Higher-order nonlinear correction of electron bunch compression toward laser power enhancement at SACLA

In order to increase pulse energy of an x-ray free-electron laser light, the electrons that contribute x-ray laser emission must be increased, namely the bunch length of the electron beam must be expanded longitudinally while keeping a high peak current and low slice emittance. For this purpose, longitudinal phase space distribution of the electron beam must be linearized by correcting the nonlinearities that are generated in the RF acceleration- and the bunch compression-processes. At SACLA, the second-order nonlinearity is corrected by a C-band correction cavity that is located at the end of the injector section; however, the third-order nonlinearity has not been corrected concretely. In this paper, we propose a new method to correct the third-order nonlinearity of the longitudinal phase space by an octupole magnet that is inserted at a dispersive section of a bunch compressor chicane. (author)