Abstract
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.
Highlights
The lasing performance of a linac driven x-ray free electron laser (FEL) strongly depends on the beam quality and capability to preserve it all along the entire acceleration and compression of the electron beam
Performance and experimental results of the synchrotron radiation monitor (SRM) in operation at the first magnetic chicane of SwissFEL injector are reported in the present work
Equipped with a periscope optics and a sCMOS camera, the SRM can image in the visible (400–700 nm) the transverse profile of the light spot emitted by the electron beam entering the third dipole of the magnetic chicane to provide a measurement of the beam mean energy and energy spread from the knowledge of the chicane dispersion
Summary
The lasing performance of a linac driven x-ray free electron laser (FEL) strongly depends on the beam quality and capability to preserve it all along the entire acceleration and compression of the electron beam. As a complementary solution, a shot-by-shot and fully nondestructive method to estimate the electron bunch length is presented Such a method can be implemented in any dispersive section of a FEL linac equipped with a noninvasive diagnostics of the beam transverse profile such as a synchrotron radiation monitor (SRM) in a magnetic chicane. This noninvasive method to determine shot-byshot the electron bunch length was developed and experimentally tested at SwissFEL. Given the compression factor R56 and the dispersion function η of the magnetic chicane, a shot-by-shot and fully noninvasive estimate of the electron bunch length can be obtained from the measurement of the horizontal profile of the SRM light spot. For different bunch-compression settings of SwissFEL, SRM based measurements of the electron-bunch length are presented and compared with related TDS measurements
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