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The high field superconducting magnets required for ongoing and planned upgrades to the Large Hadron Collider (LHC) will be wound with Nb3Sn Rutherford cables for which reason studies of Nb3Sn strand, cable, and magnet properties will continue to be needed. Of particular importance is field quality. The amplitudes of multipoles in the bore fields of dipole and quadrupole magnets, induced by ramp-rate-dependent coupling currents, are under the control of the interstrand contact resistances-crossing-strand, $R-{c}$, adjacent strand, $R-{a}$ , or a combination of them, $R-{{\text{eff}}}$. Although two decades ago it was agreed that for the LHC $R-{c}$ should be in the range 10-30 μ, more recent measurements of LHC quadrupoles have revealed $R-{c}$ values ranging from 95 to 230 μ. This paper discusses ways in which these values can be achieved. In a heavily compacted cable $R-{{\text{eff}}}$ can be tuned to some predictable value by varying the width of an included stainless steel (effectively 'insulating') core. But cables are no longer heavily compacted with the result that the crossing strands of the impregnated cable are separated by a thick epoxy layer that behaves like an insulating core. If a stainless steel core is actually present, $R-{{\text{eff}}}$ must be independent of core width. Since there is no guarantee that a fixed predetermined amount of interlayer separation could be reproduced from winding to winding it would be advisable to include a full width core.

The magnetizations of eight high-gradient quadrupole cables designated HQ and QXF and a pair of strands, identical in architecture but with different effective strand diameters extracted from an HQ and a related QXF cable, were measured. In the service of field quality assessment, the cable magnetizations and losses were measured by pickup coil magnetometry at 4.2 K in face-on fields, B m , of ± 400 mT at frequencies, f, of up to 60 mHz. Based on the coupling component of loss, Q coup , the coupling magnetization M coup = Q coup /4B m was derived for a ramp rate of 7.5 mT/s. Persistent current (shielding) magnetization and loss (M sh and Q h,strand ) were measured on short pieces of extracted strand by vibrating sample magnetometry at 4.2 K. Unpenetrated M-B loops to ±400 mT and fully penetrated loops to ±14 T were obtained. M coup can be easily controlled and reduced to relatively small values by introducing cores and adjusting the preparation conditions. But in low fields near injection Nb3Sn's high J c and correspondingly high M sh,cable may call for magnetic compensation to preserve field quality. The suitably adjusted cable and strand fully penetrated M-B loops were in reasonable accord leading to the conclusion that strand magnetization is a useful measure of cable magnetization, and that when suitably manipulated can provide input to magnet field error calculations.

Calorimetric measurement of coupling loss versus frequency has been measured on two sets of cored and uncored large Hadron Collider Accelerator Research Program high gradient quadrupole Nb3Sn Rutherford cables. Studied are the responses of the resulting interstrand contact resistances (ICR) to variation of stainless-steel (SS) core width and position and to variation of reaction-heat-treatment (RHT) condition. One pair of cables (an early HQ-series type) with and without core had received RHT under 20-MPa uniaxial face-on pressure. Another set of cables (recent QXF type) furnished with SS cores of various widths had received RHT under ambient pressure. The results were displayed as cable-cross-sectional micrographs and plots of ICR versus percent core coverage (W). The HQ cables were tightly compacted and produced results consistent with a previously expected continuous ICR versus W variation. On the other hand, the QXF cables were uncompacted such that their upper and lower layers were separated by what is referred to as a full-width 'pseudocore;' as a result, their ICRs were independent of the widths of the SS cores. Compaction versus noncompaction is discussed and future research directions are suggested.

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, particularly 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.

Multifilamentary MgB 2 strands (filament numbers 36 to 114) prepared by the in-situ power-in-tube (PIT) route with carbon doping contents of 0, 2, and 3.2% were wound on barrels for transport J c and n -value measurement at 4.2 K in fields of up to 12 T. The strand and gauge lengths were 1 m and 0.5 m. Heat treatments at 675 °C and 650 °C centered around the melting point of Mg (650 °C) and both utilized the liquid-solid reaction. A pair of strands, with and without 2% C doping exhibited the J c (B) crossover effect. Studied were the dependencies of J c on field strength, dopant concentration, and cabling and the dependence of n -value on field strength.

