HARMONIC RESONANCE OF ELECTRONS IN COMBINED HELICAL WIGGLER AND AXIAL-GUIDE FIELDS

The single-particle orbits of relativistic electrons in combined helic al 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 o f off-axis, electrons has been shown and analyzed. This resonance is f ound to occur in velocity space of electrons with a beat between OMEGA 1 and 2OMEGA2 (OMEGA1 and OMEGA2 are the harmonic of wiggler motion an d the cyclotron frequency respectively in a frame rotating with the wi ggler), in other words, there is a harmonic resonance at 2OMEGA(parall el-to)BAR = 3K(w)nu(parallel-to) (OMEGA(parallel-to)BAR is cyclotron f requency, K(w)nu(parallel-to) is the wiggling frequency). This resonan ce depends on the radial displacement of the electron from axis, but w ithout any large radial drifts in position space, which provides an ex planation of a dip in radiation power that has been observed in the ex periment.