Engineering topics
Collings, E. W.
Publications and source records attributed to Collings, E. W..
A Field Quality Modeling in Canted Cosine Theta Dipole Magnets Wound Using REBCO Cables
In accelerator magnets the magnetic field quality is an important parameter. Using REBCO tapes or cables in magnet windings affects the magnetic field quality because of a quite high magnetization generated by these conductors. In this paper we analyzed numerically (using FEM) an effect of various REBCO cables’ magnetization on the field quality of canted cosine theta (CCT) dipole magnets. The magnetic properties of the cables were taken from our measurements of their M (H) curves at 4.2 K and magnetic fields up to 12 T. Also, flux creep effects were measured and their effects on field quality analyzed. Here we present a modeling of a CCT magnet containing 1 dipole double layer wound using a CORC and STAR cable. To make the FEM modeling less computationally “expensive”, we modelled only a section of 10 turns of the magnet's central part and calculated b3 fields on a circle of 2/3 of the innermost layer center line. Special M (H) curves measured during various field cycles (the so called “pre-injection cycles”) are effective in decreasing b 3 fields of the magnets. Flux creep effects suppress the b 3 fields further. In conclusion, using these techniques b 3 fields less than 10 units can be achieved.
Critical Current Densities and n-Values of MgB 2 Conductors for SMES, MRI, and Low AC Loss Applications
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. Here studied were the dependencies of J c on field strength, dopant concentration, and cabling and the dependence of n -value on field strength.
Mechanism of enhanced critical fields and critical current densities of MgB2 wires with C/Dy$^{2}$O3 co-additions
A series of monofilamentary powder-in-tube MgB2 wires were fabricated with 2 mol. % C doping and co-additions of 0–3 wt. % Dy2O3. Irreversibility fields (μ0Hirr), upper critical fields (μ0Hc2), and transport critical currents were measured, and from these quantities, anisotropies (γ) and electronic diffusivities (Dπ,σ) were estimated. The addition of 1 wt. % Dy2O3 to already optimally C-doped MgB2 wires produced higher Hc2//ab, Hc2//c, and Hirr values at 4.2 K. In addition, the critical current density, Jc, increased with Dy2O3 concentration up to 1 wt. % where non-barrier Jc reached 4.35 × 104 A/cm2 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. % Dy2O3 improved non-barrier Jc by 40% and 93% compared to 2 and 3 mol. % C doping, respectively. On the other hand, measurements of Tc showed that C/Dy2O3 co-additions increase interband scattering rates at a lower rate than C doping does (assuming C doping levels giving similar levels of low-T μ0Hc2 increase as co-addition). Comparisons to a two-band model for μ0Hc2 in MgB2 allowed us to conclude that the increases in Hc2//ab, Hc2//c, and Hirr (as well as concomitant increases in high-field Jc) with Dy2O3 addition are consistent with increases primarily in intraband scattering. This suggests C/Dy2O3 co-addition to be a more promising candidate for improving non-barrier Jc of MgB2 at temperatures above 20 K.
Stability and Current Sharing in YBCO Cables – Impact of Broken Elements – FEM Modeling
Not provided.
Flux Creep in a Bi-2212 Rutherford Cable for Particle Accelerator Applications
Bi-2212 superconducting cables are being considered for use in the high field magnets needed for the next generation of particle accelerators. Magnetization in these cables and the decay of that magnetization lead to field error and field-error drift, respectively, which need to be compensated. To study this, a segment of the winding pack was extracted from a racetrack coil made from Bi-2212 Rutherford cable. Using a Hall probe measurement technique, we measured the response of the cable's magnetization and its magnetization decay to changes in the applied magnetic field. The effect of adjustments to the cycling of the magnetic field was studied, intended to simulate the preinjection cycles of an accelerator magnet. Three M vs. H loops were constructed by sweeping the magnetic field applied to the sample from 0 to 2.5 T, then to a preinjection field 'x' (where x = 0, 0.25, 0.75 T), and finally up to 1 T. The applied field was then held at 1 T for 1500 s, and the magnetization decay was measured. Finally, the decay was found to vary from 8% to 14% after 1500 s, depending on the preinjection field cycle.
Magnetic, Mechanical and Thermal Modeling of Superconducting, Whole-Body, Actively Shielded, 3 T MRI Magnets Wound Using MgB 2 Strands for Liquid Cryogen Free Operation
Here 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.
AC losses of Roebel and CORC ® cables at higher AC magnetic fields and ramp rates
We have measured ReBCO coated conductor-based conductor on round core (CORC ® ) and Roebel cables at 77 K in a spinning magnet calorimeter, which subjected the tapes in the samples to a radial magnetic field of 566 mT (peak) at frequencies up to 120 Hz (272 T s -1 , cyclic average) with an approximately sinusoidal waveform. The samples were oriented such that the field applied to the tapes within the cables was entirely radial, simplifying subsequent analysis. An expression for loss which included hysteretic, flux creep and eddy current losses was fit to both the CORC ® and the Roebel cables. This expression allowed easy comparison of the relative influence of eddy currents and flux creep (or power-law behavior) effects. The loss of both the CORC ® and Roebel cables measured here were seen to be essentially the sum of the hysteretic loss, flux creep effects, and the normal metal eddy current losses of the individual tapes. The losses of these cables were measured at high B × d B /d t with no coupling current loss observed under the present preparation conditions. The influence of flux creep effects on loss were not negligible. The losses of the CORC ® cable per meter of tape were seen to be reduced from the case of a flat tape because of the helical geometry of the tapes.
Persistent-current magnetizations of Nb 3 Sn Rutherford cables and extracted strands
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 Nb 3 Sn'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.
Interstrand Coupling Properties of LARP High Gradient Quadrupole Cables in Response to Variations in Cable Design and Heat Treatment Condition
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 Nb 3 Sn 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. Furthermore, 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.
Recent Progress in Application of Internal Oxidation Technique in Nb3Sn Strands
The internal oxidation technique can generate ZrO2 nano particles in Nb3Sn strands, which markedly refine the Nb3Sn grain size and boost the high-field critical current density (Jc). This article summarizes recent efforts on implementing this technique in practical Nb3Sn wires and adding Ti as a dopant. It is demonstrated that this technique can be readily incorporated into the present Nb3Sn conductor manufacturing technology. Powder-in-tube (PIT) strands with fine subelements (~25 µm) based on this technique were successfully fabricated, and proper heat treatments for oxygen transfer were explored. Future work for producing strands ready for applications is proposed.