Theory and observations of microbunching instability in electron machines

Over the last years there have been several reports of quasiperiodic bursts of coherent synchrotron radiation (CSR) in electron rings in the microwave and far-infrared range. The observations were made on synchrotron radiation light sources which include the Synchrotron Ultraviolet Radiation Facility SURF II, VUV ring at the National Synchrotron Light Source at BNL, second generation light source MAX-I, BESSY II, and the Advanced Light Source at the Berkeley National Laboratory. General features of those observations can be summarized as follows. Above a threshold current, there is a strongly increased radiation of the beam in the range of wavelengths shorter than the bunch length, λ < σ₂. At large currents, this radiation is observed as a sequence of random bursts. In the bursting regime, intensity of the radiation scales approximately as square of the number of particles in the bunch, indicating a coherent nature of the phenomenon.