Systematic measurement of maximum efficiencies and detuning lengths at the JAERI free-electron laser

We made a systematic measurement of efficiency detuning curves at several gain and loss parameters. The absolute detuning length (δL) of an optical cavity was measured within an accuracy of 0.1 μm around the maximum efficiency by a pulse-stacking method using an external laser. The FEL gain was controlled by the undulator gap instead of bunch charge, because we can change the gain rapidly while maintaining constant electron bunch conditions. For the high-gain and low-loss regions, the maximum efficiency is obtained at δL=0 μm and is larger than the value derived from the theoretical scaling law in the superradiant regime, while for the low-gain region the maximum efficiency is obtained for δL shorter than 0 μm and is similar to the scaling law.