Nonlinear Mode Competition and Coherence in Low Gain FEL Oscillators

Nucl. Instrum. Methods Phys. Res. Sec. A 285, 136 (1989)https://ireap.umd.edu/10.1016/0168-9002%2889%2990440-31989Journal ArticlePlasma, Accelerator, and Nuclear Physics

One of the most important problems in the design of high power generators of coherent radiation is the competition of many eigenmodes of the resonator for the beam energy. A simple one dimensional, multimode model for a low gain FEL oscillator is used to study numerically the effect of mode competition on the coherence of the radiation. For a beam current smaller than four times the minimum start-oscillation current many single mode states are possible. However, the time required to reach these states becomes very long for FEL devices described by a small slippage parameter. Two different time scales characterize the process of reaching a single mode state. The shorter time scale is related to an establishment of a relative constant amplitude of the radiation field, while the phase of the field is still modulated. The relaxation of the phase takes a much longer time. The relevance of these results to the UC Santa Barbara FEL experiment is discussed.