Hello Yang I checked again with my field maps and I seem to be getting that we need to go 77mm in from the EFB to reach 3 parts in 10^4 . I use abs(B_mod-B_central/B_central) <.0003 for the criteria. This would imply the 1cm clearance for the beam is probably what we want. Let me know if this agrees with your calculations. Cheers Jim