DESY, Hamburg
The Andrzej Soltan Institute for Nuclear Studies, Centre Swierk, Swierk/Otwock
The Free electron LASer at Hamburg (FLASH) is a linear accelerator of 330m length. It provides laser pulses with pulse duration between 10 and hundreds fs in the soft X-ray wavelength range below 5nm produced in SASE process from electron bunches with an energy up to 1.2 GeV. FLASH works in pulse mode with repetition rate of 10 Hz where up to 800 bunches at a bunch spacing of 1 us 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 of the accelerator structures to further improve the bunch compressions, bunch arrival and bunch energy stability directly impacting the quality of the FEL photon beam. In this paper, we present the beam based signal pre-processing, the implementation into LLRF system, the mandatory exception handling for robust operation and the imbedding of the real-time ~ 2us latency fast intra-train 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.
DESY, Hamburg
Electron bunches at the free electron laser FLASH at DESY have a duration of 10 fs to 150 fs and an arrival-time jitter of about 150 fs (rms). It is anticipated that the newly installed optical synchronisation system will stabilize the seed and pump-probe lasers to within ~10 fs. In order to perform reliable and stable seeding, the electron bunch timing jitter needs to be reduced. Bunch arrival-time monitors measure the arrival-time fluctuations at different locations and are used in a beam-based feedback loop to correct the amplitude of the accelerator RF. In order to provide reliable operation and high availability of the bunch arrival-time feedback, intensive efforts have been undertaken in system automation and exception handling. This will be discussed along with the latest results and limitations on the stability of the arrival-times at FLASH.