1. Proposal for an International Single Cell Cavity R&D Plan for Electropolishing Development
P. Kneisel
JLab
Sept. 25 – 28, 2006
TTC meeting at KEK

2. Some History(1) 8th SRF Workshop, Abano Terme 1998 Sept. 25 – 28, 2006
TTC meeting at KEK

3. Some History(2)
In 1998 the “world of electropolishing of niobium cavities” was in order, because there was only one user.
Since then, with more “unexperienced” users entering the field, “confusion” started to develop
The procedures for EP were based on the original “Siemens recipe”, modified and improved for horizontal , continuous electropolishing by Kenji Saito and successfully applied to 508 MHz Tristan cavities [ K. Saito et al, 4th SRF workshop, KEK Report89-21, p.635 ff]
Optimal EP parameters such voltage, current density, bath temperature, bath composition were developed
Sept. 25 – 28, 2006
TTC meeting at KEK

4. Siemens Process(1) Acid mixture:
HF(40%):H2SO4(95-97%) = 10 : 85 by volume
Constant voltage 9 – 15 V
depending on bath temperature( 25C-35C), solution concentration and electrode geometry
The optimium polishing conditions are not in the plateau region of the polarization curve, but are characterized by damped current oscillations
Oscillations reflect the generation of the viscous layer at the anode, which builds up and partially dissolves in the HF
Voltage drop in anode layer is about 90%, no matching of cathode geometry to anode geometry necessary.
Mean current density in oscillation
~ 100 mA/cm2
Sept. 25 – 28, 2006
TTC meeting at KEK

5. Siemens Process(2)
EP generates an extremely smooth surface, but not necessarily clean surface because of residual oxides on the surface
Anodizing/oxipolishing with ammonium-hydroxide solution
Fully oxidizing of suboxides
Interface is shifted from activated surface into clean material
Ammonium-hydroxide has cleaning capacity
EP samples show contamination of sulfur in the form of sulfate and fluorine (M.Grunder, Dissertation, Karlsruhe 1977)
Oxipolished samples show no sulfur anymore,less fluorine;
boiling in water for 5 min reduces sulfur by factor of 10 and sulfate is converted to sulfid; fluorine concentration is not affected
Sept. 25 – 28, 2006
TTC meeting at KEK

6. Electropolishing/Anodizing
Multi-mode (2 -4 GHz) pill box cavity, electropolished and anodized (20 V and 60 V)
TE011 cavity, ep + anodized, 9.5 GHz(H.Diepers et al., Phys. Lett. 37A, 139 (1971)
K.Saito, PAC 2003
Sept. 25 – 28, 2006
TTC meeting at KEK

7. Application of Siemens Process(1)
Karlsruhe-Cern Superconducting RF Separator
( A. Citron et al.;Nucl.Instr.& Meth. 164(1979).p.31
30 micron ep (horizontal)
Anodizing to ~ 50 V ( 0.1 micron Nb2O5)
1850 C UHV annealing for 24 hrs
70 micron ep (horizontal)
1850 C UHV annealing for 2 hrs
Intermittent EP
Sept. 25 – 28, 2006
TTC meeting at KEK

8. Application of Siemens Process(2)
First results on a 500 MHz SC
Test Cavity for TRISTAN
Spinning of half cell with 3 intermediate annealing steps
Ebw of stiffening ribs
Individual EP ( 80 micron) of cavity halfes and beam pipes
EBW of parts with inside/outside welds
900 C stress annealing at 900 C
Vertical EP , 30 micron
2 x oxipolishing at 80 V
For 3-cell cavity
Poor EP at equator
Hydrogen absorption
T.Furuya et al, Jap. Journ.Appl.
Physics, 20(1981), L
Sept. 25 – 28, 2006
TTC meeting at KEK

9. Cornell Vertical EP
R. Geng
Sept. 25 – 28, 2006
TTC meeting at KEK

10. Cavity Processing: ANL Recipe for EP with TEM Cavities[M
Cavity Processing: ANL Recipe for EP with TEM Cavities[M.Kelly, TTC Frascati]
Electropolishing has been a mainstay at ANL
Based on the Siemens process
Cavity (anode) - observed no polishing rate sensitivity to electrical connection point even for large cavities
High purity Al cathode (3003 series) tailored to the cavity
Acid composition 85:10 mixture of 96% H2SO4, 40% HF, reagent grade
Temperature 28-32o C (chilled water through the cathode)
Average anode current density ~40 mA/cm2, acid replaced when value drops below 30 mA/cm2
Sept. 25 – 28, 2006
TTC meeting at KEK

