AC Loss and Contact Resistance In Copper-Stabilized Nb3Al Rutherford Cables with and without a Stainless Steel Core

Calorimetric measurements of AC loss and hence interstrand contact resistance (ICR), were measured on three samples of Rutherford cable wound with Cu-stabilized jelly-roll type unplated Nb3Al strand. One of the cable types was furnished with a thin core of AISI 316L stainless steel and the other two were both uncored but insulated in different ways. The cables were subjected to a room-temperature-applied uniaxial pressure of 12 MPa that was maintained during the reaction heat treatment (RHT), then vacuum impregnated with CTD 101 epoxy, and repressurized to 100 MPa during AC-loss measurement. The measurements were performed at 4.2 K in a sinusoidal field of amplitude 400 mT at frequencies of 1 to 90 mHz (no DC-bias field) that was applied both perpendicular and parallel to the face of the cable (the face-on, FO, and edge-on, EO, directions, respectively). For the cored cable the FO-measured effective ICR (FO-ICR), was 5.27 . Those for the uncored cables were less than 0.08 . As shown previously for NbTi- and Nb3Sn-based Rutherford cables, the FO-ICR can be significantly increased by the insertion of a core, although in this case it is still below the range recommended for accelerator-magnet use. Post-measurement dissection of one of the cables showed that the impregnating resin had permeated between the strands and coated the core with a thin, insulating layer excepting for some sintered points of contact. In the uncored cables the strands were coated with resin except for the points of interstrand contact. It is suggested that in the latter case this tendency for partial coating leads to a processing-sensitive FO-ICR.Comment: Four pages, with two figure

Transport and magnetic Jc of MgB2 strands and small helical coils

The critical current densities of MgB2 monofilamentary strands with and without SiC additions were measured at 4.2 K. Additionally, magnetic Jc at B = 1 T was measured from 4.2 K to 40 K. Various heat treatment times and temperatures were investigated for both short samples and small helical coils. SiC additions were seen to improve high field transport Jc at 4.2 K, but improvements were not evident at 1 T at any temperature. Transport results were relatively insensitive to heat treatment times and temperatures for both short samples and coils in the 700C to 900C range.Comment: 8 text pages, 1 table, 4 fig

Increases in the Irreversibility Field and the Upper Critical Field of Bulk MgB2 by ZrB2 Addition

In a study of the influence of ZrB2 additions on the irreversibility field, Birr and the upper critical field Bc2, bulk samples with 7.5 at. % ZrB2 additions were made by a powder milling and compaction technique. These samples were then heated to 700-900C for 0.5 hours. Resistive transitions were measured at 4.2 K and Birr and Bc2 values were determined. An increase in Bc2 from 20.5 T to 28.6 T and enhancement of Birr from 16 T to 24 T were observed in the ZrB2 doped sample as compared to the binary sample at 4.2 K. Critical field increases similar to those found with SiC doping were seen at 4.2 K. At higher temperatures, increases in Birr were also determined by M-H loop extrapolation and closure. Values of Birr which were enhanced with ZrB2 doping (as compared to the binary) were seen at temperatures up to 34 K, with Birr values larger than those for SiC doped samples at higher temperatures. The transition temperature, Tc, was then measured using DC susceptibility and a 2.5 K drop of the midpoint of Tc was observed. The critical current density was determined using magnetic measurements and was found to increase at all temperatures between 4.2 K and 35 K with ZrB2 doping.Comment: 15 pages, 5 figs, 1 tabl

Extracted Strand Magnetizations of an HQ Type Nb3Sn Rutherford Cable and Estimation of Transport Corrections at Operating and Injection Fields

One of the goals of the Large Hadron Collider Accelerator Research Program (LARP) is to demonstrate the feasibility of Nb3Sn technology for a proposed luminosity upgrade based on large aperture high gradient quadrupole (HQ) magnets. For such magnets field quality at the bore is a critical requirement for which reason the parasitic magnetization of the windings must be reduced to manageable limits. In other words it is necessary to minimize (i) the static intrastrand persistent current magnetization of the cable and (ii) the cable’s coupling magnetization caused by coupling currents passing through interstrand contact resistance during field ramping. This report focuses on persistent-current magnetization as measured by vibrating-sample magnetometry on pieces of strand removed from a section of heat treated HQ cable.Funding was provided by the U.S. Dept. of Energy, Office of High Energy Physics, under Grants No. DE-FG02- 95ER40900 (OSU) and DE-AC02-05CH11231 (LBNL).The bench-mark data for the NbTi-wound LHC are Msh,inj,LHC = 10.3 kA/m and Mcoup,LHC = 2.64 kA/m. The present cable, HQ1021ZB, with an 8 mm stainless steel core wound at LBNL, is similar to a previously measured LBNLwound HQ-KC3 cable. As such we would expect to find the same 8-mm-core-moderated ICR and coupling magnetization, 1.02 μ and 70.3 kA/m, respectively. Clearly such a narrow core width is inadequate to properly suppress Mcoup. With regard to persistent-current magnetization at injection, Nb3Sn’s large Jcdeff product guarantees a large value, estimated here to be Msh,cable,inj,1T = 171 kA/m. On the other hand as the magnet is ramped up to operating field Msh,cable steadily decreases such that at 15 T in the windings it hasdropped to 5.8 kA/m, clearly an acceptable value. The Mcoup results of [19] indicated that a full width corewould be needed to adequately suppress coupling magnetization and the Msh,cable data of Table II indicates that although Msh,cable is not a problem close to operating field, strong compensation will be required near injection [26][27]

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

Atmospheric conditions and their effect on ball-milled magnesium diboride

Magnesium diboride bulk pellets were fabricated from pre-reacted MgB2 powder ball milled with different amounts of exposure to air. Evidence of increased electron scattering including increased resistivity, depressed Tc, and enhanced Hc2 of the milled and heat treated samples were observed as a result of increased contact with air. These and other data were consistent with alloying with carbon as a result of exposure to air. A less clear trend of decreased connectivity associated with air exposure was also observed. In making the case that exposure to air should be considered a doping process, these results may explain the wide varibability of "undoped" MgB2 properties extant in the literature.Comment: Work presented at ASC 2006 in Seattl