Detection of Second Sound in He-II for Thermal Quench Mapping of Superconducting Radio Frequency Accelerating Cavities

The development of future particle accelerators requires intensive testing of superconducting radio frequency cavities with different sizes and geometries. Non-contact thermometry quench localisation techniques proved to be beneficial for the localisation of surface defects that can originate a quench (sudden loss of superconducting state). These techniques are based on the detection of second sound in helium II. Transition Edge Sensors (TES) are highly sensitive thin film thermometers with fast time response. In the present work, their capability as a thermal quench mapping device for superconducting radio frequency cavities is proven experimentally by detecting second sound waves emitted by SMD heaters in a He-II bath at saturated vapour pressure. A characterisation of the sensors at steady bath temperatures was conducted to calculate the thermal sensitivity. An intense metallurgical study of gold-tin TES with different compositions revealed important relations between the superconducting behaviour and the microstructure of the thin film. It was successfully demonstrated that heat treatment can be used to enhance the superconductivity of the sensors after the manufacturing process if desired. A strategy for optimal biasing of the sensors was developed to maximise the signal-to-noise-ratio of the measured second sound signal. The velocity of second sound at 1.8K could be confirmed and the position of the heaters was successfully localised by trilateration with more than 90% of success. In a dedicated testing setup, the threedimensional propagation of second sound could be investigated in terms of angle and distance between heater and sensor. Especially the influence of the heater size and the detection angle could be studied, which reveals useful information for quench detection tests. Finally, the different nature of TES and Oscillating Superleak Transmitters (OST), another common second sound detector, was investigated experimentally showing that the temperature measurement of the TES is less angle dependent than the detection of the movement of the two components in helium II by the OST.