High Tc superconductors for accelerator cavities

Abstract

In this contribution the properties of YBa 2CU 3O 7−δ in high frequency fields are discussed in order to evaluate the potential of the high T c superconductors for particle accelerators. The performance of bulk material, of unoriented and oriented polycrystalline thick films electrophoretically deposited on silver substrates as well as those of epitaxial laser ablated films on single crystal SrTiO 3 were investigated between 3 and 86.9 GHz at the University of Wuppertal. The data are combined with results from Argonne National Laboratory to obtain information on the frequency dependence of the rf surface resistance of polycrystalline material between 243 MHz and 86.9 GHz. Single crystal results from Cornell University serve to give an idea of the instrinsic properties of the new materials. The temperature dependence of the surface resistance of YBa 2CU 3O 7−δ is compared to the one of the traditional superconductor Nb 3Sn, in order to elucidate similarities and differences and to point out the important role of grain boundaries in the comparably high surface resistance of YBa 2CU 3O 7−δ. The strong increase of the surface resistance with the applied magnetic rf surface field is discouraging at first sight. Results from Argonne on polycrystalline materials nevertheless show that superconductivity prevails up to rf fields of 640 G which, if obtained in a typical accelerating structure, would correspond to accelerating fields of 8 to 15 MV/m. In single crystals 100 G have been achieved at Cornell University without increased rf losses. At present the combination of all available data shows that considerations to use the new superconductors in particle accelerators are premature. It is nevertheless believed that the remarkable progress in materials preparation technique and the promising intrinsic properties of the perovskite superconductors justifies a continued effort toward this application. The strong reduction of the rf losses with the decrease in rf frequency makes the fabrication of a prototype YBa 2CU 3O 7−δ accelerating cavity for a low β accelerator a worthwhile goal. A silver cavity covered with an electrophoretically produced coating of YBa 2CU 3O 7−δ may be one of the next steps in this direction.