11. Niobium Geometries Electropolished at ANL
Quarter-wave
We believe that electropolishing is a key step in our fabrication process.
Smaller parts use same technique as for elliptical cavity half cells
Long center conductors are rotated in the electrolyte bath with the aluminum cathode around outside
Most challenging spoke geometry – large surface area housing – critical spoke region required good polishing with no stagnant areas in the electrolyte
Devised a washing machine action together with a set of impellers – dealt with issue of moving electrical connections
Co-axial half-wave
Important EP Technical Issues:
Temperature gradients and stability
Acid flow patterns
Double spoke
Sept. 25 – 28, 2006
TTC meeting at KEK

12. EP at KEK (6)
Sept. 25 – 28, 2006
TTC meeting at KEK

13. EP at KEK (1)(K. Saito et al. , Proc, 4th SRF workshop(1989), p
EP at KEK (1)(K. Saito et al., Proc, 4th SRF workshop(1989), p.635, KEK,Tsukuba)
Sept. 25 – 28, 2006
TTC meeting at KEK

14. Voltage or Current density ? This is a coupled problem.
EP at KEK (2)
30< Is < 100 mA/cm2
Voltage or Current density ? This is a coupled problem.
Sept. 25 – 28, 2006
TTC meeting at KEK

15. Current oscillation control is not
EP at KEK (3)
KEK
Current oscillation control is not
right EP condition
Best Finishing
Fig.1 : Current oscillation and best EP finishing surface
Sept. 25 – 28, 2006
TTC meeting at KEK

16. EP at KEK (4) #s in bracket: current density Sept. 25 – 28, 2006
TTC meeting at KEK

17. EP at KEK (5)
Sept. 25 – 28, 2006
TTC meeting at KEK

18. L.Lilje , SRF 2001 paper MA009
Sept. 25 – 28, 2006
TTC meeting at KEK

19. Some History(4) EP programs within CARE DESY Multi-cell EP development
Saclay R&D/single cell system
INFN Legnaro program for industrial EP processes/single cell system
EP System at Jlab for SNS ( not used)
Vertical EP at Cornell
Design studies at FNAL/ANL for R&D/production systems
EP has become the main subject of discussions in several meetings over the last 18 – 24 months
TTC Meeting at DESY in March 2005
ILC Snowmass Aug. 2005
SMTF Meeting at FNAL Oct. 2005
TTC Meeting in Frascati Dec. 2005
Sept. 25 – 28, 2006
TTC meeting at KEK

20. Analysis of experimental results: to what extend do we understand the scatter of the results? FRASCATI
Is scatter the result of
subtleties of EP process?
Process parameters:
non-reproducibility,non-uniformity of material removal
Set-up:acid level, cathode bag, cathode shielding, current leads, T-control
“Q-disease”: unpredictable, material?
Reproducibility in acid composition (DESY)
Draining and rinsing:
overheating? for multi-cell cavities
Is scatter caused by
“environmental” problems?
Malfunction of system
Problems during rinsing and/or assembly
Vacuum problems
Problems during testing
Human errors
Sept. 25 – 28, 2006
TTC meeting at KEK

21. List and prioritize R&D activities
Re-visit residual contamination of EP surfaces: XPS,SIMS? FE
Investigate different rinsing methods:
hot water (Henkel), H2O2 + US, anodizing, oxipolishing,..
on samples, single cells: either several or reference cavity of known performance
Removal of sulfur from mixture:
filtering?, solvents,…
Implement “on line” monitoring of HF concentration and polarization curves, purity (gas chromatography)
Shaping of cathode:
more uniform material removal, more uniform polarization curves over whole surface, lower voltage to achieve required current density, more uniform T-distribution?
Does it make sense to explore other acid mixtures? Or should one concentrate on making present process “fool proof”?
Sept. 25 – 28, 2006
TTC meeting at KEK

22. Identify resources to work on these activities
Surface studies:
Saclay, Univ. Wuppertal, Jlab, INFN Genoa,Cornell?
Rinsing studies:
KEK, DESY,Jlab,Cornell,Saclay,ANL,MSU?
Electrode shaping:
INFN Legnaro, DESY/Henkel,KEK,Jlab, Cornell,ANL
Implementation of “on-line” monitoring:
DESY,KEK/Nomura Plating, Jlab,Henkel,ANL?
How does one get a commitment?
Sept. 25 – 28, 2006
TTC meeting at KEK

