Abstract: In FEL experiment with a bifilar helical wiggler, the end effect of the wiggler magnetic field caused by the lack of helical symmetry at the entrance region can se- riously influence the e-beam transportation and destroy the e-beam quility, how to releave and remove the end effect is one of main subjects in Raman FEL experiment.
In this paper, we have investigated the adiabatical entrance region of the bifilar helical wiggler with staggered loops. Numerical simulation and experiment measurement of the magnetic field at the entrance region with the loops show that the feld can adiabatically increase from zearo to constant value of the wiggler with good symmetry, and this well fit the requirement ofadiabatically transporting e- beam to the interaction re- gion, the results of e-beam transportation experiment show that about 350A e-beam current from SooA at entrance is transported to interaction action region, however, in the same experiment conditions, only about l20A e-beam has been observated at the interaction action region of helical wiggler with flaring tapper entrance.