Development of Transition Edge Sensors for Quench Localization on SRF Cavities

The development of future Superconducting Radio Frequency (SRF) cavities for particle accelerators requires their intensive cryogenic testing. Localizing punctual surface defects that could originate a “quench” (loss of superconductivity) is crucial. This can be done by non-contact thermometry: the detection of a temperature wave in superfluid helium (T<2.17 K), known as second sound, originated locally by the fast quench heat transfer transient. A temperature perturbation in the order of milliKelvins travels at a speed of around 20 m/s. The trilateration of the wave detection is used as a GPS-like system to localize the defects. Recent development of Transition Edge Sensors (TES) at CERN has provided a new tool (other than the classical Oscillating Superleak Transducers - OST) to accomplish this local- ization. TESs are thermometers made of a superconducting narrow thin film on glass, biased within the superconducting-to-normal transition. Within the transition temperature range, a very large resistance change occurs from zero to the normal value, which makes the TES very sensitive. Sensors can transduce the tiny and fast temperature variations associated with second sound in He-II into proportional changes of the voltage drop or current across the film strip. A reliable method to produce TES by microfabrication and a validated test stand to evaluate their performance, along with data treatment algorithms had been established at CERN. Existing TESs are performant to detect the second sound. However, their time- response is limited by the excessive thermal contact with the amorphous substrate. A better time response and signal-to-noise ratio may be attained by shaping the substrate in micro- beams, thus reducing the thermal contact cross section. In this project, the focus is to fabricate TESs by depositing the thin film on substrate-released micro-beams on a glass wafer. This would allow for a better sensitivity to short and low intensity temperature-wave signals. Production trials of two designs by a novel method and performance characterization of the manufactured sensors were conducted. Microfabrication took place at CMi of EPFL and the cryogenic tests at CERN’s cryogenics laboratory.