Thermally-controlled flux avalanche dynamics in bulk NbTi superconductor
Irina Abaloszewa, Viktor V. Chabanenko, Aleksander Abaloszew
TL;DR
This work demonstrates that bulk NbTi superconductors with poor thermal coupling enter a thermally-limited avalanche regime, where flux avalanches propagate at tens of meters per second and decelerate due to heat accumulation. Using high-speed magneto-optical imaging, the authors directly visualize avalanches in a NbTi disk, quantify the threshold field $H_{\mathrm{th}}(T)$, and establish a hierarchy of electromagnetic and thermal timescales that distinguish this regime from the electromagnetically-driven thin-film case. A universal normalized velocity–distance scaling is observed across events, and the estimated thermal boundary conductance $h$ places the system near a critical boundary between thermally-limited and electromagnetically-controlled dynamics. These results sharpen our understanding of thermomagnetic instabilities in bulk superconductors and inform quench protection and thermal management strategies for NbTi-based magnets.
Abstract
We report the first direct visualization of flux avalanche propagation dynamics in bulk superconducting NbTi, tracking individual events and measuring their velocities using high-speed magneto-optical imaging. Unlike thin films with electromagnetic avalanches at km/s speeds, we observe velocities of 15--25 m/s, which are orders of magnitude slower. Analysis of characteristic timescales reveals that these avalanches are governed by local heating and limited heat dissipation through the adhesive layer, establishing a fundamentally different, thermally limited propagation regime. The threshold field for avalanche nucleation decreases with temperature, contrary to the increasing trend in thin films with efficient cooling - a behavior consistent with slow heat removal and thermal runaway in our system. All observed avalanches exhibit universal normalized velocity-distance scaling despite varying morphologies, confirming the robustness of thermal control. These findings reveal that bulk superconductors with poor thermal coupling operate in a previously uncharacterized avalanche regime, with direct implications for flux stability and quench protection in NbTi-based magnets, as well as a broader understanding of thermomagnetic instabilities in technological superconductors.