Harmonic gyroresonance of electrons in combined helical wiggler and axial guide magnetic fields

A driven-oscillator model is developed to examine the orbital-velocity harmonics of off-axis electrons in combined helical wiggler and axial guide magnetic fields. The gyroresonance effect is found to occur at harmonics of the wiggler frequency in addition to the well-known fundamental-harmonic resonance. Resonance at the first negative harmonic is potentially important for providing an explanation of the unexpected observations of a recent free-electron-laser experiment. Exploration of the gyroresonance effect as a harmonic selection mechanism for the free-electron laser is discussed. (Author)