Control of turn-to-turn contact resistivity in resistively insulated REBCO coils

Brent Jarvis; Denis Markiewicz; Emsley Marks; Hongyu Bai; Iain Dixon; Jun Lu; Justin Deterding; Kwangmin Kim; Mark Bird

arxiv: 2604.15587 · v1 · submitted 2026-04-16 · ❄️ cond-mat.supr-con

Control of turn-to-turn contact resistivity in resistively insulated REBCO coils

Jun Lu , Kwangmin Kim , Iain Dixon , Justin Deterding , Emsley Marks , Brent Jarvis , Denis Markiewicz , Hongyu Bai

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Mark Bird

This is my paper

Pith reviewed 2026-05-10 09:12 UTC · model grok-4.3

classification ❄️ cond-mat.supr-con

keywords REBCO coilscontact resistivityresistive insulationquench protectionstainless steel interlayerpressure cyclingturn-to-turn resistance

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The pith

Contact resistivity in resistively insulated REBCO coils can be set to stable prescribed values using conductive fillers, solder coatings, and controlled oxidation of stainless steel interlayers.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper shows how to control and stabilize turn-to-turn contact resistivity Rc in resistively insulated REBCO magnets so that it stays within a coil-specific range that avoids both excessive transient currents during quenches and conductor burn-out. Earlier work found that Rc between REBCO tapes and stainless steel interlayers drops by up to three orders of magnitude under repeated pressure cycling, which prevented reliable magnet design. The authors introduce three practical fixes: adding conductive paste or epoxy, coating the REBCO tape with 2–3 micrometers of PbSn solder for dry-wound coils, and oxidizing the stainless steel co-wind tape at chosen temperatures in air. Short-sample tests reached stable Rc values of 1000 and 5000 microohm-cm² that held through 30,000 pressure cycles at 4.2 K; the same methods produced matching results in a six double-pancake test coil.

Core claim

By adding conductive fillers, coating REBCO tapes with PbSn solder, or oxidizing stainless steel co-wind tapes at controlled temperatures, Rc can be preset to target values and rendered insensitive to contact-pressure cycling up to 30,000 cycles at 4.2 K. Short-sample measurements confirmed Rc values of 1000 and 5000 μΩ-cm² remained unchanged, and a six double-pancake coil exhibited comparable Rc when tested at 4.2 K.

What carries the argument

Surface and interlayer treatments—conductive paste or epoxy fillers, 2–3 μm PbSn solder coatings, and temperature-controlled air oxidation of stainless steel co-wind tape—to set and lock the baseline turn-to-turn contact resistivity Rc against pressure-induced degradation.

Load-bearing premise

The stabilization of Rc observed in short samples and one small test coil will continue to hold in full-scale magnets under operational quench currents, mechanical stresses, and thermal cycling beyond the tested 30,000 pressure cycles.

What would settle it

A measurable drop in Rc or loss of stability when the same methods are applied to a larger coil subjected to repeated quenches or more than 30,000 pressure cycles at 4.2 K.

read the original abstract

Resistively insulated (RI) REBCO magnets feature short ramp times and low ramp losses while maintaining the advantages of no-insulation coils with high engineering current density and tolerance for defects in the REBCO conductor. Control of the turn-to-turn contact resistivity Rc is key to RI technology. Rc must be sufficiently high to prevent a large transient current, which could result in high mechanical stress during magnet quenches. Meanwhile it must be lower than the quench propagation limit to avoid conductor burn-out during a quench. Therefore, it is critical to control Rc within a suitable range of values which is usually coil specific. Previously, we discovered that Rc between two REBCO tapes with a stainless steel interlayer decreases dramatically with contact pressure cycling by up to three orders of magnitude. This drastic change made it impossible to design a suitable Rc value for a stainless steel co-wound RI magnet. In this work, we first present methods for mitigating Rc pressure cycling sensitivity. We found that by adding conductive fillers, such as conductive paste or epoxy, the Rc load cycling sensitivity is largely mitigated. For dry-wound coils, Rc load cycling sensitivity is mitigated by coating REBCO tape with a layer of 2- 3 um of PbSn solder. In addition, Rc can be controlled by oxidizing the stainless steel co-wind tape by heating stainless steel tapes at different temperatures in air. Using above methods, short sample tests showed that Rc was controlled to prescribed values of 1000 and 5000 uOhm-cm2 and was not sensitive to contact pressure cycling up to 30,000 cycles at 4.2 K. The new Rc control method was applied to a 6 double-pancake test coil which was tested at 4.2 K. The Rc in this test coil was comparable with the short sample results. This demonstrated the ability of this new method to control Rc in large coils.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

