Hardness and Tensile Strength of Multifilamentary Metal-matrix Composite Superconductors for the Large Hadron Collider (LHC)

Conventional indentation hardness measurements to obtain load independent Vickers hardness values for the different phases in multifilamentary superconducting (SC) wires are described. The concept of composite hardness is validated for a binary metal-matrix metal-filament Nb-Ti/Cu composite wire. The tensile materials properties of the individual wire components are estimated from their indentation hardness. The potential and limitations of this approach are critically discussed, based on a comparison with tensile test results obtained for wires and extracted Nb-Ti filaments

Periodic squeezing in a polariton Josephson junction

The use of a Kerr nonlinearity to generate squeezed light is a well-known way to surpass the quantum noise limit along a given field quadrature. Nevertheless, in the most common regime of weak nonlinearity, a single Kerr resonator is unable to provide the proper interrelation between the field amplitude and squeezing required to induce a sizable deviation from Poissonian statistics. We demonstrate experimentally that weakly coupled bosonic modes allow exploration of the interplay between squeezing and displacement, which can give rise to strong deviations from the Poissonian statistics. In particular, we report on the periodic bunching in a Josephson junction formed by two coupled exciton-polariton modes. Quantum modeling traces the bunching back to the presence of quadrature squeezing. Our results, linking the light statistics to squeezing, are a precursor to the study of nonclassical features in semiconductor microcavities and other weakly nonlinear bosonic systems.Comment: 6 pages, 4 figure

A low-complexity channel training method for efficient SVD beamforming over MIMO channels

Singular value decomposition (SVD) beamforming is an attractive tool for reducing the energy consumption of data transmissions in wireless sensor networks whose nodes are equipped with multiple antennas. However, this method is often not practical due to two important shortcomings: it requires channel state information at the transmitter and the computation of the SVD of the channel matrix is generally too complex. To deal with these issues, we propose a method for establishing an SVD beamforming link without requiring feedback of actual channel or SVD coefficients to the transmitter. Concretely, our method takes advantage of channel reciprocity and a power iteration algorithm (PIA) for determining the precoding and decoding singular vectors from received preamble sequences. A low-complexity version that performs no iterations is proposed and shown to have a signal-to-noise-ratio (SNR) loss within 1 dB of the bit error rate of SVD beamforming with least squares channel estimates. The low-complexity method significantly outperforms maximum ratio combining diversity and Alamouti coding. We also show that the computational cost of the proposed PIA-based method is less than the one of using the Golub–Reinsch algorithm for obtaining the SVD. The number of computations of the low-complexity version is an order of magnitude smaller than with Golub–Reinsch. This difference grows further with antenna array size

Critical Current Test Facilities for LHC Superconducting NbTi Cable Strands

The Rutherford-type superconducting Cu/NbTi cables of the LHC accelerator are currently mass-produced by a few industrial firms. As a part of the acceptance tests, the critical current of superconducting multifilamentary wires is systematically measured on virgin strands to qualify the wires and on extracted strands to qualify the cables. For this purpose, four test stations are in operation at CERN to measure the critical current of strands at both 4.2 K and 1.9 K in magnetic fields in the 6-11 T range. The measurement setup and procedures of these facilities are reported in this article. The quality of the critical current test is guaranteed by supervising the SPC (Statistical Process Control) charts of a reference sample. The measurement repeatability and reproducibility of the stations are found to be excellent. Moreover, the measured critical current of a strand is found to be almost independent of the test station in which the measurement is performed

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

Reservoir-induced decoherence of resonantly excited confined polaritons

We report on the effect of decoherence on polariton bistability. The polariton hysteresis loop is shown to collapse in a similar way when increasing the temperature or under nonresonant excitation power. The hysteresis upward threshold is pulled to lower excitation power, whereas the downward threshold remains almost constant. This effect is explained by the population of an incoherent reservoir that induces dephasing and repulsive interaction that saturates at large densities. All experimental findings are accurately simulated with the excitonic Bloch equations and indicate that reservoir-induced dephasing can be dominant over the reservoir-induced energy blueshift

Spatial multistability induced by cross interactions of confined polariton modes

We demonstrate the occurrence of spatial multistability using laterally confined microcavity exciton-polaritons. By coherently exciting with a blue detuned laser a series of confined polariton modes, we investigate the effects of multistability on the transmitted laser beam as a function of the excitation power. At each threshold of the hysteresis loop, a switching of the mode profile of the laser beam is associated with a significant energy jump of each of the confined polariton modes in the mesa. A simulation of this behavior is achieved with a multimode generalization of the Gross-Pitaevskii equations in the exciton photon basis. The mechanism behind the spatial multistability is identified as a repulsive cross interaction between polaritons in different modes

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