23. Schedule milestones, coordinate joint effort
How can the output of multi-cell cavity tests be increased?
Is it worthwhile and possible to share facilities and testing?
e.g EP.barrel polishing of cavities (“Ichiro”) at KEK, testing at DESY,Jlab…
What can we learn from simulation calculations (DESY purchased program)?
Is more input from “electro-chemistry professionals” needed (DESY is in contact with Univ. Bruxelles) ?
Sept. 25 – 28, 2006
TTC meeting at KEK

24. Action Item
We promised to write a draft report of the WG1 meeting discussions with a prioritized list of R&D issues in early January 2006
Sept. 25 – 28, 2006
TTC meeting at KEK

25. After the Frascati TTC Meeting
Sept. 25 – 28, 2006
TTC meeting at KEK

26. Purpose of Proposal
Capability to explore with a simpler system the various issues plaguing EP
Take advantage of “Quick turn-around” tests (VTA with T-mapping)
However, try to simulate the complexity of multi-cell EP
Based on the findings, provide input to multi-cell procedures
Areas identified:
Contamination and Rinsing Studies
Hydrogen dissolution: “Q-disease”
Monitoring and Control ( “QA”)
Acid Composition
“Streamlining of Procedures”
Sept. 25 – 28, 2006
TTC meeting at KEK

27. Contamination and Rinsing Studies(1)
Sulfur Contamination
Has been reported already in 1977 (M.Grunder, Thesis)
Eliminated by oxipolishing, partially by boiling in water
1989 during Tristan R&D/production K. Saito could reduce the precipitation of sulfur by rinsing the system with CCl4,CS2, acetone, filtration with activated carbon.
Peroxide rinsing was used as a rinsing step
In SRF 2005 F. Eazenou et al reported sulfur contamination on ep’d surfaces , encountered during an optimization study of EP in the frame of the CARE/JRA1 program
Sulfur found at DESY and Cornell
It is proposed to investigate electropolished surfaces on samples with appropriate surface analytical methods and to apply different rinsing procedures (oxipolishing, peroxide rinsing, boiling in water, ultra/mega sound..) to confirm the results from 3 decades ago
Sept. 25 – 28, 2006
TTC meeting at KEK

28. Contamination and Rinsing Studies(2)
These surface studies should be complemented with rinsing studies on
single cell cavities, most desirably with T-mapping capability
Proposed method:
Use 6 brand new cavities, which have to be qualified to a reasonable baseline performance ( e.g. Eacc ~ 30 MV/m)
Each cavity will be electropolished by a standard amount, e.g. 10 micron and rinsed by different methods:
HPR only, oxipolishing + HPR, Peroxide rinse + HPR, boiling in high purity water +HPR, HF rinse + HPR, surfactant rinse+HPR…
These procedures will be repeated e.g. 5 times each to get some limited statistics, especially , if T-mapping is applied, these tests should result in a reasonable understanding of the contamination issues.
Total of 30 tests with single cell cavities
Sept. 25 – 28, 2006
TTC meeting at KEK

29. Hydrogen Pick-UP, Q-disease(1)
A. Matheisen reported at the TTC meeting in Frascati, that Q -degradations were encountered unpredictibly on 9-cell TESLA cavities after identical treatments.
Hydrogen degassing , rapid cooldown, applying a potential to the sample and addition of an oxidizer to the acid mixture during the “mounting and de-mounting “ phase [S.Higuchi, K.Saito SRF2003] can prevent these additional losses, however, it is important to understand the possible pick-up mechanism
Shielding of the cathode with a cathode bag with the proper geometry is supposedly a very crucial element in avoiding hydrogen pick-up [T.Saeki,SMTF meeting Oct. 2005]
Sept. 25 – 28, 2006
TTC meeting at KEK

30. Hydrogen Pick-UP, Q-disease(2)
Possible sources of Hydrogen pick-up during EP:
Inappropriate shielding of the cathode
Cathode to anode distance wrongly adjusted
Surface temperature during EP too high (~45 C)
Exothermic reaction of residual acid on surface/in piping..
during rinsing, raising the surface temperature on the niobium
Acid film on surface removes protective oxide layer; a potential on the niobium could prevent the pick-up of hydrogen
Sept. 25 – 28, 2006
TTC meeting at KEK

31. Hydrogen Pick-UP, Q-disease(3)
Proposed cavity program:
Each electropolished cavity should be tested for Q-disease
Conduct several single cell tests ( EP 10 micron e.g.5 times), where the rinsing with water after acid drainage is delayed to simulate the possible heating of the surface as well as the effect of the dissolution of the protective oxide layer.
Conduct several single cell tests(EP 10 micron e.g.5 times) with a potential applied to the cavity during mounting and de-mounting from the EP set-up
Conduct several polishing cycles with an oxidizer, e.g HNO3 added to the polishing solution
Conduct a series of tests with modified cathode to anode distance (e.g.5)
Total of 20 tests with single cell cavities
Sept. 25 – 28, 2006
TTC meeting at KEK

