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Training and field limits of superconducting accelerator magnets

The magnets that steer high-energy beams are superconducting coils that often lose superconductivity (quench) below their design current and reach it only after many repeated quenches, called training. Why they train, and what limits their field, is not understood well enough to design around.

Why it matters

Reachable dipole field and training time set the energy reach and cost of future hadron colliders.

Review

D. Schoerling, A. V. Zlobin (editors), Nb3Sn Accelerator Magnets: Designs, Technologies and Performance, Springer, 2019. https://doi.org/10.1007/978-3-030-16118-7 reference checked

Timestamp record

Signed record: a83e07dd6840…
Public log: Entry 74, in checkpoint 2,123
In the log by: Block 969,698, 2026-10-03

This text was in the PubPhys log by that date. The date says nothing about when the problem was first posed.

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3 problems

Field errors from screening currents in REBCO accelerator magnets

Open

Magnets wound from flat high-temperature superconducting tape carry circulating currents inside the tape that distort the guiding field and drift in time.

Transverse-stress limit for irreversible degradation of $\mathrm{Nb_3Sn}$ cables

Open

$\mathrm{Nb_3Sn}$ is brittle, and the huge magnetic forces in a high-field magnet squeeze the cable until its ability to carry current is permanently damaged; where that threshold lies and what breaks first is uncertain.

Microscopic origin of quench training in $\mathrm{Nb_3Sn}$ accelerator magnets

Open

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.