Sheet-beam wave interaction modeling using NEPTUNE

Summary form only given. We describe recent progress in the development of the NEPTUNE code (nonlinear evolution of particle trajectories under non-stationary excitation) for large-signal simulation of sheet-beam amplifiers. The NEPTUNE model uses a new formulation of the ADI-FDTD method to represent three- dimensional electromagnetic fields in the beam tunnel of a sheet-beam structure. Recently, two new features have been introduced. First, a model for particle trajectory integration has been implemented to represent a spatially distributed beam. This is based on a related scheme used previously in the successful TESLA code for round-beam amplifier simulation. Second, a number of additional boundary conditions have been implemented to allow for coupling to either external resonators or waveguides for incoming and outgoing waves. We describe the models used and verify simulation results for a number of examples. Finally we discuss ongoing efforts to optimize the algorithms and improve the performance of the code.