Interfaces and communication protocols in ATCA-based LLRF control systems

Linear accelerators driving Free Electron Lasers (FELs), such as Free Electron Laser in Hamburg (FLASH) or X-ray Free Electron Laser (XFEL), require a sophisticated Low Level Radio Frequency (LLRF) control system. The controller of the LLRF system should stabilize the phase and amplitude of the field in accelerating modules below 0.02 % of amplitude and 0.01 degree for phase tolerances to produce ultra stable electron beam that meets required conditions for Self-Amplified Spontaneous Emission (SASE). Since the LLRF system for XFEL must work for the next 20 years, it should be reliable, reproducible and upgradeable. Having in mind all the requirements of LLRF control system, the Advanced Telecommunications Computing Architecture (ATCA) was chosen to build a prototype LLRF system for FLASH accelerator that is able to supervise 32 cavities of one accelerating section. The LLRF controller takes advantage of the features offered by the ATCA standard. The LLRF system consists of a few ATCA carrier blades, Rear Transition Modules (RTM) and several Advanced Mezzanine Cards (AMCs) that provide all necessary digital and analogue hardware components. The distributed hardware of the LLRF system requires a number of communication links that should provide different latencies, bandwidths and protocols. The paper presents the general view of the ATCA-based LLRF system, discusses the requirements and proposes application for various interfaces and protocols in the distributed LLRF control system.