I use field maps to learn beam optics in exact magnetic field of my element. I input dense data in the magetic field, so the calculation speed is low. I need to repeat my element several times. Twice is OK. However, if I let the beam pass through the same element 3 times, the software tells me that "

**Reference particle outside the field map, from [element]... from [calc]...**". I wonder what is the reason behind this errror. Maybe 3 elements needs a large amount of calculation. 3 elements are separate. Once the particle come out from one, it enters the next element. All particles are not lost in my element. Why is this error related to the reference paricle?

BTW, I find the field map is so fast and convenient. Could you please clarify the algorithm? Is it diffrential algebra or Lie algebra? If the mesh of my field is 1mm, what is the accuracy of my results?

Kind regards,

Jie Li