Design of the JLC/NLC RDDS Structure Using Parallel Eigensolver Omega3p

The complexity of the Round Damped Detuned Structue (RDDS) for the JLC/NLC main linac is driven by the considerations of rf efficiency and dipole wakefield suppression. As a time and cost saving measure for the JLC/NLC, the dimensions of the 3D RDDS cell are being determined through computer modeling to within fabrication precision so that no tuning may be needed once the structures are assembled. The tolerances on the frequency errors for the RDDS structure are about one MHz for the fundamental mode and a few MHz for the dipole modes. At the X-band frequency, these correspond to errors of a micron level on the major cell dimensions. Such a level of resolution requires highly accurate field solvers and vast amount of computer resources. A parallel finite-element eigensolver Omega3P was developed at SLAC that runs on massively parallel computers such as the Cray T3E at NERSC. The code was applied in the design of the RDDS cell dimensions that are accurate to within fabrication precision. We will present the numerical approach of using these codes to determine the RDDS dimensions and compare the numerical predictions with the cold test measurements on RDDS prototypes that are diamond-turned using these dimensions.