Thermally-controlled flux avalanche dynamics in bulk NbTi superconductor

TL;DR

This study visualizes flux avalanche dynamics in bulk NbTi superconductors, revealing thermally limited propagation at much slower speeds than thin films, with implications for flux stability and quench protection.

Contribution

First direct visualization and measurement of flux avalanche propagation in bulk NbTi, highlighting thermal limitations distinct from thin film behavior.

Findings

Avalanche velocities are 15-25 m/s, much slower than in thin films.

Avalanches are governed by local heating and poor heat dissipation.

Universal velocity-distance scaling observed across different avalanche morphologies.

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.