Studies of the performance of the CMS drift tube barrel muon system are described, with results based on data collected during the CMS Cosmic Run at Four Tesla. For most of these data, the solenoidal magnet was operated with a central field of 3.8 T. The analysis of data from 246 out of a total of 250 chambers indicates a very good muon reconstruction capability, with a coordinate resolution for a single hit of about 260 µm, and a nearly 100% efficiency for the drift tube cells. The resolution of the track direction measured in the bending plane is about 1.8 mrad, and the efficiency to reconstruct a segment in a single chamber is higher than 99%. The CMS simulation of cosmic rays reproduces well the performance of the barrel muon detector.
The MIP Timing Detector will provide additional timing capabilities for detection of minimum ionizing particles (MIPs) at CMS during the High Luminosity LHC era, improving event reconstruction and pileup rejection. The central portion of the detector, the Barrel Timing Layer (BTL), will be instrumented with LYSO:Ce crystals and Silicon Photomultipliers (SiPMs) providing a time resolution of about 30 ps at the beginning of operation, and degrading to 50-60 ps at the end of the detector lifetime as a result of radiation damage. In this work, we present the results obtained using a 120 GeV proton beam at the Fermilab Test Beam Facility to measure the time resolution of unirradiated sensors. A proof-of-concept of the sensor layout proposed for the barrel region of the MTD, consisting of elongated crystal bars with dimensions of about 3 × 3 × 57 mm 3 and with double-ended SiPM readout, is demonstrated. This design provides a robust time measurement independent of the impact point of the MIP along the crystal bar. We tested LYSO:Ce bars of different thickness (2, 3, 4 mm) with a geometry close to the reference design and coupled to SiPMs manufactured by Hamamatsu and Fondazione Bruno Kessler. The various aspects influencing the timing performance such as the crystal thickness, properties of the SiPMs (e.g. photon detection efficiency), and impact angle of the MIP are studied. A time resolution of about 28 ps is measured for MIPs crossing a 3 mm thick crystal bar, corresponding to a most probable value (MPV) of energy deposition of 2.6 MeV, and of 22 ps for the 4.2 MeV MPV energy deposition expected in the BTL, matching the detector performance target for unirradiated devices.
The influence of impurities on the physical properties of YBa2Cu3O7−δ superconducting ceramics is investigated. Samples of yttrium ceramics with different Ni, Zn, Co, Fe, and Ga impurity contents were prepared for this study, and the superconducting fraction, the low-frequency susceptibility in an external dc field, and the temperature dependence of the magnetic susceptibility in a weak ac field were measured for each sample. The last of these measurements permits a more precise determination of the critical temperature (the start of the transition) without regard for percolation effects. The experimental results show a stronger suppression of the superconducting state by nonmagnetic impurities, thus confirming the presence of d-state pairs in these superconductors. The degree of suppression of superconductivity by magnetic impurities depends on the preference of the impurities to locate in Cu(1) or Cu(2) sites.
Experiments on resistive tearing mode control in a reversed field pinch by applying a rotating magnetic field are presented. After a brief survey of tearing mode characteristics, the actions taken to prevent their detrimental effects are described. These actions consist in the generation of a rotating magnetic field able to modify the dynamic of the m = 0, n = 1 tearing mode. The rotating field is generated by superimposing an alternating current component to the current circulating in the toroidal field winding. To achieve this, the toroidal field circuit was appropriately modified. Under some conditions, the rotating field exerts a dragging torque on the helical deformation of the plasma column, produced by phase-locked m = 1 modes, so that the deformation can be either localized in a selected toroidal position or put into a continuous toroidal rotation.
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