Longitudinal diagnostics for beam-based intra bunch-train feedback at FLASH and the European XFEL

Diagnostics and control of the electron beam parameters in order to provide stable and reproducible experimental conditions for the photon users are a crucial part of operating a free-electron laser. For the beam-based longitudinal feedback at FLASH and the European XFEL, three monitor types are discussed: bunch arrival time monitor (BAM), bunch compression monitor (BCM) and synchrotron radiation monitor (SRM). The in uence of the amplitudes and phases of the accelerating modules on the electron beam properties is studied analytically, with simulations and in measurements. For the simulations, models of the monitors are integrated into an existing longitudinal particle tracking code and a beam-based feedback framework is developed. An upgrade of the electro-optical units for the BAMs at FLASH and the European XFEL as well as the development of a BCM for the European XFEL are required. All monitors need to be installed, commissioned and characterised. The bunch pattern at FLASH and the European XFEL allowing for the generation of many thousand photon pulses per second with repetition rates up to several megahertz makes great demands on the beam diagnostics and controls. An intra-train feedback is required for stabilising the electron beam parameters along the bunch train. The feedback performance must be evaluated and the control parameters need to be optimised.