Effects of Sextupolar Undulator Magnetic Field Contributions on Inverse Free Electron Laser Accelerator

In this paper, we include the important effects of sextupolar field contributions on saturation gain and length of a linear polarized undulator driven inverse free electron laser (IFEL) that arise when the electrons are injected off axis. The modified IFEL equations are derived and analyzed. Both the accelerating gradient and the saturation length of the IFEL are influenced by the imperfect beam trajectory. For a fixed laser wavelength, the effects are more pronounced in the case of short period undulator in comparison with a larger period undulator IFEL. A useful empirical fit is prescribed to estimate the tolerance of the IFEL for imperfect beam trajectory.