A series of monofilamentary powder-in-tube MgB 2 wires were fabricated with 2 mol. % C doping and co-additions of 0-3 wt. % Dy 2 O 3 . Irreversibility fields ( μ 0 H irr ), upper critical fields ( μ 0 H c 2 ), and transport critical currents were measured, and from these quantities, anisotropies ( γ ) and electronic diffusivities ( D π , σ ) were estimated. The addition of 1 wt. % Dy 2 O 3 to already optimally C-doped MgB 2 wires produced higher H c 2 //ab , H c 2 //c , and H irr values at 4.2 K. In addition, the critical current density, J c , increased with Dy 2 O 3 concentration up to 1 wt. % where non-barrier J c reached 4.35 × 10 4  A/cm 2 at 4.2 K, 10 T. At higher temperatures, for example, 20 K and 5 T, co-additions of 2 mol. % C and 2 wt. % Dy 2 O 3 improved non-barrier J c by 40% and 93% compared to 2 and 3 mol. % C doping, respectively. On the other hand, measurements of T c showed that C/Dy 2 O 3 co-additions increase interband scattering rates at a lower rate than C doping does (assuming C doping levels giving similar levels of low-T μ 0 H c 2 increase as co-addition). Comparisons to a two-band model for μ 0 H c 2 in MgB 2 allowed us to conclude that the increases in H c 2 //ab , H c 2 //c , and H irr (as well as concomitant increases in high-field J c ) with Dy 2 O 3 addition are consistent with increases primarily in intraband scattering. This suggests C/Dy 2 O 3 co-addition to be a more promising candidate for improving non-barrier J c of MgB 2 at temperatures above 20 K., (© 2023 Author(s).)

Superconducting magnets used for Magnetic Resonance Imaging (MRI) scanners need to keep temperature gradients minimized in order to retain thermal and operating current margin. We have used 3D finite element analysis (FEA) simulation in COMSOL Multiphysics software that includes both conductive heat transfer and radiative heating to calculate the temperature distribution both along the winding direction and across the cross-section of an MRI segment coil at its equilibrium operating temperature. We have also modelled the evolution of the thermal properties during cool-down from ambient temperature. The heat capacity and thermal conductivity of the magnet winding were computed for use within this simulation. The heat capacity as a function of temperature was calculated using a rule of mixtures. This procedure was also used for the thermal conductivity along the direction of the wire. However, the thermal conductivity within the composite cross section ( x - and y -directions) was computed using a 2D FEA model. Based on this, a time-dependent, 3D coil model was built to calculate the coil temperature throughout the winding during cool-down in our test cryostat system. The model included a heat leak component to the coil current contacts via conduction through the current leads as well as a radiative component from the surfaces of the cryostat. A key result was that a maximum coil Δ T max = 5.1 K (=maximum temperature within the winding -minimum temperature in the winding) was seen and a coil I c margin of 12.75 A was predicted at steady state, with our first current lead design. A second set of more optimized current leads significantly lowered the Δ T max within the coil at the steady state. The coil I c margin has been analyzed for different current lead designs., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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 MgB 2 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 MgB 2 short (1.37 m length) MRI magnet based on the performance parameters of existing MgB 2 wire. 3 Tesla MRI magnet can operate at 20 K at 67 % of its critical current.

