Achievement of 35 MV/ m in the superconducting nine-cell cavities for TESLA

The Tera Electronvolt Superconducting Linear Accelerator (TESLA) is the only linear electron-positron collider project based on superconductor technology for particle acceleration. In the first stage with 500 GeV center-of-mass energy an accelerating field of 23.4 MV/ m is needed in the superconducting niobium cavities which are operated at a temperature of 2 K and a quality factor Q₀ of 10 ¹⁰. This performance has been reliably achieved in the cavities of the TESLA Test Facility (TTF) accelerator. The upgrade of TESLA to 800 GeV requires accelerating gradients of 35 MV/ m. Using an improved cavity treatment by electrolytic polishing, it has been possible to raise the gradient to 35- 43 MV/ m in single cell resonators. Here we report on the successful transfer of the electropolishing technique to multi-cell cavities. Presently 4 nine-cell cavities have achieved 35 MV/ m at Q₀⩾5×10 ⁹, and a fifth cavity could be excited to 39 MV/ m. In two high-power tests it could be verified that EP-cavities preserve their excellent performance after welding into the helium cryostat and assembly of the high-power coupler. One cavity has been operated for 1100 h at the TESLA-800 gradient of 35 MV/ m and 57 h at 36 MV/ m without loss in performance.