  In recent synchrotron radiation facilities, the use of short (picosecond)
electron bunches is a powerful method for producing giant pulses of Terahertz
Coherent Synchrotron Radiation (THz CSR). Here we report on the first direct
observation of these pulse shapes with a few picoseconds resolution, and of
their dynamics over a long time. We thus confirm in a very direct way the
theories predicting an interplay between two physical processes. Below a
critical bunch charge, we observe a train of identical THz pulses (a broadband
Terahertz comb) stemming from the shortness of the electron bunches. Above this
threshold, a large part of the emission is dominated by drifting structures,
which appear through spontaneous self-organization. These challenging
single-shot THz recordings are made possible by using a recently developed
photonic time stretch detector with a high sensitivity. The experiment has been
realized at the SOLEIL storage ring.
