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Automatic Control Strategies for the Swiss Free Electron Laser


A. Rezaeizadeh


This thesis illustrates the application of automation and control in high-energy physics (HEP) discipline with the goal of enhancing the performance. In particular, the control methods are developed for the Swiss Free Electron Laser (SwissFEL) machine, but they are transferable to other systems as well. The SwissFEL at Paul Scherrer Institut (PSI) in Villigen will, by the end of 2018, produce very bright and extremely short X-ray pulses enabling novel experimental capabilities in various areas of science. An injector system, triggered by an optical laser, generates an electron bunch with an energy of 330 MeV. The electron bunch is compressed and then accelerated by a pulsed radio frequency (RF) driven linear accelerator (linac). The resulting relativistic electron beam will have an energy up to 5.8 GeV and will be passed through the FEL magnetic undulator lines. The undulator drives the electron beam pulse in a transverse oscillation in synchrony with a self-amplified spontaneous emission (SASE) generated photon beam. Throughout this process, a part of the electron beam energy is transferred to the photon beam resulting in a short, intense X-ray pulse. The motor-controlled undulators give this opportunity to produce coherent optical pulses with wavelengths ranging from 1 to 7 Ň in one undulator (referred to as Aramis) and 7 to 70 Ň in the other (referred to as Athos). The injector and linac RF drives operate in a pulsed mode using normal conducting accelerating structures. The pulsed mode of operation gives a compact and efficient design but introduces higher requirements on the stability and repeatability of the electron beam control system, particularly for the injector and linac RF drives. The RF accelerating structures will be operated in a pulsed mode as power consumption and heat dissipation prevents the use of the more easily controlled continuous wave operation mode. One known detrimental factor is the strong temperature dependence on the characteristics of many components of the system. Even though the temperature is controlled to an extent, the remaining variability is too large to meet the beam stability (repeatability) specifications. In addition, less well understood effects are also likely to reduce the electron beam performance. Therefore, precision control of the beam is needed to meet the operational requirements. The injector and linac consist of 113 accelerating structures, driven by 33 high power klystron amplifiers. Each klystron and associated accelerating structures are collectively referred to as one RF station. For each RF station a control system is designed to ensure precise and stable RF voltage at the accelerating module. As the overall beam characteristics are a function of the entire RF driven injector/linac system a beam-based feedback loop is required to coordinate between the RF station controllers. For this purpose, a centralized supervisory control architecture is developed which receives specific information from the controllers at a timescale faster than the beamís pulse repetition rate (10 milliseconds in the SwissFEL design). Altogether, this thesis addresses a number of control problems in an accelerator machine and introduces control and optimization strategies to increase stability and repeatability of the system. The experimental parts of the control methods are mostly conducted at the SwissFEL Injector Test Facility, in operation from 2010 to 2014, for prototyping purposes and proof of concept.

Further Information

Type of Publication:

(03)Ph.D. Thesis

R. S. Smith

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  author      = {Amin Rezaeizadeh},
  title   = {Automatic control strategies for the {Swiss Free Electron Laser} },
  school   = {ETH Zurich},
  year        = {2016},
  doi = {10.3929/ethz-a-010653872},
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