Step II - The Electron Beam
After the energy, energy spread and emittances are defined a good value for the beam size and the twiss parameters ALPHAX, ALPHAY are critical for a good performance. GENESIS 1.3 does not support matching and these values have to be found by external programs. Anyhow if the FODO lattice start with half a quadrupole or the field strength of all quadrupoles is set to zero than the beam is going through a waist at the undulator entrance for best matching (ALPHAX, ALPHAY=0). In this case initial runs of GENESIS 1.3 allows for an iterative method to find the right values for the beam size while ignoring the radiation results.
It is recommended that the number of macro particles (NPART) is at least 2048 (powers of two numbers allows a better performance). For simulation of wavelengths below 10 nm the number must be even higher to exclude numerical noise.