The paper begins by emphasizing the clinical and commercial importance of proton or other charged particle such as carbon ion therapy, refers to the manufacturers of such systems of which more than 120 are installed or under construction worldwide by April 2021. A general review of charged particle therapy systems refers to six manufacturers and provides in tabular form some details of systems installed in the US, Europe, Asia, and elsewhere. In a description of the principles of particle beam therapy a comparison is made of the properties of photons (x-rays) versus protons and protons versus carbon ions. A brief discussion of accelerators in general is followed by descriptions of cyclotrons (including the isosynchronous cyclotron and the synchrocyclotron) and synchrotrons. An interesting case study describes the evolution of a normal-conducting 220 ton cyclotron into an iron-free synchrocyclotron weighing only 5 tons. The general principles of beam handling and gantry design are described. Subsequent sections describe gantry magnets in detail - normal conducting gantry magnets, superconducting gantry magnets for proton- and carbon therapy. Mention is made of a novel CERN-designed superconducting toroidal gantry for hadron therapy, GaToroid. This device, operating under steady state current and magnetic field, is able to deliver a beam at discrete angles over a range of treatment energies. Also considered are low temperature superconducting (LTS) and high temperature superconducting (HTS) magnet windings, and the choice of REBCO conductors for cryogen-free carbon-ion gantries. Finally, the paper mentions an important "Prospect for Improvement", viz: the introduction of MRI image guidance. A well-known property of the particle beam as it passes through tissue is its energy dependent absorption that rises to a pronounced peak (the Bragg peak) at the end of its range. In order to take advantage of this effect the exact targeting of the tumor and positioning of the patient should be guided by imaging visualization using X-ray, CT, and hopefully advanced MRI. Unlike MRI-guided photon therapy the direct interaction of the magnetic field with the charged particle beam presents a huge challenge such that MRI image-guided proton/particle therapy has not yet been available in clinical practice. Modeling studies have been undertaken on the general topic of beam-line/magnetic field interaction using, for example, the software GEANT4 (GEometry And Tracking) a platform for simulating the passage of charged particles through matter using a Monte Carlo method., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2022 Collings, Lu, Gupta and Sumption.)

Bulk samples of magnesium diboride (MgB 2 ) doped with 0.5 wt% of the rare earth oxides (REOs) Nd 2 O 3 and Dy 2 O 3 (named B-ND and B-DY) prepared by standard powder processing, and wires of MgB 2 doped with 0.5 wt% Dy 2 O 3 (named W-DY) prepared by a commercial powder-in-tube processing were studied. Investigations included x-ray diffractometry, scanning- and transmission electron microscopy, magnetic measurement of superconducting transition temperature ( T c ), magnetic and resistive measurements of upper critical field ( B c2 ) and irreversibility field ( B irr ), as well as magnetic and transport measurements of critical current densities versus applied field ( J cm ( B ) and J c ( B ), respectively). It was found that although the products of REO doping did not substitute into the MgB 2 lattice, REO-based inclusions resided within grains and at grain boundaries. Curves of bulk pinning force density ( F p ) versus reduced field ( b = B / B irr ) showed that flux pinning was by predominantly by grain boundaries, not point defects. At all temperatures the F p ( b ) of W-DY experienced enhancement by inclusion-induced grain boundary refinement but at higher temperatures F p ( b ) was still further increased by a Dy 2 O 3 additive-induced increase in B irr of about 1 T at all temperatures up to 20 K (and beyond). It is noted that Dy 2 O 3 increases B irr and that it does so, not just at 4 K, but in the higher temperature regime. This important property, shared by a number of REOs and other oxides promises to extend the applications range of MgB 2 conductors.

Three (MgB 2 ) 1- x (SnO 2 ) x samples with x ranging from 0 to 5 wt% were prepared by the in situ route to study the effect of tin dioxide additions on the superconducting properties of MgB 2 bulk materials. All of the reacted samples were slightly Mg deficient although the starting Mg:B precursor powder ratio was 1:2. A heat treatment (HT) temperature of 700 °C with a dwell time of 30 min was used. XRD results showed evidence of peak shifts for MgB 2 phases with SnO 2 addition. The magnitude of the a -axis lattice constant change (0.361 ± 0.075 %) calculated for the 3 wt% doped samples is comparable in magnitude to that seen previously for the C-doped MgB 2 bulks which exhibited enhanced B C2 . The upper critical fields ( B C2 ) and the irreversibility fields ( B irr ) were measured resistively in fields up to 14 T at 5 K to T c . The best B C2 value at 20 K (15.2 T based on extrapolation) was seen for sample IS3 (x = 3 wt%), and was comparable to the best B C2 values (≈ 15 T at 20 K) seen for C-doped MgB 2 bulks. IS3 had a corresponding B irr = 10.8 T (20 K). The superconducting transition temperature ( T c ) appeared to increase slightly with doping, although within the range of error bars (37.4 K to 37.6 K for 1.6 T B C2 increase at 20 K), in contrast to C doping which is accompanied by a significant decrease in T c (39 K to 36 K for 3.8 % C doped MgB 2 bulk). We attribute the observed increase in both B C2 and B irr for SnO 2 -additions to lattice strain caused by the introduction of precipitates within the grains., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. (Yes, we have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.)

