MgB2 Strands Research Articles

Development and testing of a three-period, subsize 2G AIMI MgB2 planar undulator

Compared to Nb3Sn- and NbTi-wound superconducting undulators (SCUs), MgB2-wound SCUs are of interest for future electron synchrotron beam light sources owing to their higher temperature operating margin and associated stability. In this study, a three-period undulator consisting of twelve racetrack coils wound with 2nd generation (2 G) multifilamentary advanced-internal-magnesium-infiltration MgB2 strands were fabricated and tested in liquid and gaseous helium (He) over a temperature range of 4.2 K–20 K. The coil winding cross sections (in each coil) were 5 mm wide and 4.8 mm thick. At 4.2 K, a critical current (I c) of 325.7 Amps produced a maximum undulator bore field of 1.19 T. It should be noted that the short, 3-period nature of the coil led to an asymmetry in the field profile (the maximum positive field was 1.19 T, the maximum negative was −0.25 T), suggesting a peak field of 0.72 T in the absence of end effects. Finite element modeling (FEM) results of simulations for a one meter long undulator of otherwise identical design gave 0.85 T (larger because of higher currents enabled by the lower field). But in any case, the I c value coil reached is 94% of that of the short sample (dictated by the 1.19 T positive field for the coil as tested). FEM was performed to study the magnetic field profile, which was validated experimentally. The magnetic field was measured using a Hall probe which was translated along the beam axis during measurement to explore the spatial field variation along the beam travel direction. The spatially alternating field was asymmetric, and the maximum field was more prominent in the positive direction than in the negative direction, the difference being due to broken symmetry, that is, short coil end effects. In this work, we show useful fields are possible for MgB2 undulators; the use of such conductors can allow a larger thermal margin and enable conduction-cooled operation.

Magnetic, Mechanical and Thermal Modeling of Superconducting, Whole-body, Actively Shielded, 3 T MRI Magnets Wound Using MgB2 Strands for Liquid Cryogen Free Operation.

we present magnetic, mechanical and thermal modeling results for a 3 Tesla actively shielded whole body MRI (Magnetic Resonance Imaging) magnet consisting of coils with a square cross section of their windings. The magnet design was a segmented coil type optimized to minimize conductor length while hitting the standard field quality and DSV (Diameter of Spherical Volume) specifications as well as a standard, compact size 3 T system. It had an overall magnet length and conductor length which can lead to conduction cooled designs comparable to NbTi helium bath cooled 3 T MRI magnets. The design had a magnetic field homogeneity better than 10 ppm (part-per-million) within a DSV (Diameter of Spherical Volume) of 48 cm and the total magnet winding length of 1.37 m. A new class of MgB2 strand especially designed for MRI applications was considered as a possible candidate for winding such magnets. This work represents the first magnetic, mechanical and thermal design for a whole-body 3 T MgB2 short (1.37 m length) MRI magnet based on the performance parameters of existing MgB2 wire. 3 Tesla MRI magnet can operate at 20 K at 67 % of its critical current.

The Role of CHPD and AIMI processing on enhancing JC and transverse connectivity of in-situ MgB2 strand

Research into in-situ MgB2 strand has been focused on improvements in JC through reduction of porosity. Both of cold-high-pressure-densification (CHPD) and advanced-internal-magnesium-infiltration (AIMI) techniques can effectively remove the voids in in-situ MgB2 strands. This study shows the nature of the reduced porosity for in-situ MgB2 strands lies on increases in transverse grain connectivity as well as longitudinal connectivity. The CHPD method bi-axially applying 1.0 GPa and 1.5 GPa yielded 4.2 K J CM║s of 9.6 × 104 A/cm2 and 8.5 × 104 A/cm2 at 5 T, respectively, with compared with 6.0 × 104 A/cm2 for typical powder-in-tube (PIT) in-situ strand. Moreover, AIMI-processed monofilamentary MgB2 strand obtained even higher J Cs and transverse grain connectivity than the CHPD strands.

