MTCA.4-Based Digital LLRF Control System for CW SRF Linacs

Stable and reproducible generation of a photon beam at Free Electron Lasers (FELs) necessitates a low energy spread of the electron beam. This spread is disturbed by various factors. A low level radio frequency (LLRF) control system stabilizes the RF field inside accelerating modules, as amplitude and phase fluctuations are the main bunch-to-bunch energy spread sources. This paper describes the architecture of the LLRF control system based on the Micro-Telecommunications Computing Architecture (MTCA.4) platform developed by the PCI Industrial Computer Manufacturers Group (PICMG). This architecture offers manageability, reliability, and scalability which are crucial for high energy physics experiments. The hardware modules such as digitizers, down-converters (DWC), or vector modulators (VM) have been adopted from multi-channel, vector sum based systems and optimized. The field detection scheme utilizes the analog frequency shifting to an intermediate frequency (IF). Analog-to-digital conversion and digital complex frequency down-conversion are applied. The firmware has been optimized for minimal latency. The software communicates with the firmware using a PCIe bus, reads waveforms for diagnostic purposes, and abstracts hardware and firmware settings for the paneling system. Distinctive characteristics of the CW mode of operation affecting the LLRF system are discussed. The system design process and the important parameter value selection criteria are presented. The noise contribution of hardware subcomponents and other limitations of the field detection are analyzed. The digital signal processing path is split into atomic operations and examined. Interfacing with other systems is discussed. Proposals for future developments are given.