Broadband Self-amplified Spontaneous Coherent Synchrotron Radiation in a Storage Ring

Bursts of coherent synchrotron radiation at far-infrared and millimeter wavelengths have been observed at several storage rings. A microbunching instability has been proposed as the source for the bursts but the mechanism has yet to be elucidated. We provide the first evidence that the bursts are due to a microbunching instability driven by the emission of synchrotron radiation in the bunch. Observations made at the Advanced Light Source are consistent with the values predicted by a model for the instability proposed by Heifets and Stupakov. These results demonstrate a new instability regime for high energy synchrotron radiation sources and will impact the design of future sources.