Research and Scholarship (CSMM)

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Magnetization and Loss Measurements of YBCO CORC and Roebel Cables for Accelerators using ±T dipole Susceptometer
(2018) Sumption, M.; Kovacs, C.; Myers, C.; Majoros, M.; Collings, E.
Magnetization and Flux Penetration of YBCO CORC Cable Segments at the Injection fields of Accelerator Magnets
(2018) Sumption, M.; Myers, C.; Collings, E.
Development of APC (ZrO2) Nb3Sn Multifilamentary and Ternary Conductor
(2017) Sumption, M.; Rochester, J.; Collings, E.
Magnetization and Loss of Superconducting Cables with Helical (CORC) and Twisted Stacked Geometries -- FEM and Analytical Modelling for Accelerator Magnets
(2018) Sumption, M.; Kun, D.; Majoros, M.; Kovacs, C.; Myers, C.; Collings, E.
Motivation – accelerator quality; Analysis of CORC and Twist Stack Cable Magnetization; Comparison of Data and Theory; Application to accelerators.
Unravelling the Magnetization of CORC, TWST, and Roebel Cables for HEP applications and Associated Error fields
(2018) Sumption, M.; Kun, D.; Majoros, M.; Kovacs, C.; Myers, C.; Collings, E.
Motivation – accelerator quality; Analysis of CORC and Twist Stack Cable Magnetization; Results from 12 T cryogen free system; Results from 3 T Dipole System; Drift Suppression; Comparison of Data and Theory.
Influence of field cycle on coated conductor magnetization and decay for accelerator applications
(2017) Sumption, M.; Myers, C.; Majoros, M.; Collings, E.
Analysis of Magnetization effects for HTS conductors for HEP magnets
(2017) Sumption, M.; Kovacs, C.; Majoros, M.; Collings, E.
Motivation – accelerator quality Comparison of accelerator and b3 Magnetization of Tape vs Cable Magnetization of various cable types Coupling -- Magnetization -- loss? Decay and its implications
Stability and currents sharing in a conduction cooled racetrack coil wound of YBCO coated conductor tape – FEM modeling
(2017) Majoros, M.; Sumption, M.; Collings, E.
We present a Finite Element Method (FEM) analysis of stability and current sharing in a conduction cooled race track coil at 20 K. The coil was assumed to be wound using YBCO coated conductor tape either non-insulated or insulated by a kapton tape. An anisotropic continuum model of the winding for thermal propagation, with input parameters taken from experiments, was developed and adopted in computations. Both coils – with non-insulated as well as kapton insulated tape – have nearly the same critical current. The coil with non insulated tape needs more YBCO tape but can operate at lower current. As a consequence of this the non-insulated coil has a higher operational temperature margin. The coil wound using the non-insulated tape also shows a higher degree of electromagnetic stability because of possible current sharing among the turns within the winding. Stress–strain modeling showed that due to a strong anisotropy of Jc in YBCO film, the critical current of the coils is not limited by mechanical stresses, but by the radial magnetic field component in the winding, i.e. by the field component parallel to c-axis of the YBCO film. Using a power supply in the constant current mode the non insulated tape shows a pronounced current sharing while the insulated tape shows a high degree of power concentrated around the defect region (the so called “hot spot”) with power loss density more than three orders of magnitude higher than the power loss density in coil wound using the non-insulated YBCO tape. Magnet length = 0.5 m, magnet width = 0.25 m, magnet thickness = 0.025 m.
Validation of Finite-Element Models of Persistent-Current Effects in Nb3Sn Accelerator Magnets
(IEEE, 2015) Wang, X.; Ambrosio, G.; Chlachidze, G.; Collings, E.; Dietderich, D.; DiMarco, J.; Felice, H.; Ghosh, A. K.; Godeke, A.; Gourlay, S. A.; Marchevsky, M.; Prestemon, S. O.; Sabbi, G.; Sumption, M.; Velev, G. V.; Xu, X.; Zlobin, A. V.
Persistent magnetization currents are induced in superconducting filaments during the current ramping in magnets. The resulting perturbation to the design magnetic field leads to field quality degradation, in particular at low field where the effect is stronger relative to the main field. The effects observed in NbTi accelerator magnets were reproduced well with the critical state model. However, this approach becomes less accurate for the calculation of the persistent-current effects observed in Nb3Sn accelerator magnets. Here a finite-element method based on the measured strand magnetization is validated using three state-of-the-art Nb3Sn accelerator magnets featuring different subelement diameters, conductor critical currents, magnet designs and test temperatures. The temperature dependence of the persistent current effects is reproduced. Based on the validated model, the impact of conductor design on the persistent-current effects is discussed. The strengths, limitations and possible improvements of the approach are also discussed.
