RF Control Optimization and Automation for Normal Conducting Linear Accelerators

Normal conducting linear accelerators are widely used in many free-electron laser (FEL) machines all over the world. They normally consist of multiple RF stations either with standing wave cavities (e.g., RF Gun) or with traveling wave accelerating structures working at different frequencies. Low-level radio frequency (LLRF) systems are used to measure the RF fields in cavities or structures and correct the fluctuations in RF fields with pulse-to-pulse feedback controllers. To facilitate the operations of multiple RF stations, the LLRF system should also provide algorithms and procedures to automate the setup, calibration, and optimization of the RF systems. In this paper, several typical algorithms and procedures will be described, such as calibrating the digital-to-analog converters offset to reduce the RF leakage from vector modulator, calibrating the RF signal group delay, and flattening the intrapulse phase distribution with an adaptive feed forward. The algorithms and procedures have been implemented as an LLRF high-level applications (HLAs) software for SwissFEL, which is an FEL machine under commissioning at Paul Scherrer Institut, Switzerland. The architecture of the LLRF HLA software will be introduced, and the test results at SwissFEL will also be described.