This document summarizes research into electropolishing niobium cavities using an ionic liquid electrolyte without fluorine. Key findings include: 1) A mixture of choline chloride and urea was found to successfully etch niobium without fluorine and provide good results on test samples and cavities. 2) Adding 30g/L of sulfamic acid to the ionic liquid gave the best surface quality with minimal roughness and defects. 3) Tests on 6GHz cavities showed promising results, though non-uniform heating remained a challenge, and different cathode and flux configurations are still being optimized.

1. V.Rampazzo Superconductivity Laboratory Legnaro National Laboratories National Institute of Nuclear Physics, Italy Electropolishing of Niobium 6GHz cavities inCholine chloride – Urea melt 2010

2. Goals Find solution/melt recipe for electropolishing Nb without using F—-ions Put this recipe on application to 6GHz cavities 2

3. Electrochemistry and properties of Nb Nb -> Nb3+ +3e- E0 = -1.1 V Nb -> Nb5+ +5e- E0 = -0,96 V Perniobates 2Nb + 5Cl2 -> 2NbCl5 NbCl5 + 4Н2О -> 5HCl + Н3NbO4 Niobium acid A(+): Niobate K(-): 3

4. IonicLiquid: Composition A Ionicliquidis a mixtureoftwosalt, thatdissolvesitself at a temperature lowerthan the fusionpointof single salt Heating the salts, those dissociate itselfintoions and assume the liquid state After the formationof IL, thisremainliquidwhencooled down 4

5. Structural formulas of mixture We have found mixture which can etch Nb without using F--ions and gives good result on application to cavities 5 Urea CholineChloride Sulfamic acid

6. Investigation samples system Holder Cathode Thermocouple Anode (Sample) Stirrer 6 Electrolyses time – 5 min

7. We were using our automatic method to find correct electrical characteristic of electropolishing EP plateau 7

8. Electropolishing of samples Front Back 8

9. Surface quality analyze with profilometer 9 where m – quantity of measurements which where taken to calculation Scan 1 Scan 6 Scan 2 Scan 5 Scan 3 Scan 4

10. Surface quality as function ofsulfamic acid concentration 10

11. Surface quality as function of sulfamic acid concentration 11

12. Changing quantities of melt compounds we have found correct recipe Because of high relatively power of process solution gets big heat. 12

13. Ra – f(C(SA)) Roughness: classical EP versus Ionic liquid EP 13 Adding 30g/l Sulfamic acid in 4:1 Choline Chloride Urea melt gives possibility to obtain brightness surface, without spots and pitting on sides of the sample The best result of IL is comparable with the result of classical EP The back roughness is the same of the front : good current distribution around the sample

14. Surface quality as a function of process time 14

15. Results in different treatment duration (min) 15 Front side 5 60 30 20 10 5 60 30 20 10 Back side

16. Surface characterization with profilometer Classical EP, 10min Raw IL EP, 60min IL EP, 10min 16

17. 17 EP samples in process…

18. EP on 6GHz cavities After the goodresult on samples, we start toapply the EP on real 6GHz cavities Cathodes, flux system, concentration, newadditionwerestudiedtofind the best EP We are stillworking… 18

19. Improvement road Toimprove the EP westudy some possibilities: Alternative tosulfammicacid Differentflux inside cavity Differentorientationofcavity 19

21. High activity formation of cathode gas brings to saturation of electrolyte with H2

22. IL comes from flanges and goes out from the cathode.

23. The cathode makes from tube 8mm in diameter with holes.

24. During the pumping electrolyte goes through the holes inside the tube20

25. 21 Cavity 6 GHz EP system with IL Holed cathode Output flange Input flange Solution collector Pump

26. Vertical EP: holedcathode 22 Dopo Prima 1:4 CholineChloride-Urea Sulfammicacid: 30 g/L T: 150°C 0,3 -0,4 A/cm2

27. Q-factorresult 23

29. Differentflux Brokencathodes: the shapeofcathodeschangedtoget more uniformity on IL flux 25

30. Twopossibilitiesofflux 26 OUT OUT IN IN Fromcathodestoflanges Fromflangestocathodes

31. New shapedcathodes 27

32. Sulfamic Acid: fromflangestocathodes 28 Solution: 1:4 CholineChloride – Urea, Sulfammic Acid :30 g/L T:120-160°C The best surface quality appeared on bottom cutoff part.

33. Sulfammic Acid:fromcathodestoflanges The oppositeconfigurationbrings a lotofbubbles and the cavityweren’t electropolished 29

34. Alternative tosulfammic acid The best result on sampleswerereachedwithsulfammic acid, butcavityisquitedifferentenvironment Wechecked the performancesofvariousregulatorcontaining the group (–NHx) on samples 30

35. Comparisonbetweenregulator (–NH4) 31

36. AmmoniumPersulfate The addidionofAmmoniumPersulfatedecreases the high initialvoltagenecessarytodisrupt the oxidefilm Butthiscompoundincreases the roughness and pitting Possibility: can Sulfammic acid and AmmoniumPersulfate work together? 32

37. AmmoniumPersulfate and Sulfammic Acid On samples, agood compromise is the proportion 30 g/L ofSulfammic acid and 2.5 g/L ofAmmoniumPersulfate On cavity, the best concentrationwerefound mixing 1.5 g/L ofAmmoniumPersulfate and 30 g/L ofSulfammic Acid 33

38. Vertical EP: PA+SA Vertical ChCl:Urea 1:4 c(SA) = 30 g/l c(PS) = 1.5 g/l Distancebetweencathodes: 10 mm Bright, withfluxline 34

39. Vertical EP: PA+SA Vertical ChCl:Urea 1:4 c(SA) = 30 g/l c(PS) = 1.5 g/l Some irregularsurface Distance: 5 mm 35

40. Horizontal EP: PA+SA Horizontal ChCl:Urea 1:4 c(SA) = 30 g/l c(PS) = 1.5 g/l Distance from cathodes: 4 mm Some passivation zone 36

41. In future Set up offluxsistemof EP CalculationofQ-factorofILsEpcavities Test toget the result on 1.5 /1.3 GHz 37

42. Advantages and disadvantages 38

43. Thankstoattention! 39

44. Surface quality as function of current density 40

45. Surface quality as function of current density 41

46. Best results obtained in current density 0,33 A/cm2. This current density provides temperature equilibrium in range 150C, and stable yellow viscose film around the anode and “protect” from oxidizing. Distribution of current is similar on both sides which may give good surface quality inside cell and cutoff parts of the cell. 42 Ra – f(i)

47. Surface quality as a function of quantity Nb-ions in the melt 43

48. Surface quality as a function of quantity Nb-ions in the melt 44

49. Previous slide illustrates necessity of dissolved Nb ions inside the electrolyte. 45 Ra–f(C(NbDissolved))

50. From samples to Cavities… We tried to work in two geometrical performances: horizontal and vertical Horizontal EP didn’t give content results because of very fast temperature rising, after 1 minute temperature inside the cavity was more 190C which brought to degradation of electrolyte and to formation white viscous mass. In that places was done note a polishing but oxidizing. Cutoff part of cavity has enough good view. 46

51. Cavity after EP in IL: horizontal type 47 Cutoff part Cell part Cell part

52. Dissolving speed 6,2 um/min (in our laboratory on cavities in classical EP we have 0,5 um/min) 48

53. Conventions 49