9,772 research outputs found
High-quality ion beams by irradiating a nano-structured target with a petawatt laser pulse
We present a novel laser based ion acceleration scheme, where a petawatt
circularly polarized laser pulse is shot on an ultra-thin (nano-scale)
double-layer target. Our scheme allows the production of high-quality light ion
beams with both energy and angular dispersion controllable by the target
properties. We show that extraction of all electrons from the target by
radiation pressure can lead to a very effective two step acceleration process
for light ions if the target is designed correctly. Relativistic protons should
be obtainable with pulse powers of a few petawatt. Careful analytical modeling
yields estimates for characteristic beam parameters and requirements on the
laser pulse quality, in excellent agreement with one and two-dimensional
Particle-in Cell simulations.Comment: 18 pages, 7 figures, accepted in New. J. Phy
Habitatâdependent occupancy and movement in a migrant songbird highlights the importance of mangroves and forested lagoons in Panama and Colombia
Climate change is predicted to impact tropical mangrove forests due to decreased rainfall, seaâlevel rise, and increased seasonality of flooding. Such changes are likely to influence habitat quality for migratory songbirds occupying mangrove wetlands during the tropical dry season. Overwintering habitat quality is known to be associated with fitness in migratory songbirds, yet studies have focused primarily on territorial species. Little is known about the ecology of nonterritorial species that may display more complex movement patterns within and among habitats of differing quality. In this study, we assess withinâseason survival and movement at two spatioâtemporal scales of a nonterritorial overwintering bird, the prothonotary warbler (Protonotaria citrea), that depends on mangroves and tropical lowland forests. Specifically, we (a) estimated withinâpatch survival and persistence over a sixâweek period using radioâtagged birds in central Panama and (b) modeled abundance and occupancy dynamics at survey points throughout eastern Panama and northern Colombia as the dry season progressed. We found that site persistence was highest in mangroves; however, the probability of survival did not differ among habitats. The probability of warbler occupancy increased with canopy cover, and wet habitats were least likely to experience local extinction as the dry season progressed. We also found that warbler abundance is highest in forests with the tallest canopies. This study is one of the first to demonstrate habitatâdependent occupancy and movement in a nonterritorial overwintering migrant songbird, and our findings highlight the need to conserve intact, mature mangrove, and lowland forests
Magnetic-field-induced superconductivity in layered organic molecular crystals with localized magnetic moments
The synthetic organic compound lambda-(BETS)2FeCl4 undergoes successive
transitions from an antiferromagnetic insulator to a metal and then to a
superconductor as a magnetic field is increased. We use a Hubbard-Kondo model
to clarify the role of the Fe(3+) magnetic ions in these phase transitions. In
the high-field regime, the magnetic field acting on the electron spins is
compensated by the exchange field He due to the magnetic ions. This suggests
that the field-induced superconducting state is the same as the zero-field
superconducting state which occurs under pressure or when the Fe(3+) ions are
replaced by non-magnetic Ga(3+) ions. We show how He can be extracted from the
observed splitting of the Shubnikov-de Haas frequencies. Furthermore, we use
this method of extracting He to predict the field range for field-induced
superconductivity in other materials.Comment: 5 page
Development and Characterisation of a Gas System and its Associated Slow-Control System for an ATLAS Small-Strip Thin Gap Chamber Testing Facility
A quality assurance and performance qualification laboratory was built at
McGill University for the Canadian-made small-strip Thin Gap Chamber (sTGC)
muon detectors produced for the 2019-2020 ATLAS experiment muon spectrometer
upgrade. The facility uses cosmic rays as a muon source to ionise the quenching
gas mixture of pentane and carbon dioxide flowing through the sTGC detector. A
gas system was developed and characterised for this purpose, with a simple and
efficient gas condenser design utilizing a Peltier thermoelectric cooler (TEC).
The gas system was tested to provide the desired 45 vol% pentane concentration.
For continuous operations, a state-machine system was implemented with alerting
and remote monitoring features to run all cosmic-ray data-acquisition
associated slow-control systems, such as high/low voltage, gas system and
environmental monitoring, in a safe and continuous mode, even in the absence of
an operator.Comment: 23 pages, LaTeX, 14 figures, 4 tables, proof corrections for Journal
of Instrumentation (JINST), including corrected Fig. 8b
Search for new resonant states in 10C and 11C and their impact on the cosmological lithium problem
The observed primordial 7Li abundance in metal-poor halo stars is found to be
lower than its Big-Bang nucleosynthesis (BBN) calculated value by a factor of
approximately three. Some recent works suggested the possibility that this
discrepancy originates from missing resonant reactions which would destroy the
7Be, parent of 7Li. The most promising candidate resonances which were found
include a possibly missed 1- or 2- narrow state around 15 MeV in the compound
nucleus 10C formed by 7Be+3He and a state close to 7.8 MeV in the compound
nucleus 11C formed by 7Be+4He. In this work, we studied the high excitation
energy region of 10C and the low excitation energy region in 11C via the
reactions 10B(3He,t)10C and 11B(3He,t)11C, respectively, at the incident energy
of 35 MeV. Our results for 10C do not support 7Be+3He as a possible solution
for the 7Li problem. Concerning 11C results, the data show no new resonances in
the excitation energy region of interest and this excludes 7Be+4He reaction
channel as an explanation for the 7Li deficit.Comment: Accepted for publication in Phys. Rev. C (Rapid Communication
Computationally efficient methods for modelling laser wakefield acceleration in the blowout regime
Electron self-injection and acceleration until dephasing in the blowout
regime is studied for a set of initial conditions typical of recent experiments
with 100 terawatt-class lasers. Two different approaches to computationally
efficient, fully explicit, three-dimensional particle-in-cell modelling are
examined. First, the Cartesian code VORPAL using a perfect-dispersion
electromagnetic solver precisely describes the laser pulse and bubble dynamics,
taking advantage of coarser resolution in the propagation direction, with a
proportionally larger time step. Using third-order splines for macroparticles
helps suppress the sampling noise while keeping the usage of computational
resources modest. The second way to reduce the simulation load is using
reduced-geometry codes. In our case, the quasi-cylindrical code CALDER-CIRC
uses decomposition of fields and currents into a set of poloidal modes, while
the macroparticles move in the Cartesian 3D space. Cylindrical symmetry of the
interaction allows using just two modes, reducing the computational load to
roughly that of a planar Cartesian simulation while preserving the 3D nature of
the interaction. This significant economy of resources allows using fine
resolution in the direction of propagation and a small time step, making
numerical dispersion vanishingly small, together with a large number of
particles per cell, enabling good particle statistics. Quantitative agreement
of the two simulations indicates that they are free of numerical artefacts.
Both approaches thus retrieve physically correct evolution of the plasma
bubble, recovering the intrinsic connection of electron self-injection to the
nonlinear optical evolution of the driver
Chenoua
Le massif et sa population (Ph. Leveau) A lâest de la Mitidja, le ChĂ©noua est un petit massif de forme grossiĂšrement ovale mesurant 13 km dâest en ouest, 8 km du nord au sud et baignĂ© au nord par la mer. Un couloir de plateaux et la plaine alluviale de lâoued el-Hachem le sĂ©parent des montagnes telliennes du sud. Câest un massif original qui, comme le cap TenĂšs Ă lâouest, appartient Ă la zone primaire kabyle (zone I de Glangeaud). Une chaĂźne calcaire formĂ©e de lias massif et culminant Ă 904 m..
Non-volatile molecular memory elements based on ambipolar nanotube field effect transistors
We have fabricated air-stable n-type, ambipolar carbon nanotube field effect
transistors (CNFETs), and used them in nanoscale memory cells. N-type
transistors are achieved by annealing of nanotubes in hydrogen gas and
contacting them by cobalt electrodes. Scanning gate microscopy reveals that the
bulk response of these devices is similar to gold-contacted p-CNFETs,
confirming that Schottky barrier formation at the contact interface determines
accessibility of electron and hole transport regimes. The transfer
characteristics and Coulomb Blockade (CB) spectroscopy in ambipolar devices
show strongly enhanced gate coupling, most likely due to reduction of defect
density at the silicon/silicon-dioxide interface during hydrogen anneal. The CB
data in the ``on''-state indicates that these CNFETs are nearly ballistic
conductors at high electrostatic doping. Due to their nanoscale capacitance,
CNFETs are extremely sensitive to presence of individual charge around the
channel. We demonstrate that this property can be harnessed to construct data
storage elements that operate at the few-electron level.Comment: 6 pages text, 3 figures and 1 table of content graphic; available as
NanoLetters ASAP article on the we
Temporary Intervention and Long Term Legacy: Lessons from London Case Studies
The paper explores the issue of temporary projects on vacant land focusing on London in the 2007-2012 downturn. Using a case study approach, a link has been identified between the success of temporary projects and a longer-term vision, as well as a move toward better integration between temporary occupants and developer/land-owner. Within this paradox the whole idea of temporariness is put under question, as is the traditional mainstream depiction of bottom-up in opposition to top-down action. These trends are contextualised within the dynamics of recession that has triggered new types of creative conversations between parties traditionally considered in opposition and may contribute to reframing urban development as an incremental, organic and collaborative process
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