MgB2 conductors for dc and ac applications

The paper presents discussion on up to date results on MgB2 conductors from the point of view of their future dc and ac applications. Basic physical parameters of MgB2 compound relevant to conductors are introduced. Different conductor preparation methods and conductor architectures are presented and attainable critical current densities discussed. Some numerical results on critical currents and ac losses of future multifilamentary MgB2 conductors with magnetic cladding of their filaments are given. Recently observed anomalous decrease of ac susceptibility at 50 K in copper clad Powder-in-tube, PIT, MgB2 wires is presented.Comment: Paper presented at EUCAS'01 conference, Copenhagen, 26-30 August 200

Magnetization and Loss Measurements of YBCO CORC and Roebel Cables for Accelerators using ±T dipole Susceptometer

This CORC sample Magnetization reaches about 900 kA/m (cable volume) and 2200 kA/m (strand volume) near injection This Roebel cable reaches about 2000 kA/m (cable volume) and 2400 kA/m (strand volume) near injection These values agree well with individual tape values if (i) we normalize to strand volume, Correct CORC for twisting These values can be compared to NbTi Minj = 10 kA/m b3 = 3-6 units Nb3Sn Minj = 100 kA/m b3 = 30 units Here Minj is 100 X higher than NbTi and 10 X higher than NbTi Not insurmountable, but must be considered when thinking about particle beam steering magnet qualit

Co-creating at the threshold : a dialogical approach to festival planning at a Cape Town Waldorf school

Includes bibliographical references (leaves 160-166).Waldorf schools were first established in Germany in 1919 under the guidance of Rudolf Steiner, with the intention of educating children for the renewal of society. Since the spread of Waldorf schools to South Africa in the 1950's, South African Waldorf teachers have been faced with the challenge of localizing the pedagogy to meet the needs of modern South African children. One arena for this challenge is in planning the school festivals. Through data derived from ethnographic observation of festival planning and enactment at Michael Oak Waldorf School in Cape Town, South Africa, I show that Michael Oak teachers consider the celebration of school festivals to be intrinsic to the education of the children, and that in adapting the festivals to their own context they are confronted with conflicting opinions and ideas about how to juxtapose the Christian and seasonal festivals, how to negotiate religious differences, and to what extent to adapt the festivals to reflect specific aspects of South African culture. Using data obtained from participant observation, predominantly semi-structured, unstructured, and informal interviews with more than seventy people (including Michael Oak teachers, former pupils, and past and present parents), along with background reading and study, I show how the these teachers, recreating each festival anew every year instead of relying solely on established traditions, took a dialogical approach to conceptualizing and planning their festivals - one that, though time-consuming and sometimes complicated, was itself a ritual meaningful to the teachers. This dialogical approach was outwardly manifest in the festival's ritual symbols, particularly the use of time and space, and the objects and performance filling them. It was also observed in the planning meetings and was described by the Michael Oak teachers in interviews. Through this dialogical approach, the teachers experienced what Victor Turner calls communitas, a liminal, threshold state of creativity, changed relationships, and potentiality. I demonstrate through teachers' statements that by remaining on the threshold of these often conflicting ideas, the teachers found in themselves a creative energy that extended to the children as the teachers included them in festival preparation and enactment

