978 research outputs found
A portable platform for accelerated PIC codes and its application to GPUs using OpenACC
We present a portable platform, called PIC_ENGINE, for accelerating
Particle-In-Cell (PIC) codes on heterogeneous many-core architectures such as
Graphic Processing Units (GPUs). The aim of this development is efficient
simulations on future exascale systems by allowing different parallelization
strategies depending on the application problem and the specific architecture.
To this end, this platform contains the basic steps of the PIC algorithm and
has been designed as a test bed for different algorithmic options and data
structures. Among the architectures that this engine can explore, particular
attention is given here to systems equipped with GPUs. The study demonstrates
that our portable PIC implementation based on the OpenACC programming model can
achieve performance closely matching theoretical predictions. Using the Cray
XC30 system, Piz Daint, at the Swiss National Supercomputing Centre (CSCS), we
show that PIC_ENGINE running on an NVIDIA Kepler K20X GPU can outperform the
one on an Intel Sandybridge 8-core CPU by a factor of 3.4
Low energy physical properties of high-Tc superconducting Cu oxides: A comparison between the resonating valence bond and experiments
In a recent review by Anderson and coworkers\cite{Vanilla}, it was pointed
out that an early resonating valence bond (RVB) theory is able to explain a
number of unusual properties of high temperature superconducting (SC)
Cu-oxides. Here we extend previous calculations \cite{anderson87,FC
Zhang,Randeria} to study more systematically low energy physical properties of
the plain vanilla d-wave RVB state, and to compare results with the available
experiments. We use a renormalized mean field theory combined with variational
Monte Carlo and power Lanczos methods to study the RVB state of an extended
model in a square lattice with parameters suitable for the hole doped
Cu-oxides. The physical observable quantities we study include the specific
heat, the linear residual thermal conductivity, the in-plane magnetic
penetration depth, the quasiparticle energy at the antinode , the
superconducting energy gap, the quasiparticle spectra and the Drude weight. The
traits of nodes (including , the Fermi velocity and the velocity
along Fermi surface ), as well as the SC order parameter are also
studied. Comparisons of the theory and the experiments in cuprates show an
overall qualitative agreement, especially on their doping dependences.Comment: 12 pages, 14 figures, 1 tabl
Shunt dysfunction patterns after transjugular intrahepatic portosystemic shunt creation using a combination of a generic stent-graft and bare-stents.
Even though transjugular intrahepatic portosystemic shunt (TIPS) using Fluency Stent-grafts provides good shunt patency rates, shunt dysfunction is a great concern after TIPS creation, occurring in up to 20% of cases within one year. The objective of this study was to describe shunt dysfunction patterns after TIPS creation using a combination of generic stent-grafts/bare-stents.
Single-center retrospective study of all TIPS revisions between January 2005 and December 2020. TIPS revision angiograms were analyzed for stents' positions, stenoses' diameters, and stenoses' locations.
Out of 99 TIPS, a total of 33 TIPS revisions were included. The median time to TIPS revision was 10.4 months. Angiograms showed four patterns of TIPS dysfunction-associated features (DAF), defined as follows: Type 1 was defined as stenosis located after the stent end in the hepatic vein (HV), type 2 as intra-stent stenosis located in the hepatic vein, type 3 as intra-stent stenosis or a kink in the parenchymal tract or the portal vein end of the TIPS, and type 4 as a complete TIPS occlusion. Types 1, 2, 3, and 4 were seen in 23 (69.7%), 5 (15.2%), 2 (6.1%), and 3 (9.1%) TIPS respectively. TIPS revision was successful in 30 (90.1%) patients with median pre- and post-TIPS revision PSG of 18.5 mmHg and 8 mmHg respectively (p < .001).
