High-temperature superconductors (HTS) are considered for broad applications from medical devices and electric motors to future particle colliders and fusion reactors. The superconductors are subject to quenching, which leads to a loss of superconductivity and interruption of magnet operation. Yet, the timely detection of quench development is still on the list of “challenges to be resolved.” Microwave time and frequency domain reflectometry has recently been suggested as a possible solution enabling “noninvasive,” “operation performance time resilience,” and in-time quench monitoring. The technique considers using HTS cables’ cooling channels to measure the flowing gas temperature evolution via monitoring its electromagnetic properties. The channel surface critical temperature ($T_{\text{sc}}$), linked to HTS critical temperature and limiting the time interval for quench detection, is introduced. The two stages of the hot-spot evolution inside the gas channel during the heat release are identified and studied. The variations of the electromagnetic properties of the gas inside the channel are discussed, and experimental confirmations of the theoretical predictions are presented. We show that, depending on the heat power released and cooling gas parameters, the heating process follows different scenarios. It has been noted that the technique suggested can be used for other applications where channel-confined fluids may be affected by external changes that should be monitored.

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• Cryogenic technology
• Electromagnetic waves & oscillations
• Heat transfer
• Superconducting devices
• Ultracold gases
• Waveguides
• Numerical techniques
• Radiofrequency reflectometry