Stability, Inter-Strand Contact Resistance and AC Losses in YBCO Roebel Cables

A dc transport current was applied to the strands of a Roebel cable at 77 K in liquid nitrogen bath. The inter-strand contact resistance was measured. It was modified either by applying a pressure on the cable at 77 K in liquid nitrogen bath or using different soldering patterns between the strands of the cable. Magnetization ac losses were measured in frequency range 50 – 200 Hz in applied magnetic field 4 – 70 mT perpendicular to the broader face of the cable to test the inter-strand contact resistance effect. High stability and very low level of coupling losses was observed in the cables even with the lowest interstrand resistances.This work was supported by the U.S. Department of Energy, grant DE-FG02-07ER84913.In an effort of aiming current sharing between the strands in Roebel cables we studied experimentally the effect of different kinds of inter-strand connections on inter-strand resistances. Just applying a pressure of 8 kPa gives a maximum interstrand resistance of 105 mΩ. Using a tin foil placed on top of the Roebel cable gives the maximum inter-strand resistance of 31.5 mΩ under a pressure of 8 kPa. The lowest maximum inter-strand resistance of 0.1 mΩ was achieved by soldering copper shunts (strips 5 mm wide and 0.1 mm thick). Even such a low inter-strand resistance value does not cause any significant coupling loss increase up to 200 Hz. However, on the other hand it will allow some current sharing between the strands of the Roebel cables. Detailed experimental results on this subject will be published elsewhere

Analysis of Magnetization effects for HTS conductors for HEP magnets

Motivation – accelerator quality Comparison of accelerator and b3 Magnetization of Tape vs Cable Magnetization of various cable types Coupling -- Magnetization -- loss? Decay and its implicationsThis work was supported by the U.S. Department of Energy, Office of Science, Division of High Energy Physics, under Grant DE-SC001172

Persistent-Current Magnetization of Nb3Sn Strands: Influence of Applied Field Angle and Transport Current

For many accelerator magnets field quality at the bore is a critical requirement for which reason it is necessary to fully characterize the persistent-current magnetization of strands of the kind under consideration for these magnets. The magnetization of a strand is generally measured in a magnetometer. However, certain effects can differentiate such measurements from the true magnetizations of strands in magnets. This report focuses on persistent-current magnetization: (i) measured by vibrating-sample magnetometer on segments of strand extracted from a section of heat treated Nb3Sn cable as functions of angle of the applied field and (ii) calculated as function of applied transport current. It is found that the magnetization of a strand in a cable increases by ~10% as the field applied to the cable is shifted from edge-on to face-on, and that the difference between the current-on and current-off magnetizations is not significant until close to the operational field of a magnet.This work was supported in part by the U.S. Dept. of Energy, Office of High Energy Physics, under Grants No. DE-FG02-95ER40900 (OSU) and a DOE SBIR.When the field applied to a Rutherford cable is shifted from face-on to edge-on, the orientation of the field with respect to the stand axis shifts from 90o (FO) to tan-1(width/half-lay-pitch) = 17o for the HQ cable. Accordingly it was of interest to measure the strand’s ΔM as function of applied field angle, θ, from zero degrees (parallel to the axis) to 90o. The observed increase from the cable-relevant angles of ~73o to 90o was inthe range of 10% - not a large effect. The persistent-current magnetization of a cable in a magnet is expected to be decreased by the presence of increasing transport current, an effect initially calculated for a semi-infinite slab by LeBlanc. In studying this effect further it was found (by FEM calculation) that a sample’s shape-change from slab to cylinder resulted in a significant change in the slope of the M/M0 versus I/Ic curve. Also as computed by FEM the transport effect differed in response to the order in which currents and fields were applied to a sample in the prepenetration state (b = B/Bp < 1). The negative slope of M versus B increased in the sequence: (current first then field) < (simultaneous current and field) at an average rate, dM/db, of - 0.5 T. The report concludes with an analytic extension of the LeBlanc relationship M/M0 = 1 – (I/Ic)2 in the form of an M/M0 versus B, useful for magnet applications. In this expression, the influence of magnet load-line change can be predicted simply by changing the value of Bmax, e.g. from 12 T to 16.5 T. In the latter case it was found that the difference between the current-on and current-off magnetizations (M and M0) does not reach even 1% until B reaches 9 T. The report is a case study of some magnetic- and transport-related properties of Nb3Sn strands and provides a starting point for future work on other magnet-relevant strands such as Bi:2212 and YBCO coated conductor tapes whose like propertie

Influence of field cycle on coated conductor magnetization and decay for accelerator applications

