273 research outputs found
Parallel ion strings in linear multipole traps
Additional radio-frequency (rf) potentials applied to linear multipole traps
create extra field nodes in the radial plane which allow one to confine single
ions, or strings of ions, in totally rf field-free regions. The number of nodes
depends on the order of the applied multipole potentials and their relative
distance can be easily tuned by the amplitude variation of the applied
voltages. Simulations using molecular dynamics show that strings of ions can be
laser cooled down to the Doppler limit in all directions of space. Once cooled,
organized systems can be moved with very limited heating, even if the cooling
process is turned off
Ion dynamics in a linear radio-frequency trap with a single cooling laser
We analyse the possibility of cooling ions with a single laser beam, due to
the coupling between the three components of their motion induced by the
Coulomb interaction. For this purpose, we numerically study the dynamics of ion
clouds of up to 140 particles, trapped in a linear quadrupole potential and
cooled with a laser beam propagating in the radial plane. We use Molecular
Dynamics simulations and model the laser cooling by a stochastic process. For
each component of the motion, we systematically study the dependence of the
temperature with the anisotropy of the trapping potential. Results obtained
using the full radio-frequency (rf) potential are compared to those of the
corresponding pseudo-potential. In the rf case, the rotation symmetry of the
potential has to be broken to keep ions inside the trap. Then, as for the
pseudo-potential case, we show that the efficiency of the Coulomb coupling to
thermalize the components of motion depends on the geometrical configuration of
the cloud. Coulomb coupling appears to be not efficient when the ions organise
as a line or a pancake and the three components of motion reach the same
temperature only if the cloud extends in three dimensions
Slow and fast micro-field components in warm and dense hydrogen plasmas
The aim of this work is the investigation of the statistical properties of
local electric fields in an ion-electron two component plasmas for coupled
conditions. The stochastic fields at a charged or at a neutral point in plasmas
involve both slow and fast fluctuation characteristics. The statistical study
of these local fields based on a direct time average is done for the first
time. For warm and dense plasma conditions, typically , , well controlled molecular dynamics (MD)
simulations of neutral hydrogen, protons and electrons have been carried out.
Relying on these \textit{ab initio} MD calculations this work focuses on an
analysis of the concepts of statistically independent slow and fast local field
components, based on the consideration of a time averaged electric field. Large
differences are found between the results of these MD simulations and
corresponding standard results based on static screened fields. The effects
discussed are of importance for physical phenomena connected with stochastic
electric field fluctuations, e.g., for spectral line broadening in dense
plasmas.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
Learning to stop: a unifying principle for legged locomotion in varying environments.
Evolutionary studies have unequivocally proven the transition of living organisms from water to land. Consequently, it can be deduced that locomotion strategies must have evolved from one environment to the other. However, the mechanism by which this transition happened and its implications on bio-mechanical studies and robotics research have not been explored in detail. This paper presents a unifying control strategy for locomotion in varying environments based on the principle of 'learning to stop'. Using a common reinforcement learning framework, deep deterministic policy gradient, we show that our proposed learning strategy facilitates a fast and safe methodology for transferring learned controllers from the facile water environment to the harsh land environment. Our results not only propose a plausible mechanism for safe and quick transition of locomotion strategies from a water to land environment but also provide a novel alternative for safer and faster training of robots
Nonlinear response of electrons to a positive ion
Electric field dynamics at a positive ion imbedded in an electron gas is
considered using a semiclassical description. The dependence of the field
autocorrelation function on charge number is studied for strong ion-electron
coupling via MD simulation. The qualitative features for larger charge numbers
are a decreasing correlation time followed by an increasing anticorrelation.
Stopping power and related transport coefficients determined by the time
integral of this correlation function result from the competing effects of
increasing initial correlations and decreasing dynamical correlations. An
interpretation of the MD results is obtained from an effective single particle
model showing good agreement with the simulation results.Comment: To be published in the proceedings of the International Workshop on
Strongly Coupled Coulomb Systems, Journal of Physics
An all-endo Approach to Complete Ureteral Duplications Complicated by Ureterocele and/or Vesicoureteral Reflux: Feasibility, Limitations, and Results
Purpose. Totally endoscopic management (all-endo) of patients with a duplicated renal system (DS) associated with severe vesicoureteral reflux (VUR) or obstructive ureterocele (UC) is an attractive alternative to traditional open procedures. The authors discuss feasibility and results of an all-endo approach on a consecutive series of patients. Methods. From 1999 to 2009, all patients with a complete DS associated with UC and/or VUR were proposed for primary all-endo approach. UC puncture was performed using a 3 Fr Bugbee electrode. Deflux (dextranomer/hyaluronic acid copolymer) injection was administered for VUR. The need for secondary surgery was evaluated on followup. Results. Of the 62 patients recruited, 46 were treated using a primary all-endo approach and 16 patients received no treatment. Of the 46 treated patients with 56 affected renal units, 32 (97%) UCs collapsed following puncture and 29 (63%) VURs were resolved or downgraded. Secondary VUR occurred in 13 (39%) renal units. Secondary surgery was performed on 23 (41%) renal units. Conclusion. The all-endo approach for VUR in DS is an effective therapeutic option. UC collapse was achieved by puncture in most of the patients; secondary VUR was the main complication in a small group of extravesical UC
Structural phase transitions in multipole traps
A small number of laser-cooled ions trapped in a linear radiofrequency
multipole trap forms a hollow tube structure. We have studied, by means of
molecular dynamics simulations, the structural transition from a double ring to
a single ring of ions. We show that the single-ring configuration has the
advantage to inhibit the thermal transfer from the rf-excited radial components
of the motion to the axial component, allowing to reach the Doppler limit
temperature along the direction of the trap axis. Once cooled in this
particular configuration, the ions experience an angular dependency of the
confinement if the local adiabaticity parameter exceeds the empirical limit.
Bunching of the ion structures can then be observed and an analytic expression
is proposed to take into account for this behaviour
FORGE: An eLearning Framework for Remote Laboratory Experimentation on FIRE Testbed Infrastructure
The Forging Online Education through FIRE (FORGE) initiative provides educators and learners in higher education with access to world-class FIRE testbed infrastructure. FORGE supports experimentally driven research in an eLearning environment by complementing traditional classroom and online courses with interactive remote laboratory experiments. The project has achieved its objectives by defining and implementing a framework called FORGEBox. This framework offers the methodology, environment, tools and resources to support the creation of HTML-based online educational material capable accessing virtualized and physical FIRE testbed infrastruc- ture easily. FORGEBox also captures valuable quantitative and qualitative learning analytic information using questionnaires and Learning Analytics that can help optimise and support student learning. To date, FORGE has produced courses covering a wide range of networking and communication domains. These are freely available from FORGEBox.eu and have resulted in over 24,000 experiments undertaken by more than 1,800 students across
10 countries worldwide. This work has shown that the use of remote high- performance testbed facilities for hands-on remote experimentation can have a valuable impact on the learning experience for both educators and learners. Additionally, certain challenges in developing FIRE-based courseware have been identified, which has led to a set of recommendations in order to support the use of FIRE facilities for teaching and learning purposes
Systematic model behavior of adsorption on flat surfaces
A low density film on a flat surface is described by an expansion involving
the first four virial coefficients. The first coefficient (alone) yields the
Henry's law regime, while the next three correct for the effects of
interactions. The results permit exploration of the idea of universal
adsorption behavior, which is compared with experimental data for a number of
systems
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