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SRF Materials: First Acceleration Test of Coated Cavities

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

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SRF Materials: First Acceleration Test of Coated Cavities

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  1. SRF Materials: First Acceleration Test of Coated Cavities Pellin1, Zasadzinski2, Proslier1,2, Norem3, Cooley4, Kneisel, Rimmer5 Materials Science Division, ANL Department of Biological, Chemical and Physical Sciences, IIT High Energy Physics, ANL Technical Division, FNAL JLab ANL-LDRD

  2. XPS a Surface Probe of Nb Oxidation Dielectric Nb2O5 Nb2O5-, NbO2- are magnetic NbOx (0.2 < x < 2),metallic NbOx precipitates (0.02 < x < 0.2) Nb2O5 NbOx Nb 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. Fit Assuming BCS + Pair Breaking Surface Layer Cavity grade Nb (Measured) Fit Assuming Idealized BCS Superconductor 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 (Nb2O5-d) among the complex oxidized Nb surface • Appl. Phys. Lett. 92, 212505/1-3 2008

  4. Fit Assuming BCS + Pair Breaking Surface Layer Cavity grade Nb (Measured) Fit Assuming Idealized BCS Superconductor 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 (Nb2O5-d) among the complex oxidized Nb surface • Appl. Phys. Lett. 92, 212505/1-3 2008

  5. A Solution? Atomic Layer Deposition -> non-dissipative dielectric layer • Use ALD to synthesize a dielectric diffusion barrier on the Nb surface • Bake to “dissolve” the O associated with the Nb layer into the bulk

  6. Point Contact Tunneling (PCT) + ALD D • D (1.55meV = Nb). • G (pair breaking) • -> 500 C bake should significantly reduce dissipation • Appl. Phys. Lett. in prep

  7. Cavity Experimental Plan • Obtain a Single Cell Cavity from JLab • “good” performance • Tested several times • Coat cavity with 10 nm’s Al2O3, 3 nm Nb2O5 • Niobia to reproduce original cavity surface • Dust, clean room care • Acceleration Test at J Lab • First test of ALD on cavities • Check for “stuck” dust, high pressure rinse difficulties, material incompatibilities, etc. • Goal: No performance loss • Bake @ Fermi, retest @ JLab (in progress)

  8. J Lab Cavity: Best Previous Performance • Strong field emission for last 5 MV/m

  9. J Lab Cavity: Last Acceleration Test (Cluster Cleaning) • Cavity “as received” for ALD Cavity Treatment

  10. J Lab Cavity: After ALD Synthesis (10 nm Al2O3 + 3 nm Nb2O5) • Only last point shows detectable field emission. • 2nd test after 2nd high pressure rinse. (1st test showed field emission consistent with particulates)

  11. 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 Nb2O5 are available. • Field Emission reduction (dielectric improvement). • Alumina is a much better dielectric than than Nb2O5 • 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.

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