Optical Observations of Normal Zone Propagation in an Immersion-Cooled Superconducting Coil and of Pressure Wave Propagation Induced in Liquid Helium

This paper presents an experimental study of optical observations on normal zone propagation in an immersion-cooled superconducting coil and pressure wave propagation in liquid helium induced by quenching. The normal zone was visualized as a dark zone on the illuminated coil surface. Transient behavior of the dark zone was recorded with a high-speed video system under various transport currents. The results obtained were compared with the measured time histories of tap voltages and temperatures for a test coil. Maddock's stability criterion, or cooled end stability, was also successfully observed with a reduced transport current. Finally, the pressure wave propagation was found to be induced by the thermal shock of quenching.