Investigations on hydrogen contamination of superconducting cavities

All 16 cavities of the energy upgrading program for the HERA storage ring showed increased losses on the second or any successive cool down in contrast to superior values on the first cool down. The authors attributed this deterioration to hydrogen, which contaminates the niobium during chemical polishing. The low oxygen content of the new niobium material, which has a residual resistance ratio (RRR)=300, allows the hydrogen to diffuse to the surface and to produce NbH precipitation at low temperatures. The hydrogen degassing process at 800 degrees C cannot be applied to these cavities because of the complex construction. The authors investigated chemical polishing methods with respect to hydrogen contamination and tried to find a hydrogen cleaning procedure at room temperature.<>