Control and measurement system for high quality superconducting cavities

The measurements of superconducting cavity parameters before module assembly stage are very important, because they allow to evaluate critical properties (resonant frequency, quality factor of accelerating mods) of individual resonating cavities. Such measurements are performed in Vertical Test Stand facilities. One of such setups is Vertical Test Stand II (VTS II) at DESY. It is used for conditioning and characterization of cavities for FEL (Free Electron Laser) experiments (especially for FLASH and European XFEL). In the previous set-up cavities in VTS II were operated in Continuous Wave mode using analog control system. The core of the system was analog Phase Lock Loop used to lock phases of incident wave and transmitted wave. All measurements were done using external equipment such as oscilloscopes, spectrum network analyzers, etc. The main disadvantage of such system was lack of flexibility - each small modification required precise system tuning, no new functionality could be added and automation of measurements was hard to achieve. The paper presents new solution of the control system for VTS II based on digital hardware. It allows to overcome most of disadvantages of previous analog system. All measurements and control functions are integrated on a single (FPGA based) computation board, which is fully integrated with external control system. The usage of such system offers functionality such as long term data acquisition (due to Continuous Wave operation of the cavity), special modes of operation (like Self Excited Loop or Digital PLL - due to low bandwidth of measured resonator) and possibility of measurement process automation. Additionally developed user interfaces allow remote access to the system parameters and flexible configuration. The paper discusses each sub-system, presents measurement results and overall system performance evaluation together with some views for future functionality development.