Validation of conformal finite difference time domain method for accurate higher order mode simulations

We have performed a detailed benchmark of a conformal finite difference time domain (CFDTD) method for applications in fusion gyrotron study. The accuracy and efficiency of simulations are the main concerns as a fusion gyrotron cavity is a computationally large scale model. Therefore, the validation of a simulation method is important for us to adopt it for the future study. Here we use a simple circular waveguide model for the benchmark purpose. A specially tailored wave launcher has been implemented for studying the higher order modes in a circular waveguide. All the dominated higher order modes could be excited at once for comparing with the exact solutions. The simulation time is chosen to be longer enough to retain the FFT accuracy for the comparison. It shows that the discrepancy between the CFDTD simulations and the exact solution is larger for higher order modes. We conclude that this is due to the dispersion error caused by the requirement of Dey-Mittra algorithm to reduce a time step for stability.