  Superconducting ratio-frequency (SRF) cavities, cooled by superfluid helium-4
(He II), are key components in modern particle accelerators. Quenches in SRF
cavities caused by Joule heating from local surface defects can severely limit
the maximum achievable accelerating field. Existing methods for quench spot
detection include temperature mapping and second-sound triangulation. These
methods are useful but all have known limitations. Here we describe a new
method for surface quench spot detection by visualizing the heat transfer in He
II via tracking He$_2^*$ molecular tracer lines. A proof-of-concept experiment
has been conducted, in which a miniature heater mounted on a plate was pulsed
on to simulate a surface quench spot. A He$_2^*$ tracer line created nearby the
heater deforms due to the counterflow heat transfer in He II. By analyzing the
tracer-line deformation, we can well reproduce the heater location within a few
hundred microns, which clearly demonstrates the feasibility of this new
technology. Our analysis also reveals that the heat content transported in He
II is only a small fraction of the total input heat energy. We show that the
remaining energy is essentially consumed in the formation of a cavitation zone
near the heater. By estimating the size of this cavitation zone, we discuss how
the existence of the cavitation zone may explain a decades-long puzzle observed
in many second-sound triangulation experiments.
