboration June 09, 2010ILC ART Review at Fermilab2 FNAL, UChicago, IIT, Northwestern, Florida State –UIC not directly funded but is involved –IIT work occurs partly at ANL FNAL – Cooley –Peoples Fellow A. Romanenko (SRF surface science) –NW Ph.D. student Denise Ford (computational chemistry) UChicago – Sibener –M. Nakayama, T. Wang (oxidation fundamentals, STM) IIT – J. Zasadzinski –T. Proslier (ANL) & TBD (PCT, ESR) $1.5M / 3 years, started 9/1/09 –POs issued to collaborators on separate schedules Northwestern – D. Seidman –McCormick Appointment – Yoon-Jun Kim (LEAP microscopy of Cottrell atmospheres) Florida State – P. Lee –Z. Sung (FIB isolation and electron microscopy) UIC – R. Klie –Atomic resolution EELS

Recent achievements Subtle effects of contaminants Oxidation –Thick, stressed oxidation (stuck HPR) initiates quench (JLab result) –A grain boundary in the hot spot is also a source of magnetic scattering (IIT – at right) –Other locations seem benign – the oxide itself is not the origin of quench. Defects in the oxide continue to be plausible sources ESR data, too – Nb 4+ (IIT) Implications –Avoid stress to the natural oxide –Avoid “flash” oxidation Acid trapped in pits hit with rinse water –Hunt for the oxide defects June 09, 2010ILC ART Review at Fermilab3 Point-contact tunneling: Normal electrons at oxide hot spot Ohm’s law In hot spot Typical superconductor response at cold areas (red, green) 22 Ciovati – Jlab, Proslier – ANL, Zasadsinzki – IIT, Cooley, Romanenko – FNAL

Recent achievements Subtle effects of contaminants Hydrogen –During EP, sulfate anodization (oxidation) of niobium may produce threading dislocations (A.Romanenko) –EP also loads metal with hydrogen (unlike BCP) –Hydrogen binds to Nb vacancies, prevents removal of threading dislocations (so-called Cottrell atmosphere) –Mild baking (120°C) releases vacancies and thereby restores dislocation climb, which improves surface resistance Implications –800 °C bake to remove surface hydrogen –Final EP must be cold –Re-tool surface science to find H (e.g. LEAP) –Positron annihilation, µ-SR June 09, 2010ILC ART Review at Fermilab4 Plot: Visentin – SRF09 Cooley, Romanenko – FNAL Seidman - NWU H Vac. Baked – mobile H No bake – bound H

Materials achievements – past 12 months Understanding electrochemistry Do things go wrong? –Coupon EP is glossy, with no grain boundary contrast –Cavity EP is less glossy, with visible grain boundaries –Agitation and stirring circulate fluorine to coupon surface, producing grain-boundary contrast High temperature reduces viscosity, promotes circulation (H. Tian Ph.D. thesis) Some grains may etch quickly, leaving faceted pits Implications –Keep EP cold and don’t agitate Do not use acid flow as the coolant! Instead, apply external cold water spray to EP tool, and turn back flow. –Final EP will then be slow – use alternate process (tumbling) to make up processing time Process MUST pre-condition surface to a roughness comparable with the thickness of fluorine diffusion layer (~40 µm at 20 °C – Reece et al.) June 09, 2010ILC ART Review at Fermilab5 Non-typical EP coupon showing grains (FNAL)

Summary – Good materials science AND implications for SRF Collaboration PIs are advancing basic materials topics –Cottrell atmospheres –Spin-resonance and positron annihilation spectroscopy –Oxidation kinetics Collaboration work is stimulating new tool development –Low-T STM at UChicago –Electron di-chroism in the STEM at UIC –LSCM and EBSD at FNAL Implications of work have direct impact on SRF process –Avoid excessive oxidation, “flash” oxides, and mechanical damage to oxide layer –Take better care of hydrogen –Don’t heat or agitate the electrolyte if it can be helped – not only does this make surfaces rougher, it also promotes the subtle complications of oxide and hydride chemistries –New question: Is contamination more important than roughness? June 09, 2010ILC ART Review at Fermilab6