Development of MgB<sub>2</sub> Rutherford Cable for Large-scale SMES Coils Cooled by Liquid Hydrogen

To realize the practical application of Superconducting Magnetic Energy Storage (SMES) systems cooled by liquid hydrogen, MgB2 is promising as a material for SMES coils in terms of reducing cost in the future and enabling high critical temperature above the temperature of liquid hydrogen. Major technological development issues to realize the application of SMES coils are fabrication of the Rutherford superconducting cable with a kA-class current capacity and fabricating the coils using both Wind and React (W&R) and React and Wind (R&W) methods. Since there is little data regarding the allowable bending strain of the MgB2 strand before heat treatment, an experiment was conducted and it was determined to be approximately 4 % before the fabrication of the cable. The Rutherford cable was designed and developed based on analysis of the space curve theory because the complex bending strain is exposed to the MgB2 filaments during the cable and coil fabrication processes. However, the critical currents measured using cables developed according to the design were degraded approximately 30%. This degradation can be explained as being due to local deformation of the strands caused by compression at the cross-over point between the strands of the conductor. The broken sections of the Nb barrier, which were observed by reviewing SEM images of the strand cross-section, can cause degradation of the critical current. Two double pancake coils, which were primarily wound using the Rutherford cables and manufactured applying both the R&W and W&R methods for the purpose of comparing fabrication processes, were connected in series to excite the coils. As a result, an operating current of 600 A was successfully achieved.

Design optimization and evaluation of the 3 kA MgB2 cable at 4.3 K for the superconducting link project at CERN

High-current superconducting links (SC links) are being developed at CERN for powering the superconducting magnets of the High-Luminosity Large Hadron Collider upgrade. The links contain MgB2 cables, each about 100 m long, rated at currents of up to 18 kA. The assembly of the cables is carried out with reacted wires; cable geometry and cabling processes take into account the mechanical properties of the reacted MgB2 conductor. The production of the cables as well as their final installation requires good mechanical stability, flexibility and electrical performance. The superconductor should have good mechanical strength in view of the stresses during cabling (tension and bending), operation at cryogenic temperatures (Lorentz forces and thermal contraction) and handling installation (tension and bending) at room temperature. Moreover, the electrical integrity of the MgB2 wire (IC degradation ≤5%) should be ensured after the cabling process for the SC links. Therefore, the study of the mechanical properties of the MgB2 wire and the definition of the cable design parameters are crucial for the project. In the present work we report on the optimization and validation of the design of the 18-strand, 3 kA MgB2 cable by assessing the electromechanical behaviour of 3 kA MgB2 superconducting cables produced with different cabling parameters. The critical current (IC) of the MgB2 wires was measured after triple bending and axial tensile load, and strands extracted from the cables were tested to quantify the critical current degradation. Electrical measurements of two, 2 m long, 18-strand MgB2 cables were carried out in the FRESCA test facility at CERN and no degradation was observed. Cabling parameters such as the bending radius and the twist pitch were evaluated, and the most appropriate geometries for the MgB2 18-strand cables have been selected. Numerical simulations were performed to allow a detailed study of the strains developed in the MgB2 strands due to the mechanical loading. The present results have been successfully transferred to industry for the first industrial production in the framework of the SC Link project.

Architecture and Transport Properties of Multifilamentary MgB2 Strands for MRI and Low AC Loss Applications.

Standard in-situ type MgB2 strands manufactured by Hyper Tech Inc have 19 - 36 subelements, a monel outer sheath, and a Cu interfilamentary matrix. Typical transport Jc s of the strands are 2×105 A/cm2 with n-values of 20 - 30 at 4.2 K and 5 T. This work introduces two new MgB2 conductor designs. First, a new class of MgB2 strand is designed for magnetic resonance imaging applications. This type has a higher Cu content designed to enhance protection of a magnet wound with it, and a larger diameter to increase the critical current. Second, a new class of low AC loss MgB2 strand with high filament count and a high resistance matrix is discussed. Transport properties at 4.2 K and fields up to 10 T are reported. Optical techniques are used to study the macro- and micro-structures of these MgB2 strands.