MgB 2 superconducting wires made using a Mg infiltration method have reached a higher performance than either in-situ or ex-situ mixed powder based routes. Indeed, very high layer J c coupled with whole-strand J e (critical current per total strand cross section) exceeding 10 4 A cm -2 at 4.2 K, 10 T have been found for monocore MgB 2 wires. However, previous multicore infiltration route wires have not reached their potential for J e due to partially reacted and non-uniform MgB 2 layers. This study shows that 18-core MgB 2 AIMI wires processed using a low temperature route can attain higher and more uniform J e values due to a more uniform MgB 2 reaction layer. The formation of fully reacted, uniform MgB 2 layers is attributed to the switch from a liquid-solid to a vapor-solid reaction route.

MgB 2 superconducting wires and bulks with nano-La 2 O 3 addition have been studied. A series of MgB 2 superconducting bulk samples with nano-La 2 O 3 addition levels of 0, 5, 7, 18wt% were prepared. AC resistivity data showed slight increases of Bc 2 and unchanged B irr for the bulk samples with doping levels lower than 7 wt% and decreased critical fields for the heavily doped (18 wt%) bulk. X-ray diffraction (XRD) showed the presence of LaB 6 in the nano-La 2 O 3 doped MgB 2 bulk samples and decreased MgB 2 grain size in nano-La 2 O 3 doped bulks. Monocore powder-in-tube (PIT) MgB 2 wires without and with 5 wt% nano-La 2 O 3 addition (P-05) were prepared for transport property measurement. 2mol%C-doped Specialty Materials Inc. (SMI) boron powder was used for wire P-05 and previously prepared control wires (control wires were made without the addition of nano-La 2 O 3 powder, W-00 and P2). Low field magnetic properties were obtained from magnetization loop (M-H), transport critical current density ( J c ) was measured at 4.2 K for the nano-La 2 O 3 doped PIT wire (P-05) and the control samples (P2 and W-00). The transport critical current density J c (B) of P-05 at 4.2 K and 8 T (6.0 ×10 4 A/cm 2 ) was twice that of the control wire. The critical magnetic fields ( Bc 2 and B irr ) of P-05 and the control sample P2 were compared. The critical fields of P-05 were slightly less than those of P2. Kramer-Dew-Hughes plots indicated a change from surface pinning to a mixture of volume pinning and surface pinning. It is shown that enhancement of P-05's transport properties is due to additional flux pinning by the fine-size rare-earth borides rather than enhanced Bc 2 or B irr .

Research into in-situ MgB 2 strand has been focused on improvements in J C 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 MgB 2 strands. This study shows the nature of the reduced porosity for in-situ MgB 2 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 × 10 4 A/cm 2 and 8.5 × 10 4 A/cm 2 at 5 T, respectively, with compared with 6.0 × 10 4 A/cm 2 for typical powder-in-tube (PIT) in-situ strand. Moreover, AIMI-processed monofilamentary MgB 2 strand obtained even higher J C s and transverse grain connectivity than the CHPD strands.

