Microscopic origin of quench training in Nb3Sn accelerator magnets
In plain words
Each premature quench lets the magnet reach a slightly higher current the next time, which suggests that small mechanical events in the coil release heat, but which events and where is not established.
Precise statement
Nb3Sn dipoles and quadrupoles operating at 1.1e5 to 1.5e5 G often require tens of training quenches to approach 80 to 90 percent of the conductor short-sample limit. Identify the energy-release events that start training quenches (epoxy cracking, conductor-insulation debonding, stick-slip friction at coil-structure interfaces), their energy spectrum relative to the minimum quench energy of the cable, and a model that predicts the training curve from coil prestress and material properties.
What would settle it
Acoustic-emission and quench-antenna localization of initiating events across the training of several magnets, matched by a mechanical-thermal model that predicts the training curve of a new magnet before it is tested.
Status in the literature
Unverified note
Acoustic-emission localization of events during training was reported from 2015 onward; no model predicts training curves (moderate confidence, not rechecked for 2024-2026).