Electron spectrum measurements for a tapered-wiggler free-electron laser

An experimental investigation of basic aspects of the tapered-wiggler free-electron laser (FEL) is described. The electron energy distribution resulting from the interaction is measured in a single pass amplifier configuration using a CO2 laser. This distribution is consistent with an energy loss, for some electrons, equal to the change of resonant energy from the entrance to the exit of the wiggler. A net deceleration of 2.5 percent has been achieved, compared to an expected value of 4 percent. The discrepancy stems from an oversize electron beam which results in an imperfect electron-photon overlap and increased effective electron energy spread.