1. ANL/FNAL/UC Collaboration meeting 27 June 2008 SRF Materials: First Acceleration Test of Coated Cavities Pellin 1, Zasadzinski 2, Proslier 1,2, Norem 3, Cooley 4, Kneisel, Rimmer 5 1.Materials Science Division, ANL 2.Department of Biological, Chemical and Physical Sciences, IIT 3.High Energy Physics, ANL 4.Technical Division, FNAL 5.JLab ANL-LDRD

2. ANL/FNAL/UC Collaboration meeting 27 June 2008 XPS a Surface Probe of Nb Oxidation Nb 2 O 5 Nb NbO x Dielectric Nb 2 O 5 Nb 2 O 5- , NbO 2-  are magnetic NbO x (0.2 < x < 2),metallic NbO x precipitates (0.02 < x < 0.2) Scattering off magnetic interfaces or precipitates gives rise to Shiba states inside the gap. These cause dissipation (lowering Q). Nb samples supplied by FNAL!

3. ANL/FNAL/UC Collaboration meeting 27 June 2008 Point Contact Tunneling (PCT) Reveals the presence of dissipative Cooper pair breaking layers on the surface of cavity grade, processed Nb samples. Likely Source? Magnetic layers (Nb 2 O 5-  ) among the complex oxidized Nb surface Appl. Phys. Lett. 92, 212505/1-3 2008 Cavity grade Nb (Measured) Fit Assuming Idealized BCS Superconductor Fit Assuming BCS + Pair Breaking Surface Layer

4. ANL/FNAL/UC Collaboration meeting 27 June 2008 Point Contact Tunneling (PCT) Reveals the presence of dissipative Cooper pair breaking layers on the surface of cavity grade, processed Nb samples. Likely Source? Magnetic layers (Nb 2 O 5-  ) among the complex oxidized Nb surface Appl. Phys. Lett. 92, 212505/1-3 2008 Cavity grade Nb (Measured) Fit Assuming Idealized BCS Superconductor Fit Assuming BCS + Pair Breaking Surface Layer

5. ANL/FNAL/UC Collaboration meeting 27 June 2008 A Solution? Atomic Layer Deposition -> non-dissipative dielectric layer 1.Use ALD to synthesize a dielectric diffusion barrier on the Nb surface 2.Bake to “dissolve” the O associated with the Nb layer into the bulk

6. ANL/FNAL/UC Collaboration meeting 27 June 2008 Point Contact Tunneling (PCT) + ALD  (1.55meV = Nb).  (pair breaking) -> 500 C bake should significantly reduce dissipation Appl. Phys. Lett. in prep 

7. ANL/FNAL/UC Collaboration meeting 27 June 2008 Cavity Experimental Plan 1.Obtain a Single Cell Cavity from JLab a)“good” performance b)Tested several times 2.Coat cavity with 10 nm’s Al 2 O 3, 3 nm Nb 2 O 5 a)Niobia to reproduce original cavity surface b)Dust, clean room care 3.Acceleration Test at J Lab a)First test of ALD on cavities b)Check for “stuck” dust, high pressure rinse difficulties, material incompatibilities, etc. c)Goal: No performance loss 4.Bake @ Fermi, retest @ JLab (in progress)

8. ANL/FNAL/UC Collaboration meeting 27 June 2008 J Lab Cavity: Best Previous Performance Strong field emission for last 5 MV/m

9. ANL/FNAL/UC Collaboration meeting 27 June 2008 J Lab Cavity: Last Acceleration Test (Cluster Cleaning) Cavity “as received” for ALD Cavity Treatment

10. ANL/FNAL/UC Collaboration meeting 27 June 2008 J Lab Cavity: After ALD Synthesis (10 nm Al 2 O 3 + 3 nm Nb 2 O 5 ) Only last point shows detectable field emission. 2 nd test after 2 nd high pressure rinse. (1 st test showed field emission consistent with particulates)

11. ANL/FNAL/UC Collaboration meeting 27 June 2008 Conclusions ALD is a compatible method for SCRF Cavity Processing. No significant multipactoring. –Alumina underlayer does not enhance –Other surface choices? Many better choices than Nb 2 O 5 are available. Field Emission reduction (dielectric improvement). –Alumina is a much better dielectric than than Nb 2 O 5 –Is 10 nm optimum? Thicker, two step coating, etc. Improved Performance from last result. –200 C during layer synthesis + surface reduction Improved performance vs previous best –3x improvement in Q, slight gradient enhancement –Anneal? Cavity Annealing Coating is proceeding.