When the scaled growth rate of a free-electron laser (FEL) is plotted as a function of the scaled thermal velocity for the case of ideal beam compression, we find two distinct cases. If the FEL is in the energy-spread-dominated regime, the growth rate can be improved as a result of compression provided the scaled thermal velocity is less than unity. Surprisingly, we find that the growth rate is always improved if the FEL is in the emittance-dominated regime.
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Roberson et al. (1994) studied this question.
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