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Abstract: FLASH (Free Electron Laser in Hamburg) accelerates electron bunches to up to 750 MeV for producing intense, coherent, very short pulses of radiation. Various types of BPMs (beam position monitors) are installed in the facility: cavity and re-entrant-cavity BPMs in the accelerating cryo-modules and button and stripline BPMs in most of the room-temperature sections. The undulator section, where the FEL radiation is produced, is one of the most critical areas of the linac in terms of requirements on the position monitoring. Due to the tight space, button BPMs were chosen for this area. The electronics is based on the AM/PM principle. In the past couple of years these BPMs were commissioned and intensively studied. A few modifications have been made in the electronics, in order to deal with the small signals and the very high frequencies of the ultra-short bunches. In this paper the button-BPMs at FLASH will be presented. The studies made in the RF laboratory and the measurements made on the performance of the BPMs will be discussed.
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