A direct electron beam energy spread measurement system for beam instability and FEL research

One of critical beam parameters for the storage ring based light sources is the energy spread of the electron beam. An accurate measurement of the energy spread remains a challenge. It is well known that the energy spread of the electron beam can degrade the spontaneous radiation spectrum of an optical-klystron (OK) which consists of two wigglers separated by a buncher magnet. The reduced modulation in the spectrum can be used to determine the energy spread of the beam. This paper describes our newly developed energy spread measurement system employing a compact and versatile spectrometer. This system is used in the beam instability research and free-electron laser (FEL) research. In particular, by measuring the increase of electron beam energy spread we have studied the onset of microwave instability.