Electro-thermal quench in metal-insulated nested REBCO coils for magnets over 40 T

Superconducting high field magnets have the capability to generate over 40 T, with multiple existing practical applications globally. However, at such high magnetic fields, these magnets are prone to rapid electrothermal quench which can affect the continuous operation of such magnets. A nested stack configuration, with multiple HTS inserts inside a LTS outsert, can be used for better thermal stability and compact design. We have performed detailed multiphysics quench analysis of such a nested stack high field magnet design under SuperEMFL project using our in-house software, which considers screening currents. Through various case studies, we have identified various weak spots in such a magnet, where thermal quench can be the most detrimental for magnet operation, and various ways are suggested to overcome this important issue.

💡 Research Summary

The paper presents a comprehensive multiphysics investigation of electro‑thermal quench behavior in metal‑insulated REBCO (HTS) coils that are nested inside a low‑temperature superconductor (LTS) outsert, a configuration aimed at achieving magnetic fields above 40 T. The authors focus on a specific SuperEMFL design comprising a 15 T LTS outsert and two nested HTS stacks (HTS1 and HTS2) made of metal‑insulated pancake coils. Each turn is separated by a thin Durnomag metal layer (10⁻⁶ Ω·m²) that permits radial currents, while the REBCO tape (Theva) exhibits full Jc(B,T,θ) dependence.

Modeling framework
Electromagnetic fields are solved with the Minimum Electro‑Magnetic Entropy Production (MEMEP) method, which captures both azimuthal screening currents and radial currents arising from the metal insulation. Thermal evolution is computed with an explicit finite‑difference scheme, and the two solvers are tightly coupled (MEMEP‑FD) to account for temperature‑dependent material properties. The simulations assume adiabatic boundaries, a nominal operating current of 231.2 A at 4.2 K, and a time step of 10 ms. A “damage” scenario is introduced by reducing the critical current density of a block of ten consecutive turns (turns 41‑50) to 10 % of its original value.

Key parametric studies

1. Location of the weak spot – Damage is placed in the top, middle, or bottom pancake of the inner HTS1 stack. Because the Theva tape shows strong angular anisotropy (Jc is lowest for field angles ≈30°), the top pancake quenches fastest (≈1 s) while the bottom pancake takes the longest (≈3 s) but reaches the highest peak temperature (≈300 K). The middle pancake yields intermediate quench times but the largest radial currents and associated radial power loss, due to simultaneous upward and downward propagation.

2. Effect of screening currents – A comparison with a uniform‑current‑density model (i.e., neglecting screening currents) shows that quench propagation is dramatically slower (≈3× longer) and the predicted maximum temperature is substantially lower. The authors attribute this to the additional AC losses generated by rapidly changing screening currents, which dominate the early stage of the quench.

3. Contact resistance sensitivity – The metal‑insulation resistance is set to 10⁻⁶ Ω·m². Raising this value suppresses radial currents and reduces the total Joule heating, but it also narrows the operating window of the voltage‑limiting protection circuit.

4. Voltage‑limiting protection – A 2.5 V limit is imposed on the coil voltage. When the limit is triggered, the total current drops sharply, curbing the exponential rise of total power and preventing uncontrolled thermal runaway. Without the limit, the current continues to increase, leading to a rapid temperature rise and eventual burnout.

5. Coupling between the two HTS stacks – The quench of HTS1 induces a rapid change in the magnetic flux that couples inductively to HTS2, causing it to quench almost simultaneously. This demonstrates that in a nested configuration, a failure in one stack can cascade to the other, emphasizing the need for system‑wide protection strategies.

Conclusions and design recommendations

• The position of a local defect strongly influences both quench speed and peak temperature; bottom‑stack defects, while slower to spread, generate the most severe thermal excursions and should be a primary focus for mechanical reinforcement and thermal buffering.
• Accurate quench prediction for REBCO magnets requires inclusion of screening currents; models that assume uniform current density can underestimate both propagation speed and temperature rise by a factor of three or more.
• Metal‑insulation contact resistance must be optimized: low enough to allow radial current sharing for thermal homogenization, yet high enough to limit excessive radial Joule heating.
• A modest voltage‑limiting circuit (≈2–3 V) is an effective, fast‑acting safeguard that can arrest a quench before the coil reaches destructive temperatures.
• Because the two HTS stacks are electromagnetically coupled, protection systems must be coordinated across the entire nested assembly rather than applied to each stack in isolation.

Overall, the work delivers a detailed, physics‑based roadmap for designing, analyzing, and protecting >40 T hybrid superconducting magnets that employ metal‑insulated REBCO nested coils. The findings are directly applicable to ongoing high‑field projects such as SuperEMFL, the LBC hybrid demonstrator, and other next‑generation magnet programs worldwide.