The singlehyphen;particle orbits of relativistic electrons in combined helical wiggler and axial guide magnetic fields have been studied. Based on the numerical simulations of the Raman free electron laser experiment in Shanghai Institute of Optics and Fine Mechanics, a new resonance of offhyphen;axis electrons has been shown and analyzed. This resonance is found to occur in velocity space of electrons with a beat between OHgr;1and 2OHgr;2(OHgr;1and OHgr;2are the harmonic of wiggler motion and the cyclotron frequency respectively in a frame rotating with the wiggler), in other words, there is a harmonic resonance at 2OHgr;macr;par;=3Kwngr;par;(OHgr;macr;par;is cyclotron frequency,Kwngr;par;is the wiggling frequency). This resonance depends on the radial displacement of the electron from axis, but without any large radial drifts in position space, which provides an explanation of a dip in radiation power that has been observed in the experiment.
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