The future of surface treatment technologies for high field niobium superconducting cavities

The recent applications of superconducting cavities are directed to a TeV energy linear accelerator (TESLA) which needs the very high field gradient of > 25 MV/m and a very high current machine for B-factory which requires to accelerate the current of several amperes. On the second item, how to damp strong higher order modes which lead to beam instabilities is the main subject and the main R and D is not in cavity itself with present technologies. However, the first item requires to upgrade much more present technologies with niobium materials, cavity fabrication and preparations. This paper concentrates our interests on the first item and reports on efforts to improve cavity performances. As technologies for high field, discussions will be focused on surface treatments like chemical polishing, electropolishing, anodizing, high pressure water rinsing and ultra high vacuum annealing. The acceleration field gradient is often limited to 10-20 MV/m in L-band niobium cavities by conventional wet surface preparations. These limitations are due to field emission and two sides multipacting. Especially multipacting is a serious problem. At first concerning to multipacting, the field limitation of TRISTAN sc is reconsidered. It is presented that two sides first order multipacting level exists at 7 MV/m for KEK shape and it corresponds to 20 MV/m for L-band cavities by frequency dependence of multipacting. If the two sides multipacting is suppressed by cavity geometry, attainment of the field gradient of 25 MV/m in L-band cavities by conventional wet treatments will be no problem. (author)