  Emission of light by a single electron moving on a curved trajectory
(synchrotron radiation) is one of the most well-known fundamental radiation
phenomena. However experimental situations are more complex as they involve
many electrons, each being exposed to the radiation of its neighbors. This
interaction has dramatic consequences, one of the most spectacular being the
spontaneous formation of spatial structures inside electrons bunches. This
fundamental effect is actively studied as it represents one of the most
fundamental limitations in electron accelerators, and at the same time a source
of intense terahertz radiation (Coherent Synchrotron Radiation, or CSR). Here
we demonstrate the possibility to directly observe the electron bunch
microstructures with subpicosecond resolution, in a storage ring accelerator.
The principle is to monitor the terahertz pulses emitted by the structures,
using a strategy from photonics, time-stretch, consisting in slowing-down the
phenomena before recording. This opens the way to unpreceeded possibilities for
analyzing and mastering new generation high power coherent synchrotron sources.
