In the framework of the recent FLASH2020+ upgrade program, the longitudinal electron beam diagnostics of the FLASH accelerator had been modernized and extended by additional devices, including an electro-optical bunch length detector (EOD[1]), as well as an additional bunch compression monitor (BCM [2]) and a bunch arrival-time monitor (BAM [3]) in the new direct seeding beamline FLASH1. Also, the THz intensity spectrometer (CRISP [4]) received a modernized control interface that will allow non-experts to perform bunch profile measurements. The paper presents an overview on the current status of the longitudinal electron beam diagnostics at FLASH and the ongoing (re-)commissioning.

Pump-probe experiments at Free-Electron Laser facilities depend heavily on the relative timing precision of the pump and the probe, which determines the resolution of the observed ultrafast phenomena. The DESY's Bunch Arrival Time Monitors (BAM) are state-of-the-art sensors based on an electro-optical detection principle, that delivers information on the bunch timing with unprecedented femtosecond-level precision. Timing synchronization at machine level is achieved through a complex system of arrival time sensors, stabilized optical distribution, and feedback controls. Major advances in the performance, construction, and operation of the BAMs are discussed in detail. Integration of the sensor into the synchronization system, and important global optimization and interplay are mentioned as well. These improvements enabled a synchronization of the electron beam with a world-leading precision of less than 3fs at European XFEL.