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

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

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

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

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

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

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

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), L145-148 Sept. 25 – 28, 2006 TTC meeting at KEK

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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