ANL/FNAL/UC Collaboration meeting 27 June 2008

SRF Materials: First Acceleration Test of Coated

Cavities

Pellin1, Zasadzinski2, Proslier1,2, Norem3, Cooley4, Kneisel,

Rimmer5

1. Materials Science Division, ANL2. Department of Biological, Chemical and Physical

Sciences, IIT3. High Energy Physics, ANL4. Technical Division, FNAL5. JLab

ANL-LDRD

ANL/FNAL/UC Collaboration meeting 27 June 2008

XPS a Surface Probe of Nb Oxidation

Nb2O5

Nb

NbOx

Dielectric Nb2O5

Nb2O5-, NbO2- are magneticNbOx (0.2 < x < 2),metallicNbOx precipitates (0.02 < x < 0.2)

Scattering off magnetic interfaces orprecipitates gives rise to Shiba states inside the gap. These cause dissipation(lowering Q).

Nb samples supplied by FNAL!

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 (Nb2O5-) 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

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 (Nb2O5-) 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

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

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

ANL/FNAL/UC Collaboration meeting 27 June 2008

Cavity Experimental Plan

1. Obtain a Single Cell Cavity from JLaba) “good” performanceb) Tested several times

2. Coat cavity with 10 nm’s Al2O3, 3 nm Nb2O5

a) Niobia to reproduce original cavity surfaceb) Dust, clean room care

3. Acceleration Test at J Laba) First test of ALD on cavitiesb) Check for “stuck” dust, high pressure rinse

difficulties, material incompatibilities, etc.c) Goal: No performance loss

4. Bake @ Fermi, retest @ JLab (in progress)

ANL/FNAL/UC Collaboration meeting 27 June 2008

J Lab Cavity: Best Previous Performance

• Strong field emission for last 5 MV/m

Quench @Eacc = 32.6 MV/m

108

109

1010

1011

Q0

Eacc [MV/m]0 5 15 20 25 30 3510

Previous Best Cavity Performance (Initial Electro-Polish and Bake)

Single Cell Cavity Test (J Lab 6/27/08)Argonne Cavity Coating Procedure

ANL/FNAL/UC Collaboration meeting 27 June 2008

J Lab Cavity: Last Acceleration Test (Cluster Cleaning)

• Cavity “as received” for ALD Cavity Treatment

Quench @Eacc = 32.6 MV/m

108

109

1010

1011

Quench @Eacc = 22.7 MV/mQ0

Eacc [MV/m]0 5 15 20 25 30 3510

Previous Best Cavity Performance (Initial Electro-Polish and Bake)

Cavity As Received For Coating

Single Cell Cavity Test (J Lab 6/27/08)Argonne Cavity Coating Procedure

ANL/FNAL/UC Collaboration meeting 27 June 2008

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)

108

109

1010

1011

Quench @Eacc = 32.9 MV/m

Q0

Eacc [MV/m]0 5 15 20 25 30 3510

Atomic Layer Deposition (10 nm Al2O3 + 3 nm Nb2O5)

Previous Best Cavity Performance (Initial Electro-Polish and Bake)

Cavity As Received For Coating

Single Cell Cavity Test (J Lab 6/27/08)Argonne Cavity Coating Procedure

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 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.