Pickup concepts for ultra-low charged short bunches in X-Ray Free-Electron Lasers

TL;DR

This paper explores optimized pickup concepts for ultra-low charged, ultra-short electron bunches in X-ray free-electron lasers, aiming to enhance synchronization precision crucial for future high-resolution experiments.

Contribution

It presents a theoretical analysis and simulation of a new pickup design that significantly improves sensitivity for ultra-short, low-charge bunches in XFELs.

Findings

Simulated a new pickup concept with improved slope and voltage.

Reduced pickup distance and increased bandwidth enhance sensitivity.

The new design outperforms previous concepts in key metrics.

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.