Digital Measurement System for the HIE-Isolde Superconducting Accelerating Cavities

Extensive R&D efforts are being invested at CERN into the fundamental science of the RF superconductivity, cavity design, niobium sputtering, coating and RF properties of superconducting cavities. Fast and precise characterization and measurements of RF parameters of the newly produced cavities is essential for advances with the cavity production. The currently deployed analogue measurement system based on an analogue phase discriminators and tracking RF generators is not optimal for efficient work at the SM18 superconducting cavity test stand. If exact properties of the cavity under test are not known a traditional feedback loop will not be able to find resonant frequency in a reasonable time or even at all. This is mainly due to a very high Q factor. The resonance peak is very narrow (fraction of a Hz at 100 MHz). If the resonant frequency is off by several bandwidths, small changes of the cavity field during the tuning will not be measureable. Also cavity field will react only very slowly to any change of the drive signal. A new techniques to find and track the cavity resonance faster, as well as to keep the cavity field constant under strong microphonics and helium pressure variations must be found to meet the deadlines for the HIE Isolde machine. Therefore a new fully digital measurement and conditioning system based on the available existing hardware from the LHC and SPS must be designed and built.