Abstract
The all-optical synchronization systems used in various X-ray free-electron lasers (XFEL) such as the European XFEL observe the transient fields of passing electron bunches coupled into one or more pickups in the Bunch Arrival Time Monitors (BAM). The extracted signal is then amplitude modulated on reference laser pulses in a Mach-Zehnder type electro-optical modulator. With the emerging demand for future experiments with ultra-short FEL shots, fs precision is required for the synchronization systems even with 1 pC bunches. Since the sensitivity of the BAM depends in particular on the slope of the bipolar signal at the zero-crossing and thus, also on the bunch charge, a redesign with the aim of a significant increase by optimized geometry and bandwidth is inevitable. In this contribution the theoretical foundations of the pickup signal are aggregated and treated with a focus on ultra-short bunches as well as a general formulation. A possible new pickup concept is simulated and its performance is compared to the previous concept. A significant improvement of slope and voltage is found. The improvement is mainly achieved by the reduced distance to the beam and a higher bandwidth.
Highlights
Free-electron lasers (FELs) became an important light source for experiments in various fields since they provide short pulses with extreme brilliance in atomic length and time scales [1]
The all-optical synchronization systems used in various x-ray free-electron lasers (XFEL) such as the European XFEL usually include bunch arrival-time monitors (BAM), which observe the transient fields of passing electron bunches through their pickups
A tremendous improvement in synchronization stability and BAM resolution, surpassing rf techniques, and a reduction of arrival time as well as energy jitter was achieved by the implementation of an all-optical synchronization system with two different feedback loops [11], which was developed at the Deutsches ElektronenSynchrotron (DESY)
Summary
Free-electron lasers (FELs) became an important light source for experiments in various fields since they provide short pulses with extreme brilliance in atomic length and time scales [1]. A tremendous improvement in synchronization stability and BAM resolution, surpassing rf techniques, and a reduction of arrival time as well as energy jitter was achieved by the implementation of an all-optical synchronization system with two different feedback loops [11], which was developed at the Deutsches ElektronenSynchrotron (DESY). For the Pohang Accelerator Laboratory (PAL-XFEL) a resolution of 25 fs has been reported in 2016 for the type 1 and type 2 cavities combined with the low level rf [12] These values were found in a 20 min measurement with 200 pC bunches at the PAL-XFEL injector test facility [12]. Examining the measured arrival times for a period of 1 min gave a timing jitter of approximately 6 fs rms caused by the BAM resolution and critical parts of the reference laser distribution system [22]. While none of these designs could fully achieve the desired improvement they still offer potential for further optimization
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