Wang, H. JACoW Geneva, Switzerland 978-3-95450-180-9 10.18429/JACoW-NAPAC2016-MOPOB69 English 225-228 MOPOB69 ion cavity experiment simulation cryomodule Contribution to a conference proceedings 2017 2017-01 http://dx.doi.org/10.18429/JACoW-NAPAC2016-MOPOB69 https://jacow.org/napac2016/papers/mopob69.pdf The fabrication accuracy of a superconducting RF crab cavity for the Large Hadron Collider High Luminosity Upgrade and the future Electron Ion Collider requires the cavity's electric center line relative to the crabbing plane within sub mm offset and sub degree in rotation. It is very hard for the cavity's niobium sheet formation, high temperature bake and chemistry processes and finally cooling down in cryomodule to satisfy such tight tolerance. A new wire stretching technique combining with the RF measurement in the deflecting modes has been demonstrated on the bench to detect less than 10um resolution on the RF signal when the wire is moving away from the ideal electric center line. The foundation of this technique and its difference from the use in other applications will be reviewed. Based on this principle, the possible implementations for detecting RF leakage to the higher older mode couplers, cavity string alignment and cryomodule assembly will be discussed.