Commissioning of the electronics for HOM-based beam diagnostics at the 3.9 GHz accelerating module at FLASH

Transverse Higher Order Modes (HOM) excited by electron beams in the 3.9GHz accelerating cavities at FLASH may damage the beam quality. They can be reduced by extracting their energy through special couplers and by aligning the beam in the cavity. Electronics has been designed at FNAL for monitoring some of the potentially most damaging HOMs. This may be used for beam centering and therefore reducing the HOM effects. Moreover, the signals can be potentially calibrated into beam offset, so that they could be used as beam position monitors (HOM-BPM). The specificationsof the monitors have been defined during an extensive study on the 4-cavity accelerating module installed at FLASH. Signals around 5.46 GHz have been chosen for higher precision measurements. However these signals propagate into the entire 1.2 m long module. Therefore in addition modes at about 9.06 GHz were selected for localized measurements in each cavity. The electronics has been recently installed at FLASH. The initial experience with this electronics is presented in this paper. The signals can already be used for centering. Some instability in the signals has been observed and the cause has to be further investigated.