Hi, thank you for your reply. The simulation time is sufficiently long, I made sure that the autoshutoff level was reached before reaching the end of the simulation time. I see your first point makes some sense too. The strucutre of the first figure consists of three curved arcs and two straight waveguide with the same cross-section, would that be a problem if there is an air gap (~10nm wide) between two aircs? Thank you!