This work was funded by the U.S. Dept. of Energy, Division of High Energy Physics, under Grant No. under Grant DE-SC0011721 (OSU) (University Program)Both the magnetization and the creep of HTS materials are important at 4 K for accelerator magnet applications – the first leads to b3, the second leads to drift in b3 Both magnetization and its creep (b3 and its drift) might be minimized by an appropriate field cycle – a reduction of an order of magnitude is seen in this study The reason for this is the balanced critical state if we have a field rest within a field penetration excursion of the injection field This may be difficult to employ completely in a magnet, given the different fields the conductors are exposed to within the magnet, but the benefit should still be substantia

Effects of Core Type, Placement, and Width, on the Estimated Interstrand Coupling Properties of QXF-Type Nb3Sn Rutherford Cables

The coupling magnetization of a Rutherford cable is inversely proportional to an effective interstrand contact resistance, Reff, a function of the crossing-strand resistance, Rc, and the adjacent strand resistance, Ra. In cored cables Reff varies continuously with W, the core width expressed as percent interstrand cover. For a series of un-heat-treated stabrite-coated NbTi LHC-inner cables with stainless-steel (SS, insulating) cores Reff(W) decreased smoothly as W decreased from 100% while for a set of research-wound SS-cored Nb3Sn cables Reff plummeted abruptly and remained low over most of the range. The difference is due to the controlling influence of Rc – 2.5 μΩ for the stabrite/NbTi and 0.26 μΩ for the Nb3Sn. The experimental behavior was replicated in the Reff(W)s calculated by the program CUDI© which (using the basic parameters of the QXF cable) went on to show in terms of decreasing W that: (i) in QXF-type Nb3Sn cables (Rc = 0.26 μΩ) Reff dropped even more suddenly when the SS core, instead of being centered, was offset to one edge of the cable, (ii) Reff decreased more gradually in cables with higher Rcs, (iii) a suitable Reff for a Nb3Sn cable can be achieved by inserting a suitably resistive core rather than an insulating (SS) one.Funding was provided by the U.S. Dept. of Energy, Office of High Energy Physics, under Grants No. DE-SC0010312 & DE-SC0011721 (OSU) and DEAC02- 05CH11231 (LBNL).The coupling magnetization of a Rutherford cable is inversely proportional to an effective interstrand contact resistance, Reff, defined as Reff = [1/Rc + 20/N3Ra]-1. In uncored cables Reff is primarily controlled by Rc. The LHC magnet’s uncored NbTi cables, wound with specially heat treated stabrite-coated strands, evidently have acceptable Rcs. It has been reported that the current ramping of LHC magnets produces field errors: (i) in dipoles of about 1 unit of b1 and less than 0.1 units of cn, consistent with Rc well above 50 μΩ, (ii) in quadrupoles of about 2 units of b1 and less than 0.2 units of cn, consistent with Rc between 100 and 150 μΩ. Evidently such Rcs have contributed to the successful operation of the LHC dipoles and quadrupoles to date and hence could be thought of as new target values when designing the Nb3Sn cables for the LHC upgrades. But with measured Rcs of typically 0.3 μΩ bare Nb3Sn cables are unsuitable; the cables need to be furnished with some kind of core to separate the crossing strands. In cables with insulating cores Reff (now a function of both Rc and Ra) increases continuously with W (% core cover), with Ra eventually taking over as the controlling ICR. In seeking an optimal core width a large assortment of research cables were wound and measured over the years. The results, assembled and compared here for the first time, show Reff(W) reaching acceptable values only when W approached ~90% beyond which it increased very steeply. These experimental values were compared to modelling results using the program CUDI© choosing as our model cable a variablewidth- core version of QXF. Further application of the program demonstrated that core positioning was important, Reff decreasing by about 2½ times as the cores shifted from the center to one edge of the cable. As a result it is predicted that irregularities in core placement could produce a large scatter in Reff. The sensitivity of Reff to core width and position in the optimal large-W range leads to the suggested inclusion of a core, not of SS (which has a stable, insulating oxide surface layer), but of a resistive composite such as Cr-plated SS or Crplated Cu