Filamentary MgB2Wires With Low Magnetization AC Losses

This paper describes the development of MgB2 wires with low magnetization ac losses made by in situ or internal magnesium diffusion methods. A strong contribution of eddy currents was found in a wire with a highly conductive copper sheath, but a GlidCop sheath showed no eddy current loss addition. For minimized hysteresis loss, the filaments have to be small enough, which, however, reduces the critical current. Twisting of filaments was used to achieve a reduction in coupling loss; however, this induced mechanical strain into the filaments and decreased the critical current. Cabling of MgB2 strands decreased the coupling loss without degradation. Interfilament resistivity was tuned using different barriers (Nb, Ti, and NbTi), which allowed for an effective reduction in the coupling loss and resulted in a shift of the ac loss maximum into high frequencies.

Influence of twisting and bending on the Jc and n-value of multifilamentary MgB2 strands

The influences of strand twisting and bending (applied at room temperature) on the critical current densities, Jc, and n-values of MgB2 multifilamentary strands were evaluated at 4.2 K as function of applied field strength, B. Three types of MgB2 strand were evaluated: (i) advanced internal magnesium infiltration (AIMI)-processed strands with 18 filaments (AIMI-18), (ii) powder-in-tube (PIT) strands processed using a continuous tube forming and filling (CTFF) technique with 36 filaments (PIT-36) and (iii) CTFF processed PIT strands with 54 filaments (PIT-54). Transport measurements of Jc(B) and n-value at 4.2 K in fields of up to 10 T were made on: (i) PIT-54 after it was twisted (at room temperature) to twist pitch values, Lp, of 10–100 mm. Transport measurements of Jc(B) and n-value were performed at 4.2 K; (ii) PIT-36 and AIMI-18 after applying bending strains up to 0.6% at room temperature.PIT-54 twisted to pitches of 100 mm down to 10 mm exhibited no degradation in Jc(B) and only small changes in n-value. Both the Jc(B) and n-value of PIT-36 were seen to be tolerant to bending strain of up to 0.4%. On the other hand, AIMI-18 showed ±10% changes in Jc(B) and significant scatter in n-value over the bending strain range of 0-0.6%.

Transport Critical Current Densities and <formula formulatype="inline"><tex Notation="TeX">$n$</tex> </formula>-Values of Multifilamentary <formula formulatype="inline"><tex Notation="TeX">$\hbox{MgB}_{2}$</tex></formula> Wires at Various Temperatures and Magnetic Fields

A series of high-performing continuous-tube-filling-forming-fabricated (CTFF) powder-in-tube-type (PIT) MgB2 strands have been prepared, with critical current densities, Jcs, higher than 1.30×105 A/cm2 and n-values above 30 at 4.2 K, 5 T. The transport Jcs and corresponding n-values of a selected sample C1 were reported at temperatures, T, of 4.2-30 K and magnetic fields, B, of 0-12 T. The Jc - B data were fitted by Eisterer's percolation model and the temperature dependences of the fitting parameters were analyzed. The n - Jc curves showed a power law relationship n∞Jcm, also previously observed in state-of-the-art monocore MgB2 wires. Further analysis revealed that, like monofilamentary wires, the n(B, T) of this multifilamentary strand can also be estimated for all B and T based on the results of transport voltage-current (V- I) measurements made at one arbitrarily chosen temperature.

Numerical modeling of MgB2 conductors for high power AC transmission

Cables made of MgB2 superconductors are currently explored as a viable solution for transporting high electrical power in the AC regime. In order to be competitive against the DC solution, the cables need to have an acceptable level of AC losses. In this contribution, we discuss the main aspects relevant for designing a cable with a sufficiently low AC loss level. To this end, we perform finite-element-method (FEM) simulations to determine the current and field distributions and calculate the AC losses of such cable configuration. For current capacities of 2–5kA (peak), power cables are assembled from a relatively small number of MgB2 strands. The performance of such cables strongly depends on the current and field distributions, which are in turn influenced by the number and the arrangement of the superconducting components and also by the magnetic properties of supporting materials. Numerical simulations can help to test different cable configurations and provide important insights for optimizing the cable’s design. The numerical model includes the field dependence of the superconductor’s critical current density Jc(B) as well as the non-linear properties of magnetic materials.