32. Electropolishing Absorption of Hydrogen avoided by applying a
potential to the sample and adding an oxidizer
(HNO3) to the EP solution [S.Higuchi, K.Saito SRF2003]
Sept. 25 – 28, 2006
TTC meeting at KEK

33. Monitoring and Control(1)
Experimentally, the material removal from the cavity surface is not uniform. Whether this is caused by different ep conditions at different surface areas (different polarization curves) or by different surface temperatures or flow patterns or cathode shape/distances or external current leads is not clear
Computer simulation programs exist ( at DESY in collaboration with Uni Bruxelles, Jlab has a collaboration with Virginia Tech, within the CARE program C. Bonavolonta has an experimental program) and should be explored to their fullest possibility to simulate EP conditions
During the EP process the volatile HF component of the acid mixture is depleted and polishing conditions are changing. For maintaining reproducible EP conditions, an “on-line” monitoring of the HF concentration and of polarization curves are most desirable.
An experimental single cell program should explore the causes of non-uniform material removal
Sept. 25 – 28, 2006
TTC meeting at KEK

34. Monitoring and Control(2)
Proposed method :
Conduct a series of tests (e.g.5) with modified acid flow/flow pattern
Conduct a series of tests ( e.g.5) with modified temperature distribution
Conduct a series of tests ( e.g. 5) with modified current lead attachments
Conduct a series of tests (e.g.5) with modified/matched cathode shape(at Henkel, the cathode is “matched” to the cavity shape)
Conduct a series of tests ( e.g.5) with modified cathode shielding
Total of 25 tests
Sept. 25 – 28, 2006
TTC meeting at KEK

35. Streamlining of Procedures(1)
Presently used procedures require a total material removal of ~ 200 micron in at least 2 steps with intermediate furnace treatments
The final treatment is in “flux”: ~ 5 micron bcp at DESY,
3 micron EP with fresh acid + HF rinse at KEK; also CBP as a first mechanical smoothening step is successfully used.
Prior to testing, the cavities are baked “in situ” for 48 hrs at 120 –128 C
The procedures are rather complex and expensive
Can the procedures be simplified?
e.g. less material removal,
shorter baking times, e.g for large grain niobium cavities baking times of 3 hrs are already sufficient to remove the Q-drop
Sept. 25 – 28, 2006
TTC meeting at KEK

36. Streamlining of Procedures(2)
Recommendation:
Carry out a series of tests with e.g 3 new cavities, which are subjected to successive material removal in steps of ~ 20 micron between 60 micron and 160 micron
app. 18 tests
With 3 cavities of reasonable performance (e.g. 30 MV/m)
carry out a series of “baking tests” at 120C starting at 3 hrs and continuing to 6 hrs, 12 hrs,24 hrs, 48 hrs ( after EP of ~ 10 micron between tests)
app. 15 tests
Sept. 25 – 28, 2006
TTC meeting at KEK

37. Acid Composition
Acid compositions of pre-mixed acids need to be monitored;
DESY experienced at least on 1 occasion in summer of 2005 an
unusual composition.
Henkel, the industrial company used by DESY for single cell
electropolishing, is using – successfully - a “secret” mixture,
mixed in a facility in Austria; it is reportedly “more active” , HF is either evaporated or chemically bound in sulfonic acid (private communication,Mr. Hartmann, Henkel)
Sept. 25 – 28, 2006
TTC meeting at KEK

38. How, where and when ? How? Without funds, most likely no work Where?
Lab EP
HPR
VTA
T_Map
Surface
DESY
Yes?
yes
limited
INFN no
Saclay
KEK
Cornell
Yes, vertical
JLab
Yes, lim
FNAL/ANL
Sept. 25 – 28, 2006
TTC meeting at KEK

39. Priorities Material Removal, baking time “Streamlining” 2
Sept. 25 – 28, 2006
TTC meeting at KEK

40. When?
The community has to agree on the necessity for such a program and has to make some funding available or solicit commitments from the labs
This is – as recent attempts are showing – the toughest job.
Most important is a commitment to a schedule by individuals,who are interested in this research, with the support of their institution to carry out some work
If such a program will be at all useful, it has to match the time lines for the S0/S1effort
Sept. 25 – 28, 2006
TTC meeting at KEK