ECR plasma: a possible in-situ cavity processing technique

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ECR plasma a possible in-situ cavity processing
technique
G. Wu, W-D. Moeller, C. Antoine T. Khabiboulline,
E. Harms, Y. Terechkine, H. Edwards, D. Mitchell,
A. Rowe, C. Boffo, C. Cooper, T. Koeth, W.
Muranyi, Fermilab
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Contents

• Introduction Field emission problem
• Introduction to plasma cleaning
• General application
• SRF field activity
• ECR plasma and RF cavity
• Experimental plan and the issues to be addressed
• Potential benefit and other applications

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Field emission is a continuing problem
Red represents the FE limitation
DESY cavity experience
L. Lijies summary of DESY cavity databank, DESY,
2006
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Field emission is a continuing problem
JLAB SNS cavity experience
J. Ozelis, SRF 2005
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Field emission sources

• Inclusions from weld-prep machining, forming
• Residues from chemical processing (EP,BCP)
• Water impurity (HPWR)
• Clean room particles
• Assembly particulates

Particulates includes (Ni, Mn, In, Cu, C, F, Cl,
Ca, Al, Si), Nb, Fe, Cr, S, etc
Particle counter recording has not been very
indicative
Particle free is not guaranteed

• FE is a localized statistical problem
• Success else where does not guarantee the local
  success
• Past success does not guarantee the future
  success ?experienced in all Labs

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Introducing plasma

• Plasma induces chemical reactions in reduced
  temperatures, converts some surface materials,
  contaminants to gaseous phase
• Plasma generates accelerated ions to bombard the
  surface (including loose particles)
• Glow discharge, RF discharge and ECR plasma are
  common methods
• Noble gas, N2, O2, H2, mixtures,

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Applications 3,4

• Semiconductor industry 1,6,9
• Micro-electronics Josephson junctions 8
• Automobile industry painting 2,10
• Aircraft industry painting 11
• Medical applications pre-cleaning for coating
  and sterilization
• Optical industry
• Antique preservation surface protection
• Particle accelerator beam line components 14
• Microwave power - multiMW, large-orbit, coaxial
  gyrotron 7
• SRF Field
• Plasma etching for Nb cavity JLAB 13
• Plasma cleaning INFN/Lagnero, JLAB 12
• Coupler processing DESY (W-D. Moeller, Dennis?)

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ECR plasma and RF cavity
Figure 3 Electron being accelerated clockwise
by periodic electric field. External magnetic is
pointing out of the paper (not shown). Color
reflects energy

• ECR electron cyclotron resonance

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ECR plasma and RF cavity
n0 3.2?1017 /m3
?2.6?10-6 s
Minimum field 130 V/m for 90eV
rlt 0.3 mm
P1?10-5 torr
9-cell Cavity Eacc for 15 kilo-watt RF input
under different input coupling for cavity Q0
Eacc for 150 watt RF input under different input
coupling for cavity Q0
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x-wave
?L
?R
3.9GHz
Microwave traveling inside magnetized plasma
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Experimental plan and the issues to be addressed

• Usual cleaning
• Cold RF test to find FE limit
• Plasma processing at room temperature
• Cold RF test to verify improvement
• Gas mixtures Ar, H2, O2, He, Kr.

• Surface contamination removal?
• Ar implantation? Ar ion creates surface defects?
• Dry-oxidation afterwards?
• If Hydrogen, how about Q-disease?
• If Oxygen, oxidation compound?
• He also effective?

• Gas pressure
• Plasma density
• Temperature distribution
• Ion energy distribution
• Ion flux rate
• Chemical reaction

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Coupler Qext 3x104 2x109
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Potential benefit and other applications

• Reduce field emission, increase cavity production
  yield
• through in-situ processing, improve cryomodule
  performance
• Potential new design for cryomodule recovery
  (Build-in magnetic coil)

Processing opportunity in between
Or built-in capability

• CF4O2, Cl2 in plasma etching of niobium 15,
  16
• NbCl5 coating of niobium
• Sn/SnClx vapor Nb3Sn formation
• B2H6Mg similar to Penn State HPCVD 17

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Acknowledgement
H. Padamsee, Cornell P. Kneisel, L. Phillips, G.
Bialas, R. Rimmer, H. Wang, B. Manus, G. Slack,
JLAB