Some History of Electropolishing of Niobium 1970 – 1990 P. Kneisel Jefferson Lab Dec. 5-7, 2005 TTC Meeting, Frascati

Siemens Process(1) Dec. 5-7, 2005 TTC Meeting, Frascati

Siemens Process(2) The process was developed within a government funded collaboration agreement between Siemens AG and the Kernforschungszentrum Karlsruhe (GfK) It was based on a proprietary process used at Siemens AG for the processing of Ta for capacitors The process was subsequently used for the surface treatment of cavities ( rf separator, helix, R&D) at GfK It was subsequently “exported” to HEPL(P.Kneisel, C.Lyneis,J.P.Turneaure, IEEE Trans,Nucl,Sci NS-22,1197(1975)) , Cornell and KEK, modified by K. Saito in ~ 1980 for Tristan cavities Dec. 5-7, 2005 TTC Meeting, Frascati

Siemens Process(3) 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 Dec. 5-7, 2005 TTC Meeting, Frascati

Siemens Process(4) 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 Dec. 5-7, 2005 TTC Meeting, Frascati

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 Dec. 5-7, 2005 TTC Meeting, Frascati

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 Dec. 5-7, 2005 TTC Meeting, Frascati

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 Dec. 5-7, 2005 TTC Meeting, Frascati

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) Dec. 5-7, 2005 TTC Meeting, Frascati

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. Dec. 5-7, 2005 TTC Meeting, Frascati

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 Dec. 5-7, 2005 TTC Meeting, Frascati

EP at KEK (4) #s in bracket: current density Dec. 5-7, 2005 TTC Meeting, Frascati

EP at KEK (5) Dec. 5-7, 2005 TTC Meeting, Frascati

EP at KEK (6) Dec. 5-7, 2005 TTC Meeting, Frascati

EP at KEK (7) Dec. 5-7, 2005 TTC Meeting, Frascati

EP at KEK (8) Dec. 5-7, 2005 TTC Meeting, Frascati

EP at KEK (9) Dec. 5-7, 2005 TTC Meeting, Frascati

EP at KEK (10) Dec. 5-7, 2005 TTC Meeting, Frascati

EP at KEK (11) Dec. 5-7, 2005 TTC Meeting, Frascati

EP at KEK (12) Dec. 5-7, 2005 TTC Meeting, Frascati

Summary(1) Most important parameters: Current density: 30 – 100 mA/cm^2 Voltage: 8 – 16 V Bath temperature: 25 – 35 C Optimal HF concentration: 60 – 90 cc/L based on brightness, if concentration smaller, increase in voltage necessary, outside range of micropolishing Rotational speed:0.7 rpm for 508 MHz Acid flow rate: 60 l/min Viscous layer needs to be preserved Dec. 5-7, 2005 TTC Meeting, Frascati

Summary(2) For good results, initial roughness is important: mechanical polishing After ep the surfaces are still contaminated even after some rinsing: H2O2 at 50C+ultrasonic for 40 min; excessive rinsing afterwards Choice of materials in contact with acid mixture: PTFE,PVDF,PE Sulfur contamination: sulfur is generated during ep by reaction of hydrogen with sulfuric acid at a rate of 4 mg/l for 80 micron Sulfur can be dissolved in CCl4,CS2 or acetone some improvement with active carbon filtering Dec. 5-7, 2005 TTC Meeting, Frascati

Summary(3) Remaining issues: QA of acid: HF concentration sulfur contamination QA of work: how to avoid human mistakes how to achieve an absolute clean surface (HPR, dry ice,….) how to avoid recontamination Dec. 5-7, 2005 TTC Meeting, Frascati