“…Finally, the average and peak power are, to a large extent, determined by the accelerator technology. Superconducting energy recovery linacs are the primary drivers for high average power FEL oscillators [64][65][66][67][68][69][70]; however, other types of FEL oscillators have used various techniques to increase the output pulse energy including, but not limited to, optical klystrons [34,36,48,57,58,60,63,[71][72][73][74][75][76], cavity dumping [77][78][79][80][81], pulse stacking in an external cavity [82][83][84], electron beam energy ramping [85,86] or dynamic cavity desynchronization [87][88][89][90][91][92]. Tapering of the undulator [93][94][95] has also been used to improve the peak power in the pulse [96][97][98][99][100], however, tapering of the undulator in FEL oscillators leads to complicated dynamics and a strong dependence on system parameters…”