A single‐particle computer model is used to calculate the electron trajectories in combined helical wiggler and guide fields. The analysis considers the effects of a realizable wiggler field. Special attention is focused on the stability of the electron orbits near magnetoresonane. Upon calculating Poincaré surface‐of‐section plots and Lyapunov exponents, it is shown that the electron dynamics is chaotic. Analyzing the spectra of spontaneousemission, it is found that regular and chaotic orbits might be discriminated by optical measurements: off resonance, the spectra consist of well‐defined lines. On the other hand, near magnetoresonance, chaotic orbits are revealed by noisy spectra concentrated at low frequencies, and a sudden breakdown of the integral radiant energy.

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