Current Redistribution around the Superconducting-to-normal Transition in Superconducting Nb-Ti Rutherford Cables

Sufficient thermal-electromagnetic stability against external heat sources is an essential design criterion for superconducting Rutherford cables, especially if operated close to the critical current. Due to the complex phenomena contributing to stability such as helium cooling, inter-strand current and heat transfer, its level is difficult to quantify. In order to improve our understanding, many stability tests were performed on different cable samples, each incorporating several point-like heaters. The current redistribution around the heat front is measured after inducing a local normal zone in one strand of the cable. By using voltage taps, expansion of the normal zone is monitored in the initially quenched strand as well as in adjacent strands. An array of Hall probes positioned at the cable edge is used to scan the selffield generated by the cable by which it becomes possible to estimate the inter-strand current transfer. In this paper it is demonstrated that two different stability regimes can be distinguished depending on the local conditions for local normal zone recovery through heat and current transfer to adjacent strands. It is shown that in the first regime every normal zone will lead to a quench, while in the second regime a normal zone in one strand can recover. Combining the predictions developed using a novel version of the numerical network model CUDI and new measurement results, it is possible to derive char acteristic quench decision times as well to calculate and predict the influence of a change in cable parameters

Stability of Nb-Ti Rutherford Cables Exhibiting Different Contact Resistances

Dipole magnets for the so-called SIS-300 heavy-ion synchrotron at GSI are designed to generate 6 T with a field sweep rate of 1 T/s. It is foreseen to wind the magnets with a 36 strands Nb-Ti Rutherford cable. An important issue in the cable design is sufficiently low AC loss and stability as well. In order to keep the AC loss at low level, the contact resistance between crossing strands Rc is kept high by putting a stainless steel core in the cable. The contact resistance between adjacent strands Ra is controlled by oxidation of the Sn-Ag coating of the strands, like in the LHC. In order to investigate the effect of Ra on the stability of the cable, we prepared four samples with different Ra by varying the heat treatment and applying a soldering technique, resulting in values between 1 mW to 9 mW. The stability of each sample against transient point-like heat pulses was measured. The results of the stability experiments and a comparison with calculations using the network model CUDI are presented. It is concluded that variation of Ra has a strong influence on cable stability and that optimization of Ra is mandatory to properly design the cable for the SIS-300 magnets, or likewise for similar magnets that might be used at CERN for a possible LHC injector upgrade

Aluminum strand coating for increasing the interstrand contact resistance in Rutherford type superconducting cables

The interstrand contact resistance (Rc) in Rutherford type cables for fast cycling superconducting magnets must be sufficiently high in order to limit eddy current losses. The required value for Rc depends on the cable and magnet geometries and on the foreseen cycling rate, but is typically of the order of one mW. Such values can be reached with a dedicated strand coating or with a resistive internal cable barrier. As a possible candidate Al strand coatings have been tested. For a Rutherford type inner conductor cable of the Large Hadron Collider (LHC) made of Al coated strands Rc values higher than 500 Omega are achieved. The native Al2O3 oxide layer formed at ambient temperature in air is sufficient to reach this high contact resistance. A 6 h-200 °C oxidation heat treatment in air with 100% relative humidity further increases Rc to values above 600 μOmega . Due to the high thermal and mechanical stability of Al2O3 only a relatively moderate Rc drop of about 40 % is obtained during a 190 °C heat treatment under 50 MPa pressure (the so-called curing cycle of the coil insulation) subsequent to the 6 h-200 °C oxidation heat treatment

Models and experimental results from the wide aperture Nb-Ti magnets for the LHC upgrade

