Development and commissioning of a digital rf control system for the S-DALINAC and migration of the accelerator control system to an EPICS-based system

• Main Author: Konrad, Martin
• Format: Others
• Language: English; en
• Published: 2013
• Online Access: http://tuprints.ulb.tu-darmstadt.de/3398/7/Dissertation_Martin_Konrad_online.pdf; Konrad, Martin <http://tuprints.ulb.tu-darmstadt.de/view/person/Konrad=3AMartin=3A=3A.html> : Development and commissioning of a digital rf control system for the S-DALINAC and migration of the accelerator control system to an EPICS-based system. [Online-Edition] Technische Universität, Darmstadt [Ph.D. Thesis], (2013)

The high resolution scattering experiments conducted at the superconducting Darmstadt electron linear accelerator S-DALINAC call for a small energy spread of (ΔE/E) ≈ 1×10⁻⁴ of the beam. This requires stabilization of amplitude and phase of the electric field inside the accelerating cavities to (ΔA/A)ᵣₘₛ = 8×10⁻⁵ and (Δφ)ᵣₘₛ = 0.7°. The design and the commissioning of a new digital rf control system is the subject of this thesis. At the S-DALINAC two types of cavities are in use. The normal-conducting chopper and buncher cavities only need corrections for slow temperature drifts and can be controlled by a generator-driven resonator control algorithm. The superconducting accelerating cavities have a very high quality factor and thus are very susceptible to vibrations. Therefore they are operated in a self-excited loop. The rf control system is based on in-house developed hardware that converts the rf signal down to the baseband, digitizes it and feeds it into an FPGA. Inside this FPGA, a soft digital signal processor executes the control algorithm. The resulting correction is modulated onto the rf signal again and sent back to the cavity. All accelerator components are remote-controlled from a central room via an accelerator control system. Since complex and re-programmable devices are not supported well by the existing in-house developed control system, the design and implementation of a new accelerator control system is also subject of this thesis. Further important aspects are expandability, usability and maintainability of the system. Therefore the new accelerator control system uses the EPICS framework as a basis since it already provides much of the basic functionality like graphical user interfaces and flexible control servers that can be customized rapidly. This allowed the implementation of more advanced functionality like extensive read-out and diagnostics for the rf control system. The read out data can be visualized with a software oscilloscope and a spectrum analyzer software. Additionally the system provides on-line rms errors that can be used to optimize the control parameters very precisely and to monitor the performance of the controllers. Measurements show that the performance of the rf control system has been improved by one order of magnitude compared to the analog system, yielding a phase stability of (Δφ)ᵣₘₛ = 0.8° and an amplitude stability of (ΔA/A)ᵣₘₛ = 7×10⁻⁵ and thus meeting the specification. The described rf control system has been commissioned and successfully used for beam operation for two years. During this time the system has proven to be significantly more stable and reliable than the old analog system.