Electric fields can be shielded with a Faraday cage, which may take the form of a Ground Chamber.
A Faraday cage (Wikipedia:Faraday_cage) can also filter electromagnetic radiation by disrupting the E-field component, but this is of less concern in SIMION, which is geared toward static and/or low frequency fields (not electromagnetic radiation where electric and magnetic fields are coupled).
Earth’s magnetic field can be a significant source of unwanted magnetic fields in your instrument and is often the motivation for shielding. Earth’s B-field varies in direction and by location roughly in the range 0.25-0.65 gauss (see Wikipedia:Earth%27s_magnetic_field).
You can simulate Earth’s B-field in SIMION by adding a magnetic PA to your workbench with a uniform, where the field is defined either in the PA itself (see Magnets) or it is zero in the PA and defined instead in an mfield_adjust user program segment (like in SIMION Example: poisson/pipe_with_beam pipe_beam_bfieldearth):
simion.workbench_program() local b = 0.5 -- gauss function segment.mfield_adjust() ion_bfieldx_gu = 0 ion_bfieldy_gu = 0 ion_bfieldz_gu = b end