SUPERCONDUCTING CAVITIES FOR PARTICLE ACCELERATORS - ACHIEVEMENTS ANDPROBLEMS

Superconducting RF accelerating cavities of different frequencies are presently operating on various accelerators world wide. The large majo rity of these cavities are made of niobium sheet, by lathe spinning an d welding. In one case only, i.e. LEP2 at CERN, the majority of caviti es is made of copper, internally coated with a thin layer of sputtered niobium. The main motivation for developing Nb-coated copper cavities must be found in the higher stability against quenching, consequent t o the higher thermal conductivity of copper at liquid helium temperatu res. Additional advantages are higher Q(0) values at low fields, insen sitivity to trapped earth magnetic field and lower cost. Beside these advantages, the Nb-coated cavities unfortunately suffer from a faster Q(0) degradation with increasing the accelerating field. The performan ces of Nb-sheet and Nb-coated cavities are critically compared on the grounds of the large industrial production required by the LEP2 projec t.