Interdisciplinary Instrumentation Colloquium

Maxim Marchevsky, "A delicate sound of quench: instrumentation and diagnostics for superconducting magnets"

High-field superconducting magnets are at the core of modern accelerators, medical imaging and condensed matter research. These demanding applications push superconducting windings close to their operational limits where an interplay between stresses, thermal conditions and superconducting properties forms the basis for stable magnet operation. A single conductor disturbance depositing locally just a few mJ of energy may be sufficient to “quench” the entire magnet, releasing its stored energy (up to several MJ) into heat. Magnet quenching can be destructive unless timely detected and mitigated by an active protection circuitry. Another performance-limiting factor is the so-called “training”: to reach the design field for the first time the magnet typically undergoes a sequence of spontaneous quenches, starting sometimes as low as 50% of the design field, and increasing gradually in magnitude. The training phenomenon has baffled magnet designers for over 40 years, and is not yet fully understood. I will present instrumentation and analysis techniques developed within ATAP’s Superconductor Magnet group to detect and localize quenching, and identify various components of magnet “disturbance spectra” aiming at understanding the training mechanisms. Design and synergistic operation of inductive “quench antennas” and cryogenic acoustic emission sensors will be discussed, and diagnostics results from the recent magnet test campaigns will be shown. A striking similarity is seen between acoustic emission statistics in magnets and other physical systems exhibiting a self-organized critical behavior (such as earthquakes, magnetic domains motion, crackling noise, etc.) I will also discuss the recently developed active acoustic approach for non-intrusive sensing of hot spots (ΔT<1K) and early detection of quenches in magnets built with high-temperature superconductors.
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