Superiority of Electropolishing over Chemical Polishing on High Gradients

Electropolishing (EP) offers a smoother surface, and that makes us expect high gradients in superconducting niobium cavities. EP has a little bit complicated procedure and needs annealing to avoid the hydrogen Q-disease. On the other hand, chemical polishing (CP) is easier and cheaper, so widely used in many laboratories. For the conventional field gradients (,z~ 5 MV/m) in storage rings (TRISTAN-MR, HERA), no difference was seen on the cavity performance between EP and CP. In the high gradients research for TESLA, we have to establish the surface treatment method which has a high reliability on the high gradients over than 25 MV/m and is cheaper in the preparation cost. So far, CP has offered a nice cavity performance, but a concern on "European Headache" was developed recently on the high gradients at Saclay. KEK has intensively investigated the difference between the effect of CP and EP on high gradient performance since the last SRF workshop at Saclay. In this investigation "European Headache" was also seen in chemically polished cavities at KEK; in addition these cavities often showed a new Q-degradation without X-ray signals which seriously limited the high gradients. EP offers reliably high gradients over 30MV/m. At the moment, EP seems to have a superiority over CP for high gradients over 25 MV/m.