Our results illustrate the four angiographic patterns of TIPS DAF after TIPS creation using a combination of generic stent-grafts/bare-stents and emphasize the need for appropriate stent length extending to the HV/inferior vena cava junction
Interaction of Alfvénic modes and turbulence via the nonlinear modification of the equilibrium profiles
Nonlinear gyrokinetic PIC simulations in stellarators with the code EUTERPE
In this work, the first nonlinear particle-in-cell simulations carried out in
a stellarator with the global gyrokinetic code EUTERPE using realistic plasma
parameters are reported. Several studies are conducted with the aim of enabling
reliable nonlinear simulations in stellarators with this code. First, EUTERPE
is benchmarked against ORB5 in both linear and nonlinear settings in a tokamak
configuration. Next, the use of noise control and stabilization tools, a
Krook-type collision operator, markers weight smoothing and heating sources is
investigated. It is studied in detail how these tools influence the linear
growth rate of instabilities in both tokamak and stellarator geometries and
their influence on the linear zonal flow evolution in a stellarator. Then, it
is studied how these tools allow improving the quality of the results in a set
of nonlinear simulations of electrostatic turbulence in a stellarator
configuration.
Finally, these tools are applied to a W7-X magnetic configuration using
experimental plasma parameters.Comment: 24 pages, 19 figure
The in-plane electrodynamics of the superconductivity in Bi2Sr2CaCu2O8+d: energy scales and spectral weight distribution
The in-plane infrared and visible (3 meV-3 eV) reflectivity of
Bi2Sr2CaCu2O8+d (Bi-2212) thin films is measured between 300 K and 10 K for
different doping levels with unprecedented accuracy. The optical conductivity
is derived through an accurate fitting procedure. We study the transfer of
spectral weight from finite energy into the superfluid as the system becomes
superconducting. In the over-doped regime, the superfluid develops at the
expense of states lying below 60 meV, a conventional energy of the order of a
few times the superconducting gap. In the underdoped regime, spectral weight is
removed from up to 2 eV, far beyond any conventional scale. The intraband
spectral weight change between the normal and superconducting state, if
analyzed in terms of a change of kinetic energy is ~1 meV. Compared to the
condensation energy, this figure addresses the issue of a kinetic energy driven
mechanism.Comment: 13 pages with 9 figures include
Virtual reality, ultrasound-guided liver biopsy simulator: Development and performance discrimination
Objectives: The aim of this article was to identify and prospectively investigate simulated ultrasound-guided targeted liver biopsy performance metrics as differentiators between levels of expertise in interventional radiology.
Methods: Task analysis produced detailed procedural step documentation allowing identification of critical procedure steps and performance metrics for use in a virtual reality ultrasound-guided targeted liver biopsy procedure. Consultant (n=14; male=11, female=3) and trainee (n=26; male=19, female=7) scores on the performance metrics
were compared. Ethical approval was granted by the Liverpool Research Ethics Committee (UK). Independent t-tests and analysis of variance (ANOVA) investigated differences between groups.
Results: Independent t-tests revealed significant differences between trainees and consultants on three performance metrics: targeting, p=0.018, t=22.487 (22.040 to
20.207); probe usage time, p=0.040, t=2.132 (11.064 to 427.983); mean needle length in beam, p=0.029, t=22.272 (20.028 to 20.002). ANOVA reported significant differences across years of experience (0–1, 1–2, 3+ years) on seven performance metrics: no-go area touched, p=0.012; targeting, p=0.025; length of session, p=0.024; probe usage time, p=0.025; total needle distance moved, p=0.038; number of skin contacts, p<0.001; total time in no-go area, p=0.008. More experienced participants consistently received better performance scores on all 19 performance metrics.
Conclusion: It is possible to measure and monitor performance using simulation, with performance metrics providing feedback on skill level and differentiating levels of expertise. However, a transfer of training study is required
Linearized model Fokker-Planck collision operators for gyrokinetic simulations. II. Numerical implementation and tests
A set of key properties for an ideal dissipation scheme in gyrokinetic
simulations is proposed, and implementation of a model collision operator
satisfying these properties is described. This operator is based on the exact
linearized test-particle collision operator, with approximations to the
field-particle terms that preserve conservation laws and an H-Theorem. It
includes energy diffusion, pitch-angle scattering, and finite Larmor radius
effects corresponding to classical (real-space) diffusion. The numerical
implementation in the continuum gyrokinetic code GS2 is fully implicit and
guarantees exact satisfaction of conservation properties. Numerical results are
presented showing that the correct physics is captured over the entire range of
collisionalities, from the collisionless to the strongly collisional regimes,
without recourse to artificial dissipation.Comment: 13 pages, 8 figures, submitted to Physics of Plasmas; typos fixe
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