RF Superconducting Materials Workshop at Fermilab, May 23 & 24, 2007 Advanced Nb oxide surface modification by cluster ion beams Zeke Insepov, Jim Norem (ANL), David Swenson (TEL Epion Inc)

2 Outline Motivation Gas Cluster Ion Beam (GCIB) treatment for Q-cure Grand Challenge: GCIB + Multi-scale simulation of Nb oxides: Ab-initio – MD - Continuum scale Preliminary results of NbO electronic structure Preliminary results of MD simulation of Oxygen cluster infusion into Nb Summary

3 Quality degradation of SC FE-free cavities at high E gradients (> 30 MV/m) and peak B ~ 100 mT is the main challenge for materials science Q-drop prevents of achieving gradients 35 MV/m needed for future accelerators, such as the ILC Baking cures Q-drop but not Q-slope and there is no understanding of its causes. Many models exist but none was thoroughly proven. Many experiments need to be explained by one model. Motivation

4 Gas Cluster Ion Beam processing Epion Corporation

5 GCIB smoothing of electrodes Smoothing  Suppressing field emission from asperities and surface roughness  Remove nanoscale tips that can be ripped off by high RF fields contributing to RF breakdown  Hardening the surface – density is higher  Cleaning/Etching  Chemically altering the surface -- Oxidizing, Nitriding -- Infusing -- Deposition

6 Cluster impact on a surface Ar n + Shock waves target Surface smoothing effect GCIB removes sharp tips - the most Field- Emitting tips, and which most probably lead to the breakdown. Lateral sputtering target Cluster Impact Lateral sputtering causes surface smoothing Epion AFM image of Ta film surface Ra =12Å Ra = 4Å

7 Experimental work on GCIB Epion Corp. Removing sharp tips on steel

8 GCIB smoothing of electrodes  Smoothing –Suppressing field emission from asperities and surface roughness –Remove nanoscale tips that can be ripped off by high RF fields contributing to RF breakdown Hardening the surface – density is higher Cleaning/Etching Chemically altering the surface –Oxidizing, Nitriding –Infusing –Deposition

9 DC field emission experiments DC Field emission measurements of 116 cm 2 Stainless Steel electrodes The mechanical polish of unprocessed substrate was much better that that of the GCIB polished substrate. Currents below 1e-12 were not measurable. [Cornell (Sinclair) & Jlab measurements] GCIB makes dense surface layers that may cause the effect

10 Removing sharp tips (O 2 ) n + Nb Local  is defined by the surface curvature and it is higher for sharp tips.

11 Grand Challenge GCIB - a clean solution for oxides: NbO, NbO2, Nb2O5; removes FE; reduces the surface roughness up to atomically low level; makes more dense surface layers; modifies grains. GCIB can be a reference method: it can create Oxygen saturated areas to test cluster formation/diffusion models We need electronically aware materials science of the Nb oxides under extremely high electric and magnetic fields Theory can calculate the diffusivity and precipitation of Oxygen in Nb, - this helps understanding baking

12 NbO Structure NbO has a FCC cubic cell and space group Pm_3m (#221). Three lattice parameters: a = b = c = 4.212Å, V = bohr 3. The unit cell contains two nonequivalent atoms: Nb at {0.5;0.5;0.0}, O at {0.5;0.0;0.0}. Nb O Electronic and structural properties of NbO were not yet studied O diffusion characteristics were not studied theoretically DFT & MD calculations were not performed for electronic, structural and thermal properties of the Nb oxides

13 The LAPW method: The Linearized Augmented Plane Wave (LAPW) method is among the most accurate methods for performing electronic structure calculations for crystals. Forms of LSDA potentials exist in the literature, but recent improvements using the Generalized Gradient Approximation (GGA) are available too. For valence states relativistic effects can be included either in a scalar relativistic treatment or with the second variational method including spin-orbit coupling Core states are treated fully relativistically The Full Potential LAPW DFT method

14 Preliminary result #2: Total Energy of NbO Murnaghan EOS: a= Ry; b=874.4 c= ; d= [Teter et al, PRB 1995] Pressure: V 0 =515.1 b 3 (  = 2%) B = GPa B P = 4.4 E 0 = Ry Equation of state

15 Preliminary result #3: Sticking probability Nb (O2) n + Sticking probability was calculated as the Number of trapped versus the total number of impact ones: reflection trapped

16 Preliminary result #4: Number of infused molecules MD simulation predicts infusion as a feasible process (O2) n + Nb

17 30kV (O2)n (n= ) cluster Infusion in Nb (100) N=13 N=135 N=1055 N=2171

18 Summary Better understanding of the NbO is needed. Theoretical & experimental data on Nb x O y are very limited – GCIB can be a reference method as it is clean Ab-initio (DFT) simulations of NbO gives preliminary Etot, P(V), electronic states  build a multi-scale approach to oxygen diffusion. Our MD shows that infusion of Oxygen cluster is most probable for large (~ 2000) molecular clusters. Smaller cluster significantly damage the surface.