Quench and stability of Roebel cables at 77 K and self-field: minimum quench power, cold end cooling, and cable cooling efficiency
(Elsevier, 2018) Kovacs, C.; Majoros, M.; Sumption, M.; Collings, E.
A 9-tape, 14 mm wide ReBCO Roebel cable was soldered onto a U-shaped holder. The critical current, Ic, was measured at 77 K and self-field. The cryostability of the cable was studied in response to the application of local pulses of 1 to 14 W at several values of i = I/Ic. A detailed analysis of the cable’s cryostability was presented. With a Stekly parameter α = G/Q « 1 and a heat generation margin of ~190 kW/m2 the present ReBCO cable was shown to be ultra cryostable with respect to internally generated transport-current overload. However, the cable was much less stable against externally and locally applied disturbances because of the tendency to initiate local film boiling. A locally applied 10 W led to a prediction of a film-boiling-cooled zone with a temperature of 181 K. However, when cold-end cooling was considered, the predicted hot spot temperature decreased to 87-115 K depending on the surface-cooling efficiency. Predictions were compared to experiment extracting a cooling efficiency parameter representing the penetration of cryogen into the cable. Experiment showed the generation of time stable normal zones which were a function of disturbance power. This led to the description of the cable stability in terms of minimum quench power; the results are presented in stability diagrams.
Ternary Nb3Sn superconductors with artificial pinning centers and high upper critical fields
(IOP Publishing Ltd, 2019) Xu, X.; Rochester, J.; Peng, X.; Sumption, M.; Tomsic, M.
In this letter we demonstrate the development of ternary Nb3Sn multifilamentary conductors with artificial pinning centers (APC) which achieve high critical fields. These recently-developed conductors were tested in a 31 T magnet, and the results showed that their upper critical field (Bc2) values at 4.2 K are 27–28 T, and irreversible field (Birr) values are above 26 T, values similar to or higher than those of best rod-restack-process (RRP) conductors. The non-Cu Jc has been brought to nearly 1200 A mm−2 at 16 T and 4.2 K, comparable to RRP, in spite of the fact that the fine-grain Nb3Sn fractions in filaments are still low (20%–30%) and the grain sizes are still not fully refined (70–80 nm) due to conductor designs and heat treatments that are not yet optimized. The Nb3Sn layer Jc at 4.2 K, 16 T is 4710 A mm−2 for the APC wire with 1%Zr, about 2.5 times higher than RRP conductors, in spite of the fact that its grain size is not yet fully refined due to insufficient oxygen and unoptimized heat treatment. An analysis is presented about the non-Cu Jc that can be achieved by further optimizing the APC conductors and their heat treatments.
Magnetization and Analysis of CORC, TWST, and Roebel Cables for HEP applications and Associated Error fields
(2017) Sumption, M.; Majoros, M.; Kovacs, C.; Collings, E.
Motivation – accelerator quality Comparison of accelerator and b3 Expressions for Magnetization of Tape vs Cable Measurements and Analysis of Magnetization of various cable types Coupling -- Magnetization -- loss?
REBCO Roebel cable stability, current sharing, and quench measurements in a LHe bath and applied magnetic fields of 12 Tesla
(2017) Kovacs, C.; Majoros, M.; Sumption, M.; Collings, E.
Normal zone propagation, protection and quench measurements in a YBCO pancake coil at 77 K and at 4.2 K in magnetic fields up to 10 T
(2017) Majoros, M.; Sumption, M.; Kovacs, C.; Collings, E.
We have measured stability, and normal zone propagation (NZP) in a YBCO pancake coil in liquid nitrogen bath (77.3 K) and in liquid helium bath (4.2 K). The experiments in liquid helium bath at 4.2 K were carried out in applied magnetic fields up to 10 T and at transport currents of a certain percentage of the coil critical current at a given field. The pancake coil was instrumented for voltage and temperature measurements at several places around the winding, such that both radial and azimuthal quench propagation could be measured. A heater was placed on the inner-most part of the winding. Heat pulses of various powers and durations were generated at different coil currents to measure quench and NZP. A protection circuit with a possibility of setting different quench voltage limits was used to protect the coil after quench.
Single-Strand Excitation Rig for Probing Current Sharing and ICR in Nb3Sn Rutherford Cable at 4.2K up to 12 Tesla
(2017) Kovacs, C.; Sumption, M.; Collings, E.
Stability, quench, and current sharing in Roebel and CORC cables for HEP magnets
(2017-09) Sumption, M.; Majoros, M.; Kovacs, C.; Collings, E.
