The IR Upgrade design at Jefferson Lab uses an optical klystron in order to enhance the flexibility of the free-electron laser system and to match the efficiency to the energy recovery lattice. Most optical klystrons operate with a strong dispersion section (1). The IR upgrade design requires operation with a dispersion of only a few periods in order to allow the full range of efficiency of the FEL to be explored. This paper will study the details of an optical klystron in this small dispersion limit. The peak gain vs. dispersion section strength has an oscillatory behavior, suggesting that the dispersion section strength should be adjusted in unit steps rather than continuously. The gain vs. the effective number of periods is calculated and found to be, on average, in good agreement with theory. Finally, some comments on the relative merits of using an optical klystron or a uniform wiggler in a high power FEL will be presented.
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