80 research outputs found
The Free-Free Opacity in Warm, Dense, and Weakly Ionized Helium
We investigate the ionization and the opacity of warm, dense helium under
conditions found in the atmospheres of cool white dwarf stars. Our particular
interest is in densities up to and temperatures from 1000K to
10000K. For these physical conditions various approaches for modeling the
ionization equilibrium predict ionization fractions that differ by orders of
magnitudes. Furthermore, estimates of the density at which helium
pressure-ionizes vary from to . In this context, the
value of the electron-atom inverse bremsstrahlung absorption is highly
uncertain. We present new results obtained from a non-ideal chemical model for
the ionization equilibrium, from Quantum Molecular Dynamics (QMD) simulations,
and from the analysis of experimental data to better understand the ionization
fraction in fluid helium in the weak ionization limit.Comment: 4 pages, 3 figures, 1 table. Accepted for publication in the
Proceedings of the 14th APS Topical Conference on Shock Compression of
Condensed Matter, Baltimore, M
Star clusters dynamics in a laboratory: electrons in an ultracold plasma
Electrons in a spherical ultracold quasineutral plasma at temperature in the
Kelvin range can be created by laser excitation of an ultra-cold laser cooled
atomic cloud. The dynamical behavior of the electrons is similar to the one
described by conventional models of stars clusters dynamics. The single mass
component, the spherical symmetry and no stars evolution are here accurate
assumptions. The analog of binary stars formations in the cluster case is
three-body recombination in Rydberg atoms in the plasma case with the same
Heggie's law: soft binaries get softer and hard binaries get harder. We
demonstrate that the evolution of such an ultracold plasma is dominated by
Fokker-Planck kinetics equations formally identical to the ones controlling the
evolution of a stars cluster. The Virial theorem leads to a link between the
plasma temperature and the ions and electrons numbers. The Fokker-Planck
equation is approximate using gaseous and fluid models. We found that the
electrons are in a Kramers-Michie-King's type quasi-equilibrium distribution as
stars in clusters. Knowing the electron distribution and using forced fast
electron extraction we are able to determine the plasma temperature knowing the
trapping potential depth.Comment: Submitted to MNRA
Parallel TREE code for two-component ultracold plasma analysis
The TREE method has been widely used for long-range interaction {\it N}-body
problems. We have developed a parallel TREE code for two-component classical
plasmas with open boundary conditions and highly non-uniform charge
distributions. The program efficiently handles millions of particles evolved
over long relaxation times requiring millions of time steps. Appropriate domain
decomposition and dynamic data management were employed, and large-scale
parallel processing was achieved using an intermediate level of granularity of
domain decomposition and ghost TREE communication. Even though the
computational load is not fully distributed in fine grains, high parallel
efficiency was achieved for ultracold plasma systems of charged particles. As
an application, we performed simulations of an ultracold neutral plasma with a
half million particles and a half million time steps. For the long temporal
trajectories of relaxation between heavy ions and light electrons, large
configurations of ultracold plasmas can now be investigated, which was not
possible in past studies
Calculation of a Deuterium Double Shock Hugoniot from Ab initio Simulations
We calculate the equation of state of dense deuterium with two ab initio
simulations techniques, path integral Monte Carlo and density functional theory
molecular dynamics, in the density range of 0.67 < rho < 1.60 g/cc. We derive
the double shock Hugoniot and compare with the recent laser-driven double shock
wave experiments by Mostovych et al. [1]. We find excellent agreement between
the two types of microscopic simulations but a significant discrepancy with the
laser-driven shock measurements.Comment: accept for publication in Phys. Rev. Lett., Nov. 2001, 4 pages, 4
figure
Evolution of Ultracold, Neutral Plasmas
We present the first large-scale simulations of an ultracold, neutral plasma,
produced by photoionization of laser-cooled xenon atoms, from creation to
initial expansion, using classical molecular dynamics methods with open
boundary conditions. We reproduce many of the experimental findings such as the
trapping efficiency of electrons with increased ion number, a minimum electron
temperature achieved on approach to the photoionization threshold, and
recombination into Rydberg states of anomalously-low principal quantum number.
