3.9GHz accelerating cavity field detection hardware for the free-electron laser

The 3rd harmonic system installed in the injector at FLASH (Free-Electron Laser at Hamburg) accelerator allows to achieve ultra short electron bunches with higher peak current, which pushed the limits of high brilliance laser light generation into the so-called water window. The 3rd harmonic system operates at 3.9GHz and requires a very precise phase and amplitude control of the accelerating field below 10fs, which is done by the Low-Level RF system. One of the key component is the RF field detector that converts the detected cavity field signal to an intermediate frequency for digital sampling. The paper presents an overview of the system, particularly the low phase noise reference generator, local oscillator and multichannel down-converter. System scheme and performance are demonstrated.