Effects of carbon concentration and filament number on advanced internal Mg infiltration-processed MgB2 strands

In this paper we show that an advanced internal Mg infiltration method (AIMI) is effective in producing superconducting wires containing dense MgB2 layers with high critical current densities. The in-field critical current densities of a series of AIMI-fabricated MgB2 strands were investigated in terms of C doping levels, heat treatment (HT) time and filament numbers. The highest layer Jc for our monofilamentary AIMI strands was 1.5 × 105 A cm−2 at 10 T, 4.2 K, when the C concentration was 3 mol% and the strand was heat-treated at 675 ° C for 4 h. Transport critical currents were also measured at 4.2 K on short samples and 1 m segments of 18-filament C doped AIMI strands. The layer Jcs reached 4.3 × 105 A cm−2 at 5 T and 7.1 × 104 A cm−2 at 10 T, twice as high as those of the best powder-in-tube strands. The analysis of these results indicates that the AIMI strands, possessing both high layer Jcs and engineering Jes after further optimization, have strong potential for commercial applications.

Critical Current Density and Current Transfer Length of Multifilamentary $\hbox{MgB}_{2}$ Strands of Various Design

In this paper, a series of high-performing powder-in-tube MgB2 strands have been prepared. Transport voltage-current measurements were performed to determine the effects of C doping and strand geometry such as filament numbers. The best Jc for our samples was 1.0 × 105 A/cm2 at 4.2 K, 7 T, for a strand using B powder with 3% C addition. The current transfer length was also measured for MgB2 short wires with Nb chemical barrier and Monel outer sheath. The current transfer length ranged from 2 to 12 mm, and had a correlation with the filament numbers.

Critical current densities of doped MgB2 strands in low and high applied field ranges: The Jc(B) crossover effect

Numerous classes of dopant have been added to MgB2 in order to raise the upper critical field, Bc2, and hence to increase the field range over which the pinned superconductor has the possibility of supporting supercurrent. Thus dopant additions to grain-boundary-pinned MgB2, for example, have the effect of raising the high field critical current density, Jc(B). However, at low fields, when B is relatively small compared to Bc2, Jc(B) decreases as Bc2 increases. This leads to a low field convergence, or even the intersection, of the Jc(B) curves of a family of variously doped MgB2 strands. Two important conclusions derive from this “crossover effect”: (i) Doping-induced increases of Bc2 should be applied only if improved high field properties are required. For low field applications of MgB2 such as: low field nuclear magnetic resonance imaging (MRI), synchrotron insertion devices, and current leads, doping should be avoided since not only would the increased Bc2 degrade Jc, but the possible chemical byproducts of doping may reduce connectivity; (ii) If an across-the-board increase in Jc(B) is desired there is no substitute for increased connectivity, in regard to which densification of the MgB2 layer (the subject of a separate report) is recommended.

Influence of Strand Design, Boron Type, and Carbon Doping Method on the Transport Properties of Powder-in-Tube $\hbox{MgB}_{2-{\rm X}}\hbox{C}_{\rm X}$ Strands

