Measurement of thermo-mechanical properties of NbTi windings for accelerator magnets

In the framework of the development of superconducting accelerator magnets, one has to determine the apparent elastic modulus and thermal contraction of NbTi windings. This knowledge is required to calculate the prestress needed to compensate thermal shrinkage differentials during cool-down and stress redistribution due to Lorentz forces during excitation. A compression mold was developed to measure the apparent elastic modulus of ten alternately stacked insulated superconducting cables, both at room temperature and in a cryostat at liquid helium temperature. The thermal contraction is measured in another stainless steel mold, designed to simulate the prestress application. A force sensor based on a strain gage measurement technique is inserted between the conductor stack and the cover of the mold to monitor the applied prestress during cool-down. The force sensor is calibrated at room and liquid helium temperature. Reference samples made with known materials such as stainless steel, copper, aluminum and titanium have been also measured to calibrate the test apparatus.