In addition, many of these effects establish themselves very early in the
plasma evolution ( ns) before present experimental observations begin.Comment: 4 pages, 3 figures, submitted to PR
Neutral Plasma Oscillations at Zero Temperature
We use cold plasma theory to calculate the response of an ultracold neutral
plasma to an applied rf field. The free oscillation of the system has a
continuous spectrum and an associated damped quasimode. We show that this
quasimode dominates the driven response. We use this model to simulate plasma
oscillations in an expanding ultracold neutral plasma, providing insights into
the assumptions used to interpret experimental data [Phys. Rev. Lett. 85, 318
(2000)].Comment: 4.3 pages, including 3 figure
The melting curve of iron at extreme pressures: implications for planetary cores
Exoplanets with masses similar to that of Earth have recently been discovered
in extrasolar systems. A first order question for understanding their dynamics
is to know whether they possess Earth like liquid metallic cores. However, the
iron melting curve is unknown at conditions corresponding to planets of several
times the Earth's mass (over 1500 GPa for planets with 10 times the Earth's
mass (ME)). In the density-temperature region of the cores of those
super-Earths, we calculate the iron melting curve using first principle
molecular dynamics simulations based on density functional theory. By comparing
this melting curve with the calculated thermal structure of Super Earths, we
show that planets heavier than 2ME, have solid cores, thus precluding the
existence of an internal metallic-core driven magnetic field. The iron melting
curve obtained in this study exhibits a steeper slope than any calculated
planetary adiabatic temperature profile rendering the presence of molten
metallic cores less likely as sizes of terrestrial planets increase
On the Specific Features of Temperature Evolution in Ultracold Plasmas
A theoretical interpretation of the recent experimental studies of
temperature evolution in the course of time in the freely-expanding ultracold
plasma bunches, released from a magneto-optical trap, is discussed. The most
interesting result is finding the asymptotics of the form T_e ~ t^{-(1.2 +/-
0.1)} instead of t^{-2}, which was expected for the rarefied monatomic gas
during inertial expansion. As follows from our consideration, the substantially
decelerated decay of the temperature can be well explained by the specific
features of the equation of state for the ultracold plasmas with strong
Coulomb's coupling, whereas a heat release due to inelastic processes (in
particular, three-body recombination) does not play an appreciable role in the
first approximation. This conclusion is confirmed both by approximate
analytical estimates, based on the model of "virialization" of the
charged-particle energies, and by the results of "ab initio" numerical
simulation. Moreover, the simulation shows that the above-mentioned law of
temperature evolution is approached very quickly--when the virial criterion is
satisfied only within a factor on the order of unity.Comment: LaTeX + 3 eps figures, 16 pages. Plasma Physics Reports, v.37, in
press (2011
CariesCare practice guide : consensus on evidence into practice
This CariesCare practice guide is derived from the International Caries Classification and Management System (ICCMS) and provides a structured update for dentists to help them deliver optimal caries care and outcomes for their patients. This '4D cycle' is a practice-building format, which both prevents and controls caries and can engage patients as long-term health partners with their practice. CariesCare International (CCI™) promotes a patient-centred, risk-based approach to caries management designed for dental practice. This comprises a health outcomes-focused system that aims to maintain oral health and preserve tooth structure in the long-term. It guides the dental team through a four-step process (4D system), leading to personalised interventions: 1st D: Determine Caries Risk; 2nd D: Detect lesions, stage their severity and assess their activity status; 3rd D: Decide on the most appropriate care plan for the specific patient at that time; and then, finally, 4th D: Do the preventive and tooth-preserving care which is needed (including risk-appropriate preventive care; control of initial non-cavitated lesions; and conservative restorative treatment of deep dentinal and cavitated caries lesions). CariesCare International has designed this practice-friendly consensus guide to summarise best practice as informed by the best available evidence. Following the guide should also increase patient satisfaction, involvement, wellbeing and value, by being less invasive and more health-focused. For the dentist it should also provide benefits at the professional and practice levels including improved medico-legal protection
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