A New Flange Design for the Superconducting Cavities for TESLA

In order to achieve accelerating gradients of 15-25 MV/m with the superconducting cavities in the TESLA Test Facility, considerable effort is put into preparing the cavities and the auxiliary components to avoid field emission due to particle contamination. The particle cleanliness has to be preserved throughout the whole assembly procedure of the cavities. Most crucial is the mounting of the flange connections which have to be UHV-leaktight with the cavities immersed in superfluid helium. In the present design, the niobium cavities are fabricated with Nb sealing surfaces with a thickness of 3 mm. Split stainless steel rings and spring-type gaskets (Helicoflex) are used to provide UHV-tight connections to the stainless steel counter flanges. This design has several disadvantages. The gaskets can not be properly made “pa rticle-free” and the handling of the split flanges m ay contaminate the cavities during assembly. Moreover, the sealing surfaces of the Nb cavities get quite soft during the 1400˚C heat treatment, which often results in leak problems due to plastic deformations. An alternative approach are NbTi-flanges directly welded to the Nb cavities. They should allow an easier assembly of the counter flanges. Various types of spring-less gaskets are under investigation and the results will be presented.