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 this 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)