INTRA-TRAIN LONGITUDINAL FEEDBACK FOR BEAM STABILIZATION AT FLASH

Abstract

The Free electron LASer at Hamburg (FLASH) is a linear accelerator of 330 m length. It provides laser pulses with pulse duration between 10 and hundreds of fs in the soft X-ray wavelength range below 5 nm produced in SASE process from electron bunches with an energy up to 1.2 GeV. FLASH works in pulsed mode with a repetition rate of 10 Hz where up to 800 bunches at a bunch spacing of 1 μs are accelerated in one macro-pulse. The electron beam time structure is well suited for fast intra-train feedbacks using beam based measurements incorporated to the Low Level Radio Frequency (LLRF) control system. The feedback allows further improving the bunch compressions, bunch arrival and bunch energy stability which directly impact the quality of the FEL photon beam. In this paper, we present the beam based signal preprocessing, the implementation into LLRF system, the mandatory exception handling for robust operation and the imbedding of the real-time ~ 2 μs latency fast intratrain feedback with feedbacks for the removal of slow and repetitive errors. First results of the achieved intra-train bunch arrival and peak current stability will be presented together with observed limitations.