FIRST EXPERIENCE WITH DRY-ICE CLEANING ON SRF CAVITIES

The surface of superconducting (s.c.) accelerator cavities must be cleaned from any kind of contamination, like particles or chemical residues. Contaminations might act as centers for field emission, thus limiting the maximum gradient. Today's final cleaning is based on high pressure rinsing with ultra pure water. Application of dry-ice cleaning might result in additional cleaning potential. Dry-ice cleaning relies on the sublimationimpulse method and removes particulate and film contaminations without residues. As first qualifying step intentionally contaminated niobium samples were treated by dry-ice cleaning. It resulted in a drastic reduction of the particle numbers and of DC field emission up to fields of 100 MV/m. The dry-ice jet caused no observable surface damage. First cleaning tests on single-cell cavities showed Q-values at low fields up to 4x10 at 1.8K. Gradients up to 33 MV/m were achieved, but field emission still is the limiting effect. Further tests are planned to optimise the dry-ice cleaning technique.