Microbunch rotation in an x-ray free-electron laser using a first-order achromatic bend

Electrons in a x-ray free electron laser (XFEL) develop periodic density fluctuations, known as microbunches, which enable the exponential gain of x-ray power in an XFEL. When an electron beam microbunched at a hard x-ray wavelength is kicked, microbunches are often washed out due to the dispersion and R56 of the bend. An achromatic (dispersion-free) bend with small R56, however, can preserve microbunches, which rotate to follow the new trajectory of the electron bunch. Rotated microbunches can subsequently interact in a repointed undulator to produce a new beam of off-axis x-rays. In this talk, I show a demonstration of hard x-ray multiplexing in the Linac Coherent Light Source (LCLS) Hard x-ray Undulator Line (HXU) using microbunch rotation through a 10 microradian first-order-achromatic bend created by transversely offsetting quadrupole magnets in the FODO lattice. Quadrupole offsets are determined analytically from beam-matrix theory. I also discuss the application of microbunch rotation to out-coupling a cavity-based XFEL (CBXFEL).