9,334 research outputs found
Field-reversed bubble in deep plasma channels for high quality electron acceleration
We study hollow plasma channels with smooth boundaries for laser-driven
electron acceleration in the bubble regime. Contrary to the uniform plasma
case, the laser forms no optical shock and no etching at the front. This
increases the effective bubble phase velocity and energy gain. The longitudinal
field has a plateau that allows for mono-energetic acceleration. We observe as
low as 10^{-3} r.m.s. relative witness beam energy uncertainty in each
cross-section and 0.3% total energy spread. By varying plasma density profile
inside a deep channel, the bubble fields can be adjusted to balance the laser
depletion and dephasing lengths. Bubble scaling laws for the deep channel are
derived. Ultra-short pancake-like laser pulses lead to the highest energies of
accelerated electrons per Joule of laser pulse energy
Interpolation between the epsilon and p regimes
We reconsider chiral perturbation theory in a finite volume and develop a new
computational scheme which smoothly interpolates the conventional epsilon and p
regimes. The counting rule is kept essentially the same as in the p expansion.
The zero-momentum modes of Nambu-Goldstone bosons are, however, treated
separately and partly integrated out to all orders as in the epsilon expansion.
In this new scheme, the theory remains infra-red finite even in the chiral
limit, while the chiral-logarithmic effects are kept present. We calculate the
two-point function in the pseudoscalar channel and show that the correlator has
a constant contribution in addition to the conventional hyperbolic cosine
function of time t. This constant term rapidly disappears in the p regime but
it is indispensable for a smooth convergence of the formula to the epsilon
regime result. Our calculation is useful to precisely estimate the finite
volume effects in lattice QCD simulations on the pion mass Mpi and kaon mass
MK, as well as their decay constants Fpi and FK.Comment: 49 pages, 6 figures, minor corrections, references added, version to
appear in PR
Bayesian clinical trial designs : Another option for trauma trials?
The UK-REBOA Trial is funded by the National Institute for Health Research (NIHR) Health Technology Assessment (HTA) programme (project number 14/199/09). PP was supported by the MRC Network of Hubs for Trials Methodology Research (MR/L004933/1-R/N/P/B1).Peer reviewedPublisher PD
Numerical stability of the AA evolution system compared to the ADM and BSSN systems
We explore the numerical stability properties of an evolution system
suggested by Alekseenko and Arnold. We examine its behavior on a set of
standardized testbeds, and we evolve a single black hole with different gauges.
Based on a comparison with two other evolution systems with well-known
properties, we discuss some of the strengths and limitations of such simple
tests in predicting numerical stability in general.Comment: 16 pages, 12 figure
Modeling and predicting pointing errors in two dimensions
Recently, Wobbrock et al. (2008) derived a predictive model of pointing accuracy to complement Fitts â lawâs predictive model of pointing speed. However, their model was based on one-dimensional (1-D) horizontal movement, while applications of such a model require two dimensions (2-D). In this paper, the pointing error model is investigated for 2-D pointing in a study of 21 participants performing a time-matching task on the ISO 9241-9 ring-of-circles layout. Results show that the pointing error model holds well in 2-D. If univariate endpoint deviation (SDx) is used, regressing on N=72 observed vs. predicted error rate points yields R 2 =.953. If bivariate endpoint deviation (SDx,y) is used, regression yields R 2 =.936. For both univariate and bivariate models, the magnitudes of observed and predicted error rates are comparable. Author Keywords: Pointing error model, Fitts â law, metronome, movement time, error prediction, error rates
Magnetic properties of PdAs2O6: a dilute spin system with an unusually high N\'eel temperature
The crystal structure and magnetic ordering pattern of PdAs2O6 were
investigated by neutron powder diffraction. While the magnetic structure of
PdAs2O6 is identical to the one of its isostructural 3d-homologue NiAs2O6, its
N\'{e}el temperature (140 K) is much higher than the one of NiAs2O6 (30 K).
This is surprising in view of the long distance and indirect exchange path
between the magnetic Pd ions. Density functional calculations yield
insight into the electronic structure and the geometry of the exchange-bond
network of both PdAs2O6 and NiAs2O6, and provide a semi-quantitative
explanation of the large amplitude difference between their primary exchange
interaction parameters
Twentyâyear study of inâhospital and postdischarge mortality following emergency general surgical admission
We are grateful to Lizzie Nicholson, and the team at the Information Services Division, Scotland for their support in providing us with these data and the Data Safehaven Department of the University of Aberdeen for its storage. The authors would also like to thanks Dr Neil Scott and Dr Rute Vieira of the Department of Medical Statistics, University of Aberdeen for their advice in conducting this research.Peer reviewedPublisher PD
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