MQXC is a Nb-Ti quadrupole designed to meet the accelerator quality requirements needed for the phase-1 LHC upgrade, now superseded by the high luminosity upgrade foreseen in 2021. The 2-m-long model magnet was tested at room temperature and 1.9 K. The technology developed for this magnet is relevant for other magnets currently under development for the high-luminosity upgrade, namely D1 (at KEK) and the large aperture twin quadrupole Q4 (at CEA). In this paper we present MQXC test results, some of the specialized heat extraction features, spot heaters, temperature sensor mounting and voltage tap development for the special open cable insulation. We look at some problem solving with noisy signals, give an overview of electrical testing, look at how we calculate the coil resistance during at quench and show that the heaters are not working We describe the quench signals and its timing, the development of the quench heaters and give an explanation of an Excel quench calculation and its comparison including the good agreement with the MQXC test results. We propose an improvement to the magnet circuit design to reduce voltage to ground values by factor 2. The program is then used to predict quench Hot-Spot and Voltages values for the D1 dipole and the Q4 quadrupole.Comment: 8 pages, Contribution to WAMSDO 2013: Workshop on Accelerator Magnet, Superconductor, Design and Optimization; 15 - 16 Jan 2013, CERN, Geneva, Switzerlan

Temperature Induced Degradation of Nb Ti/Cu Composite Superconductors

The degradation mechanisms of state-of-the-art Nb-Ti/Cu superconductors are described, based on in-situ synchrotron X-ray diffraction measurements during heat treatment. A quantitative description of the Nb-Ti/Cu degradation in terms of critical current density, Cu stabiliser resistivity and mechanical composite strength is presented. In an applied magnetic field a significant critical current degradation is already observed after a 5-minute 400 °C heat treatment, due to variations of a-Ti precipitate size and distribution within the Nb-Ti alloy filaments. A strong degradation of the strand mechanical properties is observed after several minutes heating above 550 °C, which is also the temperature at which the formation of Cu Ti intermetallic phases is detected. Several minutes heating at 250 °C are sufficient to increase the RRR of the strongly cold work strands inside a Rutherford type cable from about 80 to about 240. Heating for several minutes at 400 °C does not cause a significant conductor degradation in self-field and, thus, leaves enough temperature margin for the electrical interconnection of Nb-Ti/Cu conductors with common low temperature solders

Power Test of the First Two HL-LHC Insertion Quadrupole Magnets Built at CERN

The High-Luminosity project (HL-LHC) of the CERN Large Hadron Collider (LHC), requires low β* quadrupole magnets in Nb$_3$Sn technology that will be installed on each side of the ATLAS and CMS experiments. After a successful shortmodel magnet manufacture and test campaign, the project has advanced with the production, assembly, and test of full-size 7.15- m-long magnets. In the last two years, two CERN-built prototypes (MQXFBP1 and MQXFBP2) have been tested and magnetically measured at the CERN SM18 test facility. These are the longest accelerator magnets based on Nb$_3$Sn technology built and tested to date. In this paper, we present the test and analysis results of these two magnets, with emphasis on quenches and training, voltage-current measurements and the quench localization with voltage taps and a new quench antenna

Progress on HL-LHC Nb3Sn Magnets

The high-luminosity Large Hadron Collider (HL-LHC) project aims at allowing to increase the collisions in the LHC by a factor of ten in the decade 2025-2035. One essential element is the superconducting magnet around the interaction region points, where the large aperture magnets will be installed to allow to further reduce the beam size in the interaction point. The core of this upgrade is the Nb Sn triplet, made up of 150-mm aperture quadrupoles in the range of 7-8 m. The project is being shared between the European Organization for Nuclear Research and the US Accelerator Upgrade Program, based on the same design, and on the two strand technologies. The project is ending the short model phase, and entering the prototype construction. We will report on the main results of the short model program, including the quench performance and field quality. A second important element is the 11 T dipole that replaces a standard dipole making space for additional collimators. The magnet is also ending the model development and entering the prototype phase. A critical point in the design of this magnet is the large current density, allowing increase of the field from 8 to 11 T with the same coil cross section as in the LHC dipoles. This is also the first two-in-one Nb Sn magnet developed so far. We will report the main results on the test and the critical aspects. 3

Electron-induced neutron knockout from 4^{4}He

The differential cross section for electron-induced neutron knockout in the reaction 4He(e,e′n)3He has been measured for the first time with a statistical accuracy of 11%. The experiment was performed in quasielastic kinematics at a momentum transfer of 300  MeV/c and in the missing-momentum range of 25–70  MeV/c. The comparison of the data with theoretical calculations shows an impressive increase of the cross section resulting from final state interaction effects. Specifically , the p−n charge-exchange process dominates the cross section in this kinematical regime. (APS