The transport properties of a number of MgB2 strands have been investigated in terms of their response to strand design, starting B powder choice, and the approach to C doping used. The strands had various designs, specifically; (i) several chemical barriers were introduced, viz: Fe and Nb, (ii) the strands were encased in various outer-sheath materials, viz.: monel, Cu+monel, monel+glidcop, Nb+monel, (iii) the filament counts were varied (1, 18, and 36), and (iv) the final strand diameter was varied. In addition, for a subset of the strand designs several B powder and C-dopant types were investigated. Specifically, two types of amorphous B powder were used: (i) Moissan based "Tangshan boron" (ii) "SMI-boron" which is produced in a plasma torch by the reduction-by-hydrogen of BCl3. Two approaches to C doping were taken: (i) "malic-acid treatment" in which C is introduced into the B powder precursor by the moderate temperature drying out a slurry of B mixed in with a malic-acid-toluene solution (during which the malic acid decomposes leaving C as the only solid residue) before the Mg powder is mixed in; (ii) direct C doping of the SMI-produced B by introducing a known percentage of CH4 into the plasma flame. Critical current densities, Jc, were measured on 1.5 m long samples at 4.2 K in fields of up to 14 T; of all the strands measured, that doped with SMI-C at a nominal 4 mol% C (in relation B) yielded the highest Jc values e.g 1.1x105 A/cm2 at 7 T, 4.5x104 at 10 T, and 2.2x104 A/cm2 at 12 T. The n-values are given for all strands at 5 and 10 T, and for a certain set of strands the magnetic field dependencies of the n-values and the influence of C-doping is presented. Finally we demonstrate that, over a wide range of B, log(Jc) decreases linearly with B with a slope -{\alpha} such that the Jc(B) of any strand can be parameterized in terms of {\alpha} and its zero-field intercept Jc(B=0).

Drawing induced texture and the evolution of superconductive properties with heat treatment time in powder-in-tube in situ processed MgB2 strands

Monocore powder-in-tube MgB2 precursor strands were cold-drawn and heat-treated at 600 and 700 °C for times of up to 71 h, and structure–property relationships examined. Drawing induced elongation of the Mg particles led, after heat treatment (HT), to a textured macrostructure consisting of elongated fine polycrystalline MgB2 fibers (or veins) separated by elongated pores. The superconducting transition temperature (Tc), critical current density (Jc) and bulk pinning force density (Fp) were correlated with the macrostructure and grain size. Grain size increased with HT time at both 600 and 700 °C. Critical current density and hence Fp decreased monotonically but not linearly with grain size. Overall, it was observed that at 700 °C the MgB2 reaction was more or less complete after as little as 30 min; at 600 °C full reaction completion did not occur until 72 h into the HT. Transport, Jct(B), was measured in a perpendicular applied field, and the magnetic critical current densities, and , were measured in perpendicular and parallel (axial) applied fields, respectively. Particularly noticeable was the premature drop-off of at fields well below the irreversibility field of Jct(B). This effect is attributed to the fibrous macrostructure and its accompanying anisotropic connectivity. Magnetic measurements with the field directed along the strand axis yielded a critical density, , for current flowing transversely to the strand axis that was less than, and dropped off more rapidly than, Jct(B). In the conventional magnetic measurement, the loop currents that support the magnetization are restricted by the lower of Jct(B) and : in the present case the latter, leading to the premature drop-off of the measured compared to Jct(B) with increasing field. This result is supported by Kramer plots of the and Jct(B) data, which lead to an irreversibility field for transverse current that is very much less than the usual transport-measured longitudinal one, Birr,t.

Critical current densities and n-values of MgB2 strands over a wide range of temperatures and fields

Transport measurements of critical current density, Jct, in monocore powder-in-tube MgB2 strands have been carried out at temperatures, T, of from 4.2 to 40 K, and in transverse fields, B, of up to 14 T. Processing methods used were conventional continuous tube forming/filling (CTFF) and internal magnesium diffusion (IMD). Strands with several powder compositions were measured, including binary (undoped) MgB2, 2% carbon doped MgB2, and 3% carbon doped MgB2. Magnetization loops (M–B) were also measured, and magnetic critical current density, Jcm, values extracted from them. The transport, Jct(B) and magnetic, Jcm(B), critical current densities were compared. Also studied was the influence of doping on the resistively measured irreversibility field, Birr, and upper critical field, Bc2. Critical current densities, Jct, and n-values were extracted from transport measurements and were found to be universally related (for all B and T) according to in which m = 0.52 ± 0.11. Likewise n was found to be related to B according to n ∝ B−p with a T-dependent p in the range of about 0.08–0.21. Further analysis of the field (B) and temperature (T) dependences of n-value resulted in an expression that enabled n(B,T), for all B and T, to be estimated for a given strand based on the results of transport Jct(B) measurements made at one arbitrarily chosen temperature.

