Development of High Pressure Ultrapure Water Rinsing
Superconducting niobium rf cavities have suffered and are still suffering from field emission loading at high electric surface fields caused by artificial contamination of the sensitive rf surfaces. Field emission will degrade the Q-value of sc cavities and limit the achievable gradients. Reduction or elimination of this contamination will result in improved cavity performance and a great deal of effort has been invested in improving cleaning and assembly procedures. One method, which has been pursued at Jlab after the initial reporting of its benefits [1] is high pressure ultrapure water rinsing (HPR), during which the interior cavity surfaces are repeatedly sprayed with ultra pure water of up to ~ 100 bar.[2]. This method turned out to be very beneficial and is being used presently by all laboratories involved in SRF technology as a final step in the surface preparation of sc cavities.
[1] Ph. Bernard et al.;Proc. 3 rd European Part.Acc.Conf., Berlin, Germany (1992), p. 1269
[2] P. Kneisel and B. Lewis; Proc. 7 th Workshop on RF Superconductivty, Gif sur Yvette, France (1995), p.311
Development of Post-purification of Niobium in the Presence of Titanium as a Solid State Getter Material
It had been well established by 1985, that defects in the niobium were the causes for pre-mature quenches in sc rf cavities. Thermal model calculations in different laboratories [e.g.1] showed that these defects – if not being eliminated – could be stabilized by improved thermal conductivity of the cavity material. The threshold field for “quench” increases proportionally to the square root of RRR (residual resistivity ratio), which is proportional to the thermal conductivity.
Improvements in the thermal conductivity of niobium can be achieved by multiple electron beam melting under good vacuum condition, which reduces the amount of interstitial impurities such as hydrogen, nitrogen, oxygen and carbon. Further improvements can be gained by “solid state” gettering, during which the niobium is heat treated under vacuum in the presence of a material such as Yttrium [2] or Ti [3] with higher affinity to these interstitial impurities than niobium.
Titanium has become the material of choice for post – purification of niobium and under certain conditions, RRR value > 1000 could be achieved.
[1] G. Mueller; Proc. 3 rd Workshop on RF Superconductivity, Report ANL-PHY-88-1, Vol I,p.331, Argonne National Lab (1987)
[2] H. Padamsee; Proc. 2 nd Workshop on RF Superconductivity, CERN, Geneva, Switzerland (1984), p. 339
[3] P. Kneisel; Journ.Less Comm.Metals 139, 179 (1988)
Q-disease Investigations
In 1989 R. Roeth [1] observed at the University of Wuppertal a significant degradation of the Q-value of a 1500 MHz cavity made from high purity niobium after it had been kept at nitrogen temperature for several days after the initial test in superfluid helium. A temperature map indicated that these unexpected losses were distributed uniformly over the whole cavity surface. The initial hypothesis expressed in [1] of precipitation of the Nb-hydride phase was supported by a variety of subsequent experiments in various laboratories around the world.
Jlab contributed to the understanding and elimination of this phenomenon by e.g., investigating the dangerous temperature region, in which the losses occur, and by quantifying the length of time in this temperature region for the losses to take place [2].
[1] R.Roeth et al.; Proc. 2 nd European Part. Acc. Conf., Nice, France,Vol.2, p. 1097 (1990)
[2] K. Saito and P. Kneisel.; Proc. 3rd European Prt.Acc.Conf. EPAC 1992 , p. 1231 (1992)
Development of In-situ Baking Procedures
RF cavities made from high purity niobium often show a degradation of the Q-value at higher gradients in the absence of field emission (Q-drop). Improvements in this behavior were first reported [1] by in situ baking at temperatures > 100C for longer periods of time. Such Q-drop was never seen in electropolished cavities tested at KEK [2], where routinely the cavities were baked at 85C for a day in order to improve the cavity vacuum.
In-situ baking was systematically investigated at Jlab [3] -among other labs- and the effect of baking temperature and duration of baking on cavity performance for both chemically polished and electropolished cavities was explored. Besides improvements in the BCS surface resistance often improvements in quench fields were found. It was also demonstrated that the surface layer effected by the in situ baking and having modified material parameters reached to a depth of ~3000 Angstrom; after removal of this layer the “original” niobium material parameters were restored.
[1] P. Charrier,B. Coadou and B.Visientin; Proc. EPAC 1998, p. 1885
[2] K. Saito et al.;Proc. 8 th workshop on RF Superconductivity, Report LNL-INFN(Rep) 133/98, p. 830, Abano Terme, Italy (1997)
[3] P. Kneisel, -Proc. 9 th Workshop on RF Superconductivity, Report LA – 13782-C, Vol.1, p. 328, Santa Fe, New Mexico (1999)