Internally Oxidized Nb3Sn Strands with Fine Grain Size and High Critical Current Density
(Wiley, 2015) Xu, X.; Sumption, M.; Peng, X.
Nb3Sn superconducting strands are the most practical conductors to generate high magnetic fields (12-16 T), and thus have significant applications in nuclear magnetic resonance (NMR), and great potential for fusion reactors and particle accelerator magnets. High critical current density (Jc) is a key parameter for such applications. Significant efforts towards optimization of various factors led to an 80% improvement in Jc from the early 1990s to 2003, when the 4.2 K, 12 T non-matrix Jc reached 3000 A/mm2 (corresponding to 5000 A/mm2 in Nb3Sn layer Jc).[1,2] However, further efforts over the past decade have failed to bring about further increase beyond this level,[3,4] leading some researchers to conclude that the Jc of conventional Nb3Sn strands had reached its maximum. Here, however, by applying an internal oxidation method, we reduce the grain size by a factor of three and nearly double the 12 T Jc. In this method, a Nb3Sn strand is fabricated with Nb-Zr alloy as starting material; with oxygen supplied properly via an oxide powder, the Zr atoms in the Nb-Zr alloy are internally oxidized, forming fine intra-granular and inter-granular ZrO2 particles in Nb3Sn layer, which effectively refine Nb3Sn grain size. At a reaction temperature of 625 °C, grain size down to 20-50 nm (36 nm on average) has been achieved. For this sample the 4.2 K, 12 T Nb3Sn layer Jc reached 9600 A/mm2.
A model for the compositions of non-stoichiometric intermediate phases formed by diffusion reactions, and its application to Nb3Sn superconductors
(Nature Publishing Group, 2016) Xu, X.; Sumption, M.
In this work we explore the compositions of non-stoichiometric intermediate phases formed by diffusion reactions: a mathematical framework is developed and tested against the specific case of Nb3Sn superconductors. In the first part, the governing equations for the bulk diffusion and interphase interface reactions during the growth of a compound are derived, numerical solutions to which give both the composition profile and growth rate of the compound layer. The analytic solutions are obtained with certain approximations made. In the second part, we explain an effect that the composition characteristics of compounds can be quite different depending on whether it is the bulk diffusion or grain boundary diffusion that dominates in the compounds, and that “frozen” bulk diffusion leads to unique composition characteristics that the bulk composition of a compound layer remains unchanged after its initial formation instead of varying with the diffusion reaction system; here the model is modified for the case of grain boundary diffusion. Finally, we apply this model to the Nb3Sn superconductors and propose approaches to control their compositions.
Stability and normal zone propagation in YBCO CORC cables
(IOP Publishing Ltd, 2016) Majoros, M.; Sumption, M.; Collings, E.; Van Der Laan, D.
In this work, a two layer CORC (Conductor On Round Core) cable was tested for stability and Normal Zone Propagation (NZP) at 77 K in a liquid nitrogen bath. The cable was instrumented with voltage taps and wires on each strand over the cable’s central portion (i.e. excluding the end connections of the cable with the outside world). A heater was placed in the central zone on the surface of the cable, which allowed pulses of various powers and durations to be generated. Shrinking (recovering) and expanding (not recovering) normal zones have been detected, as well as stationary zones which were in thermal equilibrium. Such Stationary Thermal Equilibrium zones (STE) did not expand or contract, and hit a constant upper temperature while the heater current persisted; they are essentially a form of Stekly stability. Overall, the cable showed a high degree of stability. Notably, it was able to carry a current of 0.45Ic cable with maximum temperature of 123 K for one minute without damage.
Influence of field penetration ratios and filamentation on end-effect related hysteretic loss reductions for superconducting strips
(IOP Publishing Ltd, 2017) Dong, K.; Sumption, M.; Collings, E.; Majoros, M.; Yu, H.; Hu, M.
There are a few key conductor-specific factors which influence the power loss of superconductors; these include critical current, geometry, and normal metal resistivity. This paper focuses on the influence of sample geometry on the power loss of superconducting strips and the effect of filamentation and sample length as a function of the field penetration state of the superconductor. We start with the analytical equations for infinite slabs and strips and then consider the influence of end effects for both unstriated and striated conductor. The loss is then calculated and compared as a function of applied field for striated and unstriated conductors. These results are much more general than they might seem at first glance, since they will be important building blocks for analytic loss calculations for twisted geometries for coated conductors, including helical (Conductor on Round Core, CORC), and twisted (e.g., twist stack cables) geometries. We show that for relatively low field penetration, end effects and reduced field penetration both reduce loss. In addition, for filamentary samples the relevant ratio of length scales becomes the filament width to sample length, thus modifying the loss ratios.

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