A new wire position monitor readout system for ILC cryomodules

Wire position monitors (WPMs) are used to measure the absolute position of superconductive RF cavities in prototype cryomodules developed for electron linacs such as FLASH and XFEL at DESY or the future ILC. The WPM consists of four metallic strips equally spaced in azimuth on a 12 mm diameter cylinder approximately coaxial with a reference wire excited with a 140 MHz RF signal. The position of the cylinder axis with respect to the wire can be deduced from the analysis of the signals induced on the strips. In this paper we present a new approach to the readout, digitization and processing of the WPM signals which is shown to produce position resolutions at the 1 micrometer level with a bandwidth of a few kHz. The latter is important to measure typical mechanical vibrations of the cold masses.