  At SwissFEL, a fully non-invasive characterization of the beam energy
distribution can be performed by means of a Synchrotron-Radiation-Monitor (SRM)
imaging in the visible the transverse profile of the electron beam in a
magnetic chicane. Under conditions of off-crest acceleration in the SwissFEL
injector, according to a first order Taylor series expansion, the distribution
of the relative energy of the electron beam can be linearly expressed as a
function of the distribution of the electron longitudinal coordinates via the
coefficient of the rf induced beam energy chirp. It is hence possible to
express the distribution of the electron longitudinal coordinates at the
entrance of the magnetic chicane of SwissFEL as a function of the distribution
of the dispersed electron trajectories in the horizontal plane of the magnetic
chicane. Machine parameters and instrument data at SwissFEL can be
beam-synchronously acquired. A shot-by-shot correlation between the analysis
results of the SRM camera images and the linear coefficient of the beam energy
chirp resulting from the rf parameters of the injector can be hence
established. Finally, taking into account the compression factor of the
magnetic chicane, the electron bunch length at the exit of the magnetic chicane
can be expressed as a function of the horizontal size of the electron beam
imaged by the SRM. Bunch-length measurements performed by means of the SRM in
the first magnetic chicane of SwissFEL will be presented.
