Advanced Nb oxide surface modification by cluster ion beams

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Advanced Nb oxide surface modification by cluster
ion beams

• Zeke Insepov, Jim Norem (ANL),
• David Swenson (TEL Epion Inc)

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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

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Motivation

• Quality degradation of SC FE-free cavities at
  high E gradients (gt 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.

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Gas Cluster Ion Beam processing
Epion Corporation
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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

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Cluster impact on a surface
Shock waves
Lateral sputtering
Cluster Impact
Arn
target
Lateral sputtering causes surface smoothing
Ra 4Å
Surface smoothing effect GCIB removes sharp tips
- the most Field- Emitting tips, and which most
probably lead to the breakdown.
Ra 12Å
Epion AFM image of Ta film surface
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Experimental work on GCIB
Removing sharp tips on steel
Epion Corp.
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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

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DC field emission experiments
DC Field emission measurements of 116 cm2
Stainless Steel electrodes
GCIB makes dense surface layers that may cause
the effect
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
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Removing sharp tips
(O2)n
Nb
Local m is defined by the surface curvature and
it is higher for sharp tips.
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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

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NbO Structure

• 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

Nb
O

• NbO has a FCC cubic cell and space group Pm_3m
  (221). Three lattice parameters a b
• c 4.212Å, V 504.44 bohr3. The unit cell
  contains two nonequivalent atoms Nb at
• 0.50.50.0, O at 0.50.00.0.

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The Full Potential LAPW DFT method

• 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

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Preliminary result 2 Total Energy of NbO
Equation of state
Murnaghan EOS a-23398.4 Ry b874.4 c-9477.8
d31920.0
Pressure V0515.1 b3 (d 2) B 242.8 GPa BP
4.4 E0 -23374.8 Ry
Teter et al, PRB 1995
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Preliminary result 3 Sticking probability
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Preliminary result 4 Number of infused molecules
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30kV (O2)n (n13-3000) cluster Infusion in Nb
(100)
N13
N135
N2171
N1055
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Summary

• Better understanding of the NbO is needed.
  Theoretical experimental data on NbxOy 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.