The development of coils that can survive a quench is crucial for demonstrating the viability of MgB 2 -based main magnet coils used in MRI systems. Here we have studied the performance and quench properties of a large (outer diameter: 901 mm; winding pack: 44 mm thick × 50.6 mm high) conduction-cooled, react-and-wind (R&W), MgB 2 superconducting coil. Minimum quench energy (MQE) values were measured at several coil operating currents ( I op ), and distinguished from the minimum energy needed to generate a normal zone (MGE). During these measurements, normal zone propagation velocities (NZPV) were also determined using multiple voltage taps placed around the heater zone. The conduction cooled coil obtained a critical current ( I c ) of 186 A at 15 K. As the operating currents ( I op ) varied from 80 A to 175 A, MQE ranged from 152 J to 10 J, and NZPV increased from 1.3 to 5.5 cm/s. Two kinds of heater were involved in this study: (1) a localized heater ("test heater") used to initiate the quench, and (2) a larger "protection heater" used to protect the coil by distributing the normal zone after a quench was detected. The protection heater was placed on the outside surface of the coil winding. The test heater was also placed on the outside surface of the coil at a small opening made in the protection heater. As part of this work, we also developed and tested an active protection scheme for the coil. Such active protection schemes are of great interest for MgB 2 -based MRIs because they permit exploitation of the relatively large MQE values of MgB 2 to enable the use of higher J e values which in turn lead to competitive MgB 2 MRI designs. Finally, the ability to use a quench detection voltage to fire a protection heater as part of an active protection scheme was also demonstrated.

A series of SnO 2 added MgB 2 bulk superconductors were prepared by in situ route to study the effect of oxygen doping on superconducting and structural properties of MgB 2 . Several (MgB 2 ) 1- x (SnO 2 ) x samples were fabricated with x ranging from 0, 3 wt%, 4 wt%, and 6 wt%. Upper critical field ( B C 2 ) and irreversible field ( B irr ) were measured by physical property measurement system. Thermal analysis was performed on the as-received SnO 2 powder. Critical current densities ( J cm ) were obtained at 4.2 K using magnetic measurement. X-ray diffraction results showed evidence of full SnO 2 decomposition in all the doped bulk samples and a shift of a -axis in MgB 2 lattice was seen. Oxygen was successfully released during heat treatment, yet no enhancement of B C 2 or B irr was seen, indicating that oxygen atoms did not end up in the host lattice. Further exploration of different processing procedures is still needed in order to get oxygen substitution on the host lattice sites.

Standard in-situ type MgB 2 strands manufactured by Hyper Tech Inc have 19 - 36 subelements, a monel outer sheath, and a Cu interfilamentary matrix. Typical transport J c s of the strands are 2×10 5 A/cm 2 with n -values of 20 - 30 at 4.2 K and 5 T. This work introduces two new MgB 2 conductor designs. First, a new class of MgB 2 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 MgB 2 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 MgB 2 strands.

High temperatures and under pressure (HTP) processing has been used to study the effects of chemical doping in MgB2. ZrB2, TiB2 and NbB2 were selected as additives since, like MgB2, they have an AlB2-type structure and similar lattice parameters. Dy2O3 was selected as it has been reported to generate nanoscale, secondary intragrain phases in MgB2. While C is known to enter the B-sublattice readily, attempts to dope Zr and other elements onto the Mg site have been less successful due to slow bulk diffusion, low solubility in MgB2, or both. We have used high-temperature, solid-state sintering (1500 °C), as well as excursions through the peritectic temperature (up to 1700 °C), to investigate both of these limitations. Bulk MgB2 samples doped with MB2 (M = Zr, Ti and Nb) and Dy2O3 additions were synthesized and then characterized. Lattice distortion and high densities of crystal defects were observed in the MgB2 grains around nano-sized MB2 inclusions, this highly defected band contributed to a large increase in Bc2 but was not large enough to increase the irreversibility field. In contrast, distributed intragrain precipitates were formed by Dy2O3 additions which did not change the lattice parameters, Tc, Tc distribution or Bc2 of MgB2, but modified the flux pinning.

