Development of TESLA-type cavity at KEK

The development of superconducting cavity for high accelerating fields has been continued at KEK on the basis developed for TRISTAN superconducting cavity. Many attempts have been pursued to achieve high field and to understand the phenomena that limit the cavity performance at high field; investigations of niobium material, surface treatment, heat treatment, surface condition, cavity-shape and cavity-forming. Accelerating fields of more than 25 MV/m have been achieved repeatedly in the several single-cell cavities, even though we do not yet fully understand what happens at high fields. The present status will be reported.