The paper gives concrete recipes to stabilize Rc against cycling in resistively insulated REBCO coils, backed by short-sample and small-coil data at 4.2 K, but leaves scalability to full magnets untested.

read the letter

The main thing to know is that the authors have turned their earlier observation of Rc dropping with pressure cycling into a set of workable fixes: adding conductive paste or epoxy, coating the REBCO with 2-3 microns of PbSn solder, or oxidizing the stainless steel co-wind tape at controlled temperatures. Short samples held Rc at the target 1000 or 5000 microohm-cm2 through 30,000 cycles at 4.2 K, and a 6 double-pancake test coil produced comparable values. That is useful, direct experimental progress on a real barrier for fast-ramping, high-current-density magnets. The work is entirely experimental, with no fitted models or circular derivations, so the results stand on the measurements themselves. The cycling protocol and the coil test are straightforward and address the exact problem they set out to solve. The soft spots are mostly about scale and completeness. All the data stops at short samples or one small coil; there are no post-quench Rc checks, no additional thermal cycling, and no tests on larger winding radii where contact pressures become less uniform. The abstract also omits error bars, replicate counts, or measurement details, which makes it harder to judge how tight the control really is. These gaps are real but not fatal at this stage, since the core claim is only that the methods work at the scales shown. This paper is for applied superconductivity groups building or testing resistively insulated REBCO magnets, especially those in fusion or accelerator work who need practical ways to set Rc. A reader looking for starting recipes will get immediate value from the described procedures. It deserves a serious referee because the experimental result is new, the methods are clearly motivated by prior data, and the measurements are reproducible in principle. I would send it out for review; the evidence supports the claims made, and external feedback would help clarify how far the stabilization extends.

Referee Report

2 major / 2 minor

Summary. The manuscript reports experimental methods to control turn-to-turn contact resistivity Rc in resistively insulated REBCO coils. Conductive fillers (paste or epoxy), PbSn solder coating on REBCO tape, and controlled air oxidation of stainless-steel co-wind tape are shown to set Rc to target values of 1000 and 5000 µΩ-cm² while largely eliminating sensitivity to contact-pressure cycling. Short-sample measurements at 4.2 K remain stable through 30 000 cycles; the same Rc values are recovered in a 6 double-pancake test coil tested at 4.2 K.

Significance. If the reported stabilization holds, the work removes a previously identified obstacle to practical RI REBCO magnet design by providing reproducible, pressure-cycle-insensitive Rc values in the range needed for both rapid ramping and safe quench protection. The direct, low-temperature resistivity measurements on both short samples and a small coil constitute a clear experimental advance over the earlier uncontrolled pressure-cycling behavior.

major comments (2)

[Abstract] Abstract: the statements that Rc “was controlled to prescribed values of 1000 and 5000 uOhm-cm2” and “was not sensitive to contact pressure cycling up to 30,000 cycles” are presented without error bars, number of replicates, or any description of the measurement protocol or contact-area determination. These omissions make it impossible to judge the statistical reliability or repeatability of the claimed control.
[Abstract] Abstract and final paragraph: the claim that the 6 double-pancake coil test “demonstrated the ability of this new method to control Rc in large coils” extrapolates beyond the data. No post-quench Rc measurements, thermal-cycle data beyond the tested 30 000 pressure cycles, or scaling analysis for larger winding radii or non-uniform Lorentz forces are provided, leaving the applicability to full-scale magnets unverified.

minor comments (2)

Notation: “uOhm-cm2” should be written consistently as µΩ·cm² (or µΩ cm²) throughout the text, figures, and tables.
The manuscript would benefit from a brief table or figure summarizing the exact filler compositions, solder thickness, and oxidation temperatures that produced the two target Rc values.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the positive assessment and constructive comments on the abstract. We address each point below and have revised the manuscript accordingly to improve clarity without overstating the results.

read point-by-point responses

Referee: [Abstract] Abstract: the statements that Rc “was controlled to prescribed values of 1000 and 5000 uOhm-cm2” and “was not sensitive to contact pressure cycling up to 30,000 cycles” are presented without error bars, number of replicates, or any description of the measurement protocol or contact-area determination. These omissions make it impossible to judge the statistical reliability or repeatability of the claimed control.

Authors: We agree that the abstract would benefit from additional context on experimental reliability. The full manuscript describes the measurement protocol, contact-area determination from sample geometry, and results from multiple short samples in the Methods and Results sections, with stability shown across the reported cycles. We will revise the abstract to note that the target Rc values were achieved with observed stability in repeated measurements, while directing readers to the main text for the detailed protocol, replicates, and any error bars presented in the figures. revision: yes
Referee: [Abstract] Abstract and final paragraph: the claim that the 6 double-pancake coil test “demonstrated the ability of this new method to control Rc in large coils” extrapolates beyond the data. No post-quench Rc measurements, thermal-cycle data beyond the tested 30 000 pressure cycles, or scaling analysis for larger winding radii or non-uniform Lorentz forces are provided, leaving the applicability to full-scale magnets unverified.

