EFFECT OF WIGGLER ERRORS ON FREE-ELECTRON-LASER GAIN

For a free-electron laser (FEL) operating in the exponential regime before saturation, we present an analytic description of the effect on the gain of longitudinal velocity variations arising from wiggler field errors. The average gain reduction and the width of the output power distribution are expressed in terms of the mean-square average of the ponderomotive phase shift per gain length. A scheme for correcting the electron trajectory using position monitors and dipole correctors is analyzed. Our work is directly applicable to the design of FEL amplifiers, and the results are encouraging for their feasibility.