Results 1 - 10 of 49846
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[en] The latest results from the D0 Collaboration on top quark production and properties in p(bar p) collisions at a center of mass energy of 1.96 TeV are presented. The measurements were performed using approximately 1 fb-1 of D0 data taken during Run II at the Tevatron
[en] Laser- and particle beam-driven plasma wakefield accelerators produce accelerating fields thousands of times higher than radio-frequency accelerators, offering compactness and ultrafast bunches to extend the frontiers of high energy physics and to enable laboratory-scale radiation sources. Large-scale kinetic simulations provide essential understanding of accelerator physics to advance beam performance and stability and show and predict the physics behind recent demonstration of narrow energy spread bunches. Benchmarking between codes is establishing validity of the models used and, by testing new reduced models, is extending the reach of simulations to cover upcoming meter-scale multi-GeV experiments. This includes new models that exploit Lorentz boosted simulation frames to speed calculations. Simulations of experiments showed that recently demonstrated plasma gradient injection of electrons can be used as an injector to increase beam quality by orders of magnitude. Simulations are now also modeling accelerator stages of tens of GeV, staging of modules, and new positron sources to design next-generation experiments and to use in applications in high energy physics and light sources
[en] The Cp*2Yb(L) class of compounds, where Cp* = pentamethylcyclopentadienyl = C5Me5 and L is either a 1,4-diazabutadiene or bipy = 2,2'-bipyridine related ligand, have provided excellent analogies to the Kondo state on the nanoscale. Cp*2Yb(4,4'-Me2-bipy) furthers this analogy by demonstrating a valence transition as the sample is cooled below 200 K. Here, pair-distribution function (PDF) analysis of x-ray powder diffraction data demonstrate that the Cp*2Yb(4,4'-Me2-bipy) molecule is virtually unchanged through the valence transition. However, the molecule's stacking arrangement is altered through the valence transition.
[en] Future insertion quadrupoles with large apertures and high gradients will be required for the Phase II luminosity upgrade (1035 cm-2s-1) of the Large Hadron Collider (LHC). Although improved designs, based on NbTi, are being considered as an intermediate step for the Phase I upgrade, the Nb3Sn conductor is presently the best option that meets the ultimate performance goals for both operating field and temperature margin. As part of the development of Nb3Sn magnet technology, the LHC Accelerator Research Program (LARP) developed and tested several 1-meter long, 90-mm aperture Nb3Sn quadrupoles. The first two series of magnet used OST MJR 54/61 (TQ01 series) and OST RRP 54/61 (TQ02 series) strands. The third series (TQ03) used OST RRP 108/127 conductor. The larger number of sub-elements and the consequent reduction of the effective filament size, together with an increased fraction of copper and a lower Jc were expected to improve the conductor stability. The new coils were tested in the TQS03 series using a shell structure assembled with keys and bladders. The objective of the first test (TQS03a) was to evaluate the performances of the 108/127 conductor and, in particular, its behaviour at 1.9 K, while the second test (TQS03b) investigated the impact on high azimuthal pre-stress on the magnet performance. This paper reports on TQS03 fabrication, the strain gauge measurements performed during assembly, cool-down, excitation and the quench behavior of the two magnets.
[en] A variety of systems analyses have been conducted for laser driver IFE power plants being developed as part of the High Average Power Laser (HAPL) program. A key factor determining the economics attractiveness of the power plant is the net power conversion efficiency which increases with increasing laser efficiency, target gain and fusion-to-electric power conversion efficiency. A possible approach to increasing the power conversion efficiency is direct conversion of ionized target emissions to electricity. One chamber design being considered for HAPL is called the magnetic intervention approach where a cusp magnetic field is used to deflect ions into external dumps, thus protecting the chamber first wall. A possible option with such a design would be to inductively couple the expanding plasma to an external circuit allowing some of the ion energy to be directly converted to electricity. This study examines the potential benefits of increased efficiency achieved with such an approach. Results are evaluated parametrically considering the fraction of fusion energy in ions and the ion-to-electricity conversion efficiency. For base case direct-drive targets with approximately 24% of the target yield in ions, the benefits are modest, especially for chamber designs that operate at high temperature and thus already have relatively high thermal conversion efficiencies. The reduction in the projected cost of electricity is ∼5-10% assuming the cost of direct conversion is no higher than thermal conversion. Details of the systems model and parametric studies are presented
[en] We have conducted experiments on both the Vulcan and Titan laser facilities to study hot electron generation and transport in the context of fast ignition. Cu wires attached to Al cones were used to investigate the effect on coupling efficiency of plasma surround and the pre-formed plasma inside the cone. We found that with thin cones 15% of laser energy is coupled to the 40(micro)m diameter wire emulating a 40(micro)m fast ignition spot. Thick cone walls, simulating plasma in fast ignition, reduce coupling by x4. An increase of prepulse level inside the cone by a factor of 50 reduces coupling by a factor of 3.
[en] A LIFE laser driver needs to be designed and operated which meets the rigorous requirements of the NIF laser system while operating at high average power, and operate for a lifetime of >30 years. Ignition on NIF will serve to demonstrate laser driver functionality, operation of the Mercury laser system at LLNL demonstrates the ability of a diode-pumped solid-state laser to run at high average power, but the operational lifetime >30 yrs remains to be proven. A Laser Technology test Facility (LTF) has been designed to specifically address this issue. The LTF is a 100-Hz diode-pumped solid-state laser system intended for accelerated testing of the diodes, gain media, optics, frequency converters and final optics, providing system statistics for billion shot class tests. These statistics will be utilized for material and technology development as well as economic and reliability models for LIFE laser drivers.
[en] A precise, versatile, and automated method of orienting a sub-millimeter crystal in a focused neutron beam is required for e cient operation of the TOPAZ Single Crystal Di ractometer at the Spallation Neutron Source at Oak Ridge National Laboratory. To ful ll this need, a Compact Crystal Positioning System (CCPS) has been developed in collaboration with Square One Systems Design in Jackson, Wyoming. The system incorporates a tripod design with six vacuum-compatible piezoelectric linear motors capable of < 1 m resolution. National Instruments LabVIEW provides a means of system automation while at the same time accommodating the modular nature of the SNS sample environment control software for straightforward system integration. Initial results in a cryogenic test environment will be presented, as well as results from ambient tests performed at the Advanced Photon Source at Argonne National Laboratory.
[en] The range of oscillation analyses being pursued by the MiniBooNE collaboration is described. Focus is given to the various searches for electron neutrino appearance, but the disappearance of muon neutrinos and the appearance search for electron anti-neutrinos are covered as well