Stereoscopic detection of hot spots in superfluid 4He (He II) for accelerator-cavity diagnosis

• Published in: International journal of heat and mass transfer 2020-11, Vol.161, p.120259, Article 120259
• Main Authors: Bao, Shiran, Kanai, Toshiaki, Zhang, Yang, Cattafesta, Louis N., Guo, Wei
• Format: Article
• Language: English
• Subjects: Algorithms; CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; Flow visualization; Fluids; Helium isotopes; Holes; Molecular tagging velocimetry; Ohmic dissipation; PARTICLE ACCELERATORS; Q factors; Quench-spot detection; Resistance heating; Size effects; Stereoscopic molecular tagging; Stereoscopy; Substrates; Superconducting radio-frequency (SRF) cavities; Superfluid 4He; Superfluidity; Surface defects; Three dimensional motion; Tracking
• ISSN: 0017-9310; 1879-2189
• DOI: 10.1016/j.ijheatmasstransfer.2020.120259

•A stereoscopic molecular tagging method for hot-spot detection in He II is demonstrated•A heat-transfer model for analyzing the line deformation in 3D space is developed•We demonstrated an unprecedented accuracy in locating the hot spot on a solid surface•The method proves to be a powerful technique for accelerator-cavity diagnosis Superconducting radio-frequency (SRF) cavities cooled by superfluid 4He (He II) are building blocks of many modern particle accelerators due to their high quality factor. However, Joule heating from sub-millimeter surface defects on cavities can lead to cavity quenching, which limits the maximum acceleration gradient of the accelerators. Developing a non-contacting detection technology to accurately locate these surface hot spots is the key to improve the performance of SRF cavities and hence the accelerators. In a recent proof-of-concept experiment (Phys. Rev. Applied, 11, 044003 (2019)), we demonstrated that a molecular tagging velocimetry (MTV) technique based on the tracking of a He2* molecular tracer line created nearby a surface hot spot in He II can be utilized to locate the hot spot. In order to make this technique practically useful, here we describe our further development of a stereoscopic MTV setup for tracking the tracer line’s motion in three-dimensional (3D) space. We simulate a quench spot by applying a transient voltage pulse to a small heater mounted on a substrate plate. Images of the drifted tracer line, taken with two cameras from orthogonal directions, are used to reconstruct the line profile in 3D space. A new algorithm for analyzing the 3D line profile is developed, which incorporates the finite size effect of the heater. We show that the center location of the heater can be reproduced on the substrate surface with an uncertainty of only a few hundred microns, thereby proving the practicability of this method.