Technical developments on reduced-β superconducting cavities at CERN

Several authors proposed the construction of superconducting proton linacs using the LEP2 cavities once LEP will be decommissioned. However only a fraction (about half) of these cavities can be used as they are for the high-energy part (β~1) of such a linac, the low energy part requiring the development of accelerating structures optimized for lower values of the particle velocity. At CERN an R&D programme on reduced-β single-cell cavities started in 1996 in order to study and explore the limits of the technology successfully used for the production of LEP2 cavities (copper cavities niobium-plated using the magnetron sputtering technique). Four different geometries were extensively investigated, each representing part of a multicell structure optimized for particles having β=0.48, β=0.625, β=0.66 and β=0.8 respectively. The results were encouraging for the last two types and therefore a new phase of R&D aimed at the production of multicell cavities for β=0.66 and β=0.8 was started. The goal is to demonstrate simultaneously the feasibility of such cavities and the possibility of producing them by low-cost modification of LEP cavities. In the paper, after a brief review of the results obtained on the single-cell cavities, we present in more detail the procedure for the transformation of the LEP cavities, which should allow a realistic estimate of the costs of such operation