I am trying to simulate the electron bunch dynamics in the particle in cell (PIC) solver of CST Studio suite, but I observe numerical Cherenkov radiation in the electron bunch (for high bunch charges) even when it is solely travelling in the vacuum. I have tried enhancing the resolution (decreasing the mesh cell sizes) and defining a dispersive media, but it is not enough to mitigate it. How do people doing PIC simulations, reduce it?