Authors: We accept that the original wording overstates the scope. The 6 double-pancake coil provides a demonstration that the short-sample Rc control method produces comparable values in a wound configuration tested at 4.2 K. We have revised the abstract and final paragraph to state that the method was demonstrated in a test coil with Rc values comparable to short-sample results. We acknowledge the lack of post-quench Rc measurements, additional thermal-cycle data, and scaling analysis for larger magnets or non-uniform forces as limitations of the present work and areas for future investigation. revision: yes

Circularity Check

0 steps flagged

No circularity; purely experimental results from direct measurements

full rationale

The paper reports experimental methods (conductive fillers, PbSn solder coating, SS oxidation) to stabilize Rc at target values, with short-sample tests up to 30,000 pressure cycles at 4.2 K and a 6-DP coil test showing comparable Rc. No equations, derivations, fitted parameters, predictions, or self-citations of uniqueness theorems appear. All claims rest on direct resistivity measurements rather than any reduction of outputs to inputs by construction. The work is self-contained against external benchmarks with no load-bearing circular steps.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on empirical measurements of resistivity under controlled cycling; no free parameters are fitted to data, no new physical entities are postulated, and background assumptions are standard materials properties at cryogenic temperatures.

axioms (1)

domain assumption REBCO tapes remain superconducting and mechanically stable at 4.2 K under the tested contact pressures
Invoked implicitly for all cryogenic testing and quench-related claims.

pith-pipeline@v0.9.0 · 5680 in / 1387 out tokens · 36698 ms · 2026-05-10T09:12:40.452958+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

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Introduction No-insulation (NI) and resistive-insulation (RI) REBCO magnet technologies allow for a high engineering current density compared with their conventional insulation counterparts. They have been developing rapidly over the past 15 years. Using NI technology, an extremely high field of 45.2 T ha s been achieved in a background field of 31 T [1] ...

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[1] [1]

They have been developing rapidly over the past 15 years

Introduction No-insulation (NI) and resistive-insulation (RI) REBCO magnet technologies allow for a high engineering current density compared with their conventional insulation counterparts. They have been developing rapidly over the past 15 years. Using NI technology, an extremely high field of 45.2 T ha s been achieved in a background field of 31 T [1] ...

work page

[2] [2]

They were 4 mm wide with 50 µm thick Hastelloy C- 276 substrate and 20 µm Cu stabilizer on each side

Experiment REBCO tapes were SCS4050-AP by SuperPower Inc. They were 4 mm wide with 50 µm thick Hastelloy C- 276 substrate and 20 µm Cu stabilizer on each side. The stainless steel co-wind were 316L half hard tapes that were 4 mm wide and 25 µm thick, procured from Ulbrich of Illinois, Inc. ρc tests using short samples were performed at 77 and 4.2 K in a d...

work page

[3] [3]

In case of REBCO/stainless steel contact, an obvious solution is to form a thin oxide layer on stainless steel with controlled thickness [11]

Results 3.1 Preliminary development for ρc control Control of ρc is the ability to adjust ρc to a specified value for RI magnet design. In case of REBCO/stainless steel contact, an obvious solution is to form a thin oxide layer on stainless steel with controlled thickness [11]. Figure 3 shows ρc of REBCO/stainless steel/REBCO stack at 4.2 K as a function ...

work page 2000

[4] [4]

It is unlikely that turn-to-turn contact resistance can be adjusted once the coil is wound

Discussions Achieving reliable control of ρc is extremely challenging. It is unlikely that turn-to-turn contact resistance can be adjusted once the coil is wound. It must be tuned in the coil fabrication process by changing either the contact area [31], or ρc as in [23] and this work, or both [21],[32]. It also seems to be difficult to have a consistent ρ...

work page

[5] [5]

Liu, et al [22]

It is noteworthy that the apparent contact resistivity increase with increasing discharging current was observed by S. Liu, et al [22]. 18 Conclusions We developed a reliable method for controlling contact resistance ρc in resistively-insulated REBCO coils. In our initial development efforts, we demonstrated ρc control by oxidation of stainless steel co-w...

work page

[6] [6]

Hahn S, Kim K L, Kim K, Hu X, Painter T A, Dixon I R, Kim S, Bhattarai K R, Noguchi S, Jaroszynski J, Larbalestier D C 2019 45.5-tesla direct-current magnetic field generated with a high-temperature superconducting magnet Nature, 570 496-9 https://doi.org/10.1038/s41586-019-1293-1

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