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Are you using a short-pulse source or a continuous wave single-frequency source? For the pulse source, you will have to iterate until enough of that energy has left your grid. For the CW source, you can do this visually or check to see if your amplitude swings are constant everywhere.
i use cw source but my domain is large and i cant visually check that.i want measure strength of signal in whole domain but i dont know after how many time steps i can do that.my question is if i run code for a very long time step and record fields value in all iteration and then taking maximum between all time steps in each grid cell,is this steady state value or after a very long time step the electric field value in last time step is steady state???
You probably don't want to store the field values for each iteration as that will quickly eat up memory. Instead, calculate the Fourier transforms as the code is running. Therefore, all you need to store is the Fourier transform at each point you are interested. You don't have to do this everywhere. Just pick a few strategic points and Fourier transform those.
Further, since your source is CW, you only have to integrate over one wave cycle to calculate the Fourier transform. Envision doing this at some interval when you think the simulation may be finished. If the Fourier transforms are different, you still have some transient phenomenon happening.
For more information on the Fourier transforms, see Lecture 7 here: