219 research outputs found
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
Effects of Hyperbolic Rotation in Minkowski Space on the Modeling of Plasma Accelerators in a Lorentz Boosted Frame
Laser driven plasma accelerators promise much shorter particle accelerators
but their development requires detailed simulations that challenge or exceed
current capabilities. We report the first direct simulations of stages up to 1
TeV from simulations using a Lorentz boosted calculation frame resulting in a
million times speedup, thanks to a frame boost as high as gamma=1300. Effects
of the hyperbolic rotation in Minkowski space resulting from the frame boost on
the laser propagation in the plasma is shown to be key in the mitigation of a
numerical instability that was limiting previous attempts
Tunable Electron Multibunch Production in Plasma Wakefield Accelerators
Synchronized, independently tunable and focused J-class laser pulses are
used to release multiple electron populations via photo-ionization inside an
electron-beam driven plasma wave. By varying the laser foci in the laboratory
frame and the position of the underdense photocathodes in the co-moving frame,
the delays between the produced bunches and their energies are adjusted. The
resulting multibunches have ultra-high quality and brightness, allowing for
hitherto impossible bunch configurations such as spatially overlapping bunch
populations with strictly separated energies, which opens up a new regime for
light sources such as free-electron-lasers
An atmospheric perspective on North American carbon dioxide exchange: CarbonTracker
We present an estimate of net CO2 exchange between the terrestrial biosphere and the atmosphere across North America for every week in the period 2000 through 2005. This estimate is derived from a set of 28,000 CO2 mole fraction observations in the global atmosphere that are fed into a state-of-the-art data assimilation system for CO2 called CarbonTracker. By design, the surface fluxes produced in CarbonTracker are consistent with the recent history of CO2 in the atmosphere and provide constraints on the net carbon flux independent from national inventories derived from accounting efforts. We find the North American terrestrial biosphere to have absorbed –0.65 PgC/yr (1 petagram = 10^15 g; negative signs are used for carbon sinks) averaged over the period studied, partly offsetting the estimated 1.85 PgC/yr release by fossil fuel burning and cement manufacturing. Uncertainty on this estimate is derived from a set of sensitivity experiments and places the sink within a range of –0.4 to –1.0 PgC/yr. The estimated sink is located mainly in the deciduous forests along the East Coast (32%) and the boreal coniferous forests (22%). Terrestrial uptake fell to –0.32 PgC/yr during the large-scale drought of 2002, suggesting sensitivity of the contemporary carbon sinks to climate extremes. CarbonTracker results are in excellent agreement with a wide collection of carbon inventories that form the basis of the first North American State of the Carbon Cycle Report (SOCCR), to be released in 2007. All CarbonTracker results are freely available at http://carbontracker.noaa.gov
Toward regional-scale modeling using the two-way nested global model TM5:Characterization of transport using SF6
We present an evaluation of transport of sulfur hexafluoride (SF6) in the two-way nested chemistry-transport model "Tracer Model 5" (TM5). Modeled SF6 values for January 2000 to November 2003 are compared with NOAA CMDL observations. This includes new high-frequency SF6 observations, frequent vertical profiles, and weekly flask data from more than 60 sites around the globe. This constitutes the most extensive set of SF6 observations used in transport model evaluation to date. We find that TM5 captures temporal variability on all timescales well, including the relatively large SF6 signals on synoptic scales (2-5 days). The model overestimates the meridional gradient of SF6 by 19%, similar to previously used transport models. Vertical profiles are reproduced to within the standard error of the observations, and do not reveal large biases. An important area for future improvements is the mixing of the planetary boundary layer which is currently too slow, leading to modeled SF6 mixing ratios that are too large over the continents. Increasing the horizontal resolution over North America from 6×4°, to 3×2°, to even 1×1° (lon×lat) does not affect the simulated global scale SF6 distribution and potentially minimizes representation errors for continental sites. These results are highly relevant for future CO2 flux estimates with TM5, which will be briefly discussed
Upper critical field and de Haas-van Alphen oscillations in KOsO measured in a hybrid magnet
Magnetic torque measurements have been performed on a KOsO single
crystal in magnetic fields up to 35.3 T and at temperatures down to 0.6 K. The
upper critical field is determined to be 30 T. De Haas-van Alphen
oscillations are observed. A large mass enhancement of (1+) = = 7.6 is found. It is suggested that, for the large upper critical
field to be reconciled with Pauli paramagnetic limiting, the observed mass
enhancement must be of electron-phonon origin for the most part.Comment: 4 pages, 4 figures, published versio
Magnetic and charge transport properties of the Na-based Os oxide pyrochlore
Na-based osmium oxide pyrochlore was synthesized for the first time by an
ion-exchange method. KOs2O6 was used as a host compound. Elelectron probe
micro-analysis, synchrotron x-ray diffraction analysis, and thermo-gravimetric
analysis confirmed its structure not as the beta-type but as the defect-type
pyrochlore. The composition was identified as Na1.4Os2O6.H2O. Electrical
resistivity, heat capacity, and magnetization measurements of the
polycrystalline Na1.4Os2O6.H2O clarified absence of superconductivity above 2
K, being in contrast to what were found for the beta-pyrochlore AOs2O6 (A = Cs,
Rb, K). Sommerfeld coefficient of 22 mJ K-2 mol-1 of Na1.4Os2O6.H2O was
smallest among those of AOs2O6. A magnetic anomaly at ~57 K and possible
associated magnetoresistance (+3.7 % at 2 K in 70 kOe) were found.Comment: 19 pages, 6 figures, Submitted to PR
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Scaled simulations of a 10 GeV accelerator
Laser plasma accelerators are able to produce high quality electron beams from 1 MeV to 1 GeV. The next generation of plasma accelerator experiments will likely use a multi-stage approach where a high quality electron bunch is first produced and then injected into an accelerating structure. In this paper we present scaled particle-in-cell simulations of a 10 GeV stage in the quasi-linear regime. We show that physical parameters can be scaled to be able to perform these simulations at reasonable computational cost. Beam loading properties and electron bunch energy gain are calculated. A range of parameter regimes are studied to optimize the quality of the electron bunch at the output of the stage
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