  Superconducting RF cavities are used in particle accelerators to provide
energy to the particle beam. Such cavities are mostly fabricated in niobium and
often operated in superfluid helium. One of their limits of operation is the
appearance of a local quench, initiated by a local field enhancement due to a
defect, which leads to a normal conducting transition of the cavity. Localizing
the quench area can be achieved with temperature mapping systems. Another
method is the use of second sound wave propagation in superfluid helium.
Measuring the time of propagation of these waves from quench location to
special sensors, called Oscillating Superleak Transducers (OSTs), and using
their well-known velocity should allow trilateration. However, most of
experimental measurements on cavities show "premature signals", i.e. the second
sound signals arrive earlier on the OSTs than expected. This paper presents
several quench experiments on cavities equipped with OSTs and temperature
mapping quench detection systems. Two hypotheses can explain the observed
premature signals. The first one assesses faster propagation in helium. An
experimental setup has been developed for testing this hypothesis, where second
sound is created by a localized heater in a controlled environment up to 4.3
kW/cm2 and 2.8 J. Premature signals could not be verified in this setup. A
second hypothesis based on a simple model including several processes in
niobium and second sound propagation in helium is discussed. The model improves
significantly the prediction of the times of arrival of the second sound waves.
The overall study shows that the processes in niobium play a prominent role in
the second sound detection for superconducting cavities.
