I am using PIC solver to simulate an electron beam in an interaction cavity. But I find all the modes are resonant at the same frequency if I use Fourier transform to process the signal of each mode observed at the output port (waveguide port are used here). Obviously, the results are not reasonable. So, I have checked the eigenmodes of the cavity by using eigenmode solver. And all the modes are separated in the simulation frequency range, which is much reasonable. My question is why in the PIC simulation, all the modes are concentrated at one frequency?

Edu ​​​​​​​CST Studio ​​​​​​​