Measurement of static force at liquid helium temperature

The multilayer piezostacks which are integrated into a cold tuning electromechanical system are used for the compensation of the Lorentz force and damping of microphonics. Cold tuning systems are tested in the CHECHIA horizontal test stand and in the vacuum ultraviolet free electron laser VUV-FEL, both built at DESY-Hamburg. One of the most important issues for the used active element's lifetime is a correct initial preloading force. The commonly used method for measuring preload is based on a strain gauge and requires an additional sensor and wiring. The authors propose an alternative method where the piezostacks are used as force sensors. More precisely, the capacitance and the impedance change of the piezostacks were investigated. The resonance peaks, which are visible on the impedance versus frequency curve, shift when the static force is applied to piezostacks. Moreover, the value of the piezostacks capacitance increases with applied static force. A measurement performed at room and at LHe temperature (T < 4.2 K) showed that this behaviour ( frequency shift and capacitance increase) might be used for the precise measurement of static force in the temperature range 1.8 K-300 K. Using this method it is possible to estimate in situ the preload force applied to the piezoelectric element.