This study is a contribution to the development of technology for an MgB 2 -based, cryogen-free, superconducting magnet for an MRI system. Specifically, we aim to demonstrate that a react and wind coil can be made using high performance in-situ route MgB 2 conductor, and that the conductor could be operated in conduction mode with low levels of temperature gradient. In this work, an MgB 2 conductor was used for the winding of a sub-size, MRI-like coil segment. The MgB 2 coil was wound on a 457 mm ID 101 OFE copper former using a react-and-wind approach. The total length of conductor used was 330 m. The coil was epoxy impregnated and then instrumented for low temperature testing. After the initial cool down (conduction cooling) the coil I c was measured as a function of temperature (15-30 K), and an I c of 200 A at 15 K was measured.

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 inter-phase 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.

The influences of strand twisting and bending (applied at room temperature) on the critical current densities, J c , and n -values of MgB 2 multifilamentary strands were evaluated at 4.2 K as function of applied field strength, B . Three types of MgB 2 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 J c (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, L p , of 10-100 mm. Transport measurements of J c (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 J c (B) and only small changes in n -value. Both the J c (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 J c (B) and significant scatter in n -value over the bending strain range of 0-0.6%.

High quality, c -axis oriented, MgB 2 thin films were successfully grown on 6H-SiC substrates using pulsed laser deposition (PLD) with subsequent in situ annealing. To obtain high purity films free from oxygen contamination, a dense Mg-B target was specially made from a high temperature, high pressure reaction of Mg and B to form large-grained (10~50 µm) MgB 2 . Microstructural analysis via electron microscopy found that the resulting grains of the film were composed of ultrafine columnar grains of 19-30 nm. XRD analysis showed the MgB 2 films to be c -axis oriented; the a -axis and c -axis lattice parameters were determined to be 3.073 ± 0.005 Å and 3.528 ± 0.010 Å, respectively. The superconducting critical temperature, T c,onset , increased monotonically as the annealing temperature was increased, varying from 25.2 K to 33.7 K. The superconducting critical current density as determined from magnetic measurements, J cm , at 5 K, was 10 5 A/cm 2 at 7.8 T; at 20 K, 10 5 A/cm 2 was reached at 3.1 T. The transport and pinning properties of these films were compared to "powder-in-tube" (PIT) and "internal-infiltration" (AIMI) processed wires. Additionally, examination of the pinning mechanism showed that when scaled to the peak in the pinning curve, the films follow the grain boundary, or surface, pinning mechanism quite well, and are similar to the response seen for C doped PIT and AIMI strands, in contrast to the behavior seen in undoped PIT wires, in which deviations are seen at high b ( b = B/B c2 ). On the other hand, the magnitude of the pinning force was similar for the thin films and AIMI conductors, unlike the values from connectivity-suppressed PIT strands.

Significantly enhanced critical current density ( J c ) for MgB 2 superconducting wires can be obtained following the advanced internal Mg infiltration (AIMI) route. But unless suitable precautions are taken, the AIMI-processed MgB 2 wires will exhibit incomplete MgB 2 layer formation, i.e. reduced superconductor core size and hence suppressed current-carrying capability. Microstructural characterization of AIMI MgB 2 wires before and after the heat treatment reveals that the reaction mechanism changes from a "Mg infiltration-reaction" at the beginning of the heat treatment to a "Mg diffusion-reaction" once a dense MgB 2 layer is formed. A drastic drop in the Mg transport rate from infiltration to diffusion causes the termination of the MgB 2 core growth. To quantify this process, a two-stage kinetic model is built to describe the MgB 2 layer formation and growth. The derived kinetic model and the associated experimental observations indicate that fully reacted AIMI-processed MgB 2 wires can be achieved following the optimization of B particle size, B powder packing density, MgB 2 reaction activation energy and its response to the additions of dopants.

Nb3Sn strands fabricated using Nb-Zr alloy can be internally oxidized, provided that oxygen is properly supplied via an oxide powder. This allows the formation of fine intragranular and intergranular ZrO2 particles in a Nb3Sn matrix. These particles can refine the grain size by a factor of three and thereby greatly enhance the Nb3Sn critical current density., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)