Coupling loss, interstrand contact resistance, and magnetization of Nb3Sn rutherford cables with cores of MgO tape and s-glass ribbon

Multistrand cables may exhibit two classes of parasitic magnetization both of which can distort the bore-field of an accelerator magnet: (1) a static magnetization (“hysteretic”) resulting from intrastrand persistent currents, and (2) a dynamic magnetization produced by interstrand coupling currents generated during field ramping. The latter, which are moderated by the interstrand contact resistances (ICR), can be controlled by the presence of an insulating core inserted between the layers of the cable. Stainless steel ribbon (with its associated native oxide coating) is a frequently used core. Recently, however, MgO-paper tapes and woven s-glass ribbons have been suggested by LBNL (Lawrence Berkeley National Laboratory) as alternative core materials in the interests of improved flexibility and compatibility with the cabling process. This paper reports on the results of calorimetric AC loss (hence ICR) measurements on a set of four such cables and presents the results within the context of previously measured cored and uncored Nb3Sn cables. Also considered is a typical ramp-rate-induced coupling magnetization and its relationship to persistent-current magnetizations over the operating range of an accelerator magnet

Flux Jumping and a Bulk-to-Granular Transition in the Magnetization of a Compacted and Sintered MgB2 Superconductor

The recent discovery of intermediate-temperature superconductivity (ITC) in MgB2 by Akimitsu et al. and its almost simultaneous explanation in terms of a hole-carrier-based pairing mechanism by Hirsch, has triggered an avalanche of studies of its structural, magnetic and transport properties. As a further contribution to the field we report the results of field (H) and temperature (T) dependent magnetization (M) measurements of a pellet of uniform, large-grain sintered MgB2. We show that at low temperatures the size of the pellet and its critical current density, Jc(H) - i.e. its M(H) - ensure low field flux jumping, which of course ceases when M(H) drops below a critical value. With further increase of H and T the individual grains decouple and the M(H) loops drop to lower lying branches, unresolved in the usual full M(H) representation. After taking into account the sample size and grain size, respectively, the bulk sample and the grains were deduced to exhibit the same magnetically determined Jc s (e.g. 105 A/cm2, 20 K, 0T) and hence that for each temperature of measurement Jc(H) decreased monotonically with H over the entire field range, except for a gap within the grain-decoupling zone.Comment: 7 pages, 6 figures, Changes: Fig 6 Vertical scale an order of magnitude out (changed figure and associated text). Also corrected typo in last sectio

Carbon Doping of MgB2 by Toluene and Malic-Acid-in-Toluene

The decomposition of malic acid in the presence of Mg and B was studied using Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA) which revealed that malic acid reacted with Mg but not B. Also, the addition of toluene to dissolve malic acid followed by subsequent drying resulted in no reaction with Mg, indicating that the malic acid had decomposed during the dissolution/drying stage. The total carbon contributed by toluene versus a toluene/5 wt% malic acid mixture was measured using a LECO CS600 carbon analyzer. The toluene sample contained ~0.4 wt% C while the toluene/malic acid mixture had ~1.5 wt% C, demonstrating that the toluene contributed a significant amount of carbon to the final product. Resistivity measurements on powder-in-tube MgB2 monofilamentary wires established that the toluene/malic acid doped sample had the highest Bc2. However, the toluene-only sample had the highest transport Jc over most of the magnetic field range (0-9 T), equaled only by that of toluene/malic acid sample in fields above 9 T.Comment: 17 pages, 6 figures, 1 tabl

The Effect of Ta and Ti Additions on the Strain Sensitivity of Bulk Niobium-Tin

The effect of tantalum and titanium additions on the composition, the superconducting properties, and their sensitivity to strain of bulk Nb3Sn is investigated. Using heat capacity analysis and Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDX), it is found that the binary Nb3Sn bulk and Nb3Sn bulk with added titanium and tantalum consist of stoichiometric Nb3Sn and niobium(-oxide). Furthermore, it is found that the niobium-to-tin ratio decreases in the presence of tantalum and increases in the presence of titanium, which suggests that tantalum is replacing niobium and titanium is replacing tin in the A15 crystal structure. Using a 10% resistivity criterion, it is observed that the critical magnetic field of unstrained binary bulk is 26.7 T, while the presence of tantalum and titanium raises the critical magnetic field to 29.3 and 30.1 T, respectively. The curves of normalized critical magnetic field as function of strain of all three samples nearly overlap, a strong indication that the variation in strain sensitivity observed in wires is not caused by the titanium and tantalum additions. Understanding the effect of additions on the composition, superconducting properties, and strain sensitivity of Nb3Sn is important for optimizing Nb3Sn conductor technolog

Dual fast-cycling superconducting synchrotron at Fermilab and a possible path to the future of high energy particle physics

We briefly outline shorter and longer term physics motivation for constructing a dual, fast-cycling superconducting synchrotron accelerator (DSFMR - Dual Super-Ferric Main Ring) in the Tevatron tunnel at Fermilab. We discuss using this accelerator as a high-intensity dual neutrino beam source for the long-baseline neutrino oscillation search experiments, and also as a fast, dual pre-injector accelerator for the VLHC (Very Large Hadron Collider)