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Symmetric grey box identification and distributed beam-based controller design for free-electron lasers
Citation Link: https://doi.org/10.15480/882.1191
Other Titles
Symmetrische Grey Box Identifikation und verteiltes strahlbasiertes Reglerdesign für Freie-Elektronen-Laser
Publikationstyp
Doctoral Thesis
Date Issued
2014
Sprache
English
Author(s)
Advisor
Title Granting Institution
Technische Universität Hamburg
Place of Title Granting Institution
Hamburg
Examination Date
2014-02-11
Institut
TORE-DOI
The European X-ray Free-Electron Laser (XFEL) at the Deutsches Elektronen Synchtrotron (DESY) in Hamburg will, starting in 2015, open up completely new research opportunities for scientist and industrial users by exploiting ultrashort X-ray laser pulses. Bunches of electrons are accelerated by a radio frequency field inside superconducting cavities up to an energy of 17.5 GeV. A periodic arrangement of magnets forces the accelerated electrons onto a tight slalom path leading to a process in that the electrons emit extremely short and intense X-ray flashes. The generation of equidistant X-ray flashes with a constant intensity requires an extremely high precision field control in combination with beam-based signals. FLASH, which can be seen as a pilot test facility, allows to develop and test controller concepts even before the European XFEL is in operation. In this thesis it is shown that a physical white box model structure, which describes the behavior of each subsystem within the radio frequency field control loop, obeys as first-order approximation the special orthogonal group of dimension two (SO(2)). Presented is a grey box identification approach, which combines the physical model structure with general identification methods. The accelerator modules are operated in a pulsed mode. Thus, the excitation of the system and therefore the identification of the input-output behavior is only possible within a short time period. Developed is an adaptive identification approach with a specified SO(2) symmetric model structure. The proposed controller design strategy fulfills the requirements of a high precision field performance. Adapting the feedforward signal by using an iterative learning control (ILC) algorithm reduces remaining repetitive field errors from pulse to pulse. It is shown, that exploiting the SO(2) symmetric structure and using the developed tensor based ILC representation simplifies the feedforward update computation. Magnetic chicanes, so-called bunch compressors, are used for a longitudinal compression of the electron bunches. Depending on the beam energy distribution, the electrons travel on different trajectories through the bunch compressor. This allows to control the mean beam energy and energy distribution to minimize the bunch arrival time and bunch compression error at the expense of a non-constant beam energy. Besides controlling the latter beam-based signals, a distributed control scheme is presented which minimizes beam energy variations by an information exchange with neighboring controller modules, leading to an improvement of the beam energy performance.
The presented results were achieved and measurements were carried out at FLASH. Further important plant upgrades for the XFEL project are a completely new hardware platform, providing a higher sampling rate and measurement precision. The proposed system identification and controller approaches have been validated experimentally and in simulation for both hardware platforms.
The presented results were achieved and measurements were carried out at FLASH. Further important plant upgrades for the XFEL project are a completely new hardware platform, providing a higher sampling rate and measurement precision. The proposed system identification and controller approaches have been validated experimentally and in simulation for both hardware platforms.
Subjects
European XFEL
FLASH
DESY
iterative learning control
DDC Class
620:Engineering and allied operations
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