Doping effect and flux pinning mechanism of nano-SiC additions inMgB2 strands

Superconducting MgB2 strands with nanometer-scale SiC additions have been investigated systematicallyusing transport and magnetic measurements. A comparative study ofMgB2 strands with different nano-SiC addition levels has shown C-doping-enhanced critical current densityJc through enhancements in the upper critical field,Hc2, and decreased anisotropy. The critical current density and flux pinning force densityobtained from magnetic measurements were found to be substantially different from thevalues obtained through transport measurements, particularly with regards to magneticfield dependence. The differences in magnetic and transport results are largelyattributed to connectivity related effects. On the other hand, based on the scalingbehavior of the flux pinning force, there may be other effective pinning centers inMgB2 strands in addition to grain boundary pinning.

Anisotropic connectivity and its influence on critical currentdensities, irreversibility fields, and flux creep in in situ processedMgB2 strands

The anisotropy of the critical current density(Jc) and its influence on the measurement of the irreversibility field(Birr) has been investigatedfor high quality, in situ MgB2 strands. Comparison of transport and magnetization measurements hasrevealed the onset of a regime where large differences exist between transportand magnetically measured values of the critical current density andBirr. These effects, initially unexpected due to the lack of crystallinetexture in these in situ processed strands, appear to be due to a fibrousmicrostructure, connected with the details of the wire fabrication andMgB2 formation reactions.Scanning electron micrographs of in situ processedMgB2 monocore strands have revealed a fibrous microstructure. Grains(∼100 nm) are randomly oriented, and there is no apparent local texture of the grains. However, this randomlyoriented polycrystalline material has a fibrous texture at a larger length scale, with stringers ofMgB2 (∼60 µm longand ∼ 5 µm in diameter) partially separated by elongated pores—the spaces previously occupiedby stringers of elemental Mg. This leads to an interpretation of the differencesobserved between transport and magnetically determined critical currents, inparticular a large deviation between the two at higher fields, in terms of differenttransverse and longitudinal connectivities within the strand. The different valuesof the connectivity also lead to different resistive transition widths, and thusirreversibility field values, as measured by transport and magnetic techniques. Finally,these considerations are seen to influence estimated pinning potentials for thestrands.

A Model Superconducting Helical Undulator Wound Using a Wind and React ${\rm MgB}_{2}$ Multifilamentary Wire

A short model helical undulator, 250 mm long, with a period of 14 mm was designed, fabricated and then subsequently tested in a liquid helium bath. The helical coil was wound with stabilized multi-filamentary MgB2 strand, 0.5 mm in diameter, in a wind and react mode with S-glass insulation followed by vacuum epoxy impregnation. Beam aperture of the coil was 7 mm, winding bore diameter 8 mm, coil winding OD=18 mm and the 1010 Fe pole (flange) has an OD=19.05 mm. The coil was measured for I c and magnetic field in the bore at 4.2 K in a liquid helium bath. The bore field as a function of depth was also determined near the critical current at 4.2 K. The load line of the coil was compared to short sample I c values.

Numerical Modeling of the AC Limiting Properties of Insulated, Conduction Cooled ${\hbox {MgB}}_{2}$ Strands

It is known that MgB2-based strands may be used in fault current limiters as a low-cost alternative to high-temperature YBCO coated conductors. Numerical modeling of the ac limiting properties of MgB2 strands with Kapton insulation and conduction cooling have been performed using a finite element method. Two 2D ac Poisson equations, one for calculating the electric field and one for determining the heat transfer were solved simultaneously with input parameters taken from experimental results. As boundary conditions, we took the full non-linear curve of the heat flux into a cryocooler cold head as well as heat radiation from the free surfaces of the wire. The influence of different values and durations of fault electric fields were studied and the maximum temperatures, limiting currents and recovery times were calculated. Obtained results may be useful for understanding the behavior of different kinds of MgB2 multifilamentary wires under conditions encountered in fault current limiter devices.