Traditionally, superconducting cavities for large electron position colliders are fabricated from high purity niobium (Nb) sheets using lathe spinning and welding. These cavities suffer from the problem of thermomagnetic breakdown resulting from the relatively poor thermal conductivity of Nb at liquid helium temperatures. A more elegant and low cost solution to the breakdown problem is the use of Nb coated high purity copper (OFHC copper) superconducting cavities. However, the low field Q values of 2 x 109 at 4.2 K dropped rapidly with increasing field; for example at 3 MV/m the Q = 3 x 108. The loss in the Q value of Nb-coated copper cavities may be attributable to the columnar grain structure, low packing density, and surface roughness of the Nb coating. In contrast to sputtering, chemical vapor deposition (CVD) will allow deposition of excellent quality films on substrates with complex geometries, such as superconducting cavities, with relative ease. In addition, CVD of Nb will possibly give dense, smooth, and fine grain Nb films, which will definitely have much better properties than those obtained by sputtering. The Phase I objectives are: a) to determine the best precursor for CVD of Nb films on copper substrates, and b) to optimize the CVD deposition conditions for Nb films on copper substrates. The Phase II objective is to build a Nbcoated copper superconducting cavity by CVD. Anticipated Results /Potential Commercial Applications as described by the awardee:This project provides an alternative solution to the problems encountered in superconducting cavities for large electron positron colliders. In addition to the accelerator application, Nb coatings are widely used for nuclear fuel particles.
