117207881
3D Simulations for a Micron‐Scale, Dielectric‐Based Acceleration Experiment
An experimental program to demonstrate a dielectric, slab‐symmetric accelerator structure has been underway for the past two years. These resonant devices are driven by a side‐coupled 800‐nm laser and can be configured to maintain the field profile necessary for synchronous acceleration and focusing of relativistic or nonrelativistic particles. We present 3D simulations of various versions of the structure geometry, including a metal‐walled structure relevant to ongoing cold tests on resonant properties, and an all‐dielectric structure to be constructed for a proof‐of‐principle acceleration experiment.
18073215
DEVELOPING PETA-SCALABLE ALGORITHMS FOR BEAM DYNAMIC SIMULATIONS
Peta-scalable software packages for beam dynamic simulations are being developed and used at Argonne national laboratory. The standard Particle-In-Cell (PIC) method and direct Vlasov solvers in up to 4 dimensions have been developed and benchmarked with respect to each other. Both of them have successfully run on 32 thousands processors on BG/P at Argonne Leadership Computing Facility. Peta-scale computing facility at ANL provides suitable environment for simulating beams in large scale. We have simulated charged beams through traditional PIC method as well as direct solving of the Vlasov equation in higher dimensions. Several scalable Poisson solvers have been developed and incorporated with these methods. High-order numerical methods have been adopted for solving the Poisson and Vlasov equations. Preliminary results on direct Vlasov solvers have been obtained in up to 4 dimensions. Domain decomposition method has been used for the parallelization in these software packages, and good scaling has been achieved. These packages have been successfully applied in end-to-end simulation of linear accelerators and large scale accelerator design optimizations. There are still lots of places can be improved for the PIC method and lots of challenges exist in the direct Vlasov solvers, therefore, more efforts are needed in both algorithms and applications. INTRODUCTION Plasma and charged beams are of great importance in modern science and technology. Their researches become more and more rely on the simulations. Simulating plasma and charged beams has three different methods: microscopic model, kinetic model and fluid model. In the microscopic model, each charged particle is described by 6 variables (x, y, z, z y x v v v , , ). Therefore, for N particles, there are 6N variables in total. This requires solving the Vlasov equation in 6N dimensions, which exceeds the capability of current supercomputers for large N. On the other end is the fluid model which is the simplest model, because it treats the plasma as a conducting fluid with electromagnetic forces exerted on it. This leads to solving the Magneto-hydrodynamics (MHD) equations in 3D (x, y and z). MHD solves for the average quantities, such as density and charge, which makes it difficult to describe the fine structure in the plasma. Between these two models is the kinetic model, which solves for the charge density function by solving the Boltzmann or Vlasov equations in 6 dimensions (x, y, z, z y x v v v , , ). The Vlasov equation describes the evolution of a system of particles under the effects of self-consistent electromagnetic fields. The model been used in current beam dynamics simulations is the kinetic model. There are two different ways to solve it. The dominant one is the so called Particle-In-Cell (PIC) method, which utilizes the motion of the particles along the characteristics of the Vlasov equation using a Lagrange-Euler approach [1, 2]. The PIC method has the advantages of fast speed and easy implementation, but similar to MHD, it is hard to capture the fine structures in the plasma. Furthermore, there is noise associated with the finite number of particles in the simulations. With petascale computing, one-to-one simulation can be realized for 9 10 particles. But for more intense beams, PIC method still uses macro particles. The other way to solve the kinetic model is to solve the Vlasov equation directly. But the challenge is the high dimensions. For example, in order to simulate beam in 3D, Vlasov equation has to be solved in 6D. This clearly needs peta-scale computing, and the peta-scalable algorithms are critical to the success. During the past 5 years, we have developed several software packages to meet these demands. This paper presents our efforts on developing peta-scalable algorithms to simulate charged beams in linear accelerators. PARALLEL POISSON SOLVERS In both approaches, Poisson’s equation has to be solved in 2D or 3D to account for the space charge effect. In designing peta-scalable algorithms, it is usually the most challenging part. Therefore, we present our work on this first. Several different methods have been adopted and are given in following: Fourier Method This is the most standard method for solving the Poisson’s equation in box region with Cartesian coordinate system. The potential has been expanded in Fourier series in all three directions. Periodic and Dirichlet zero boundary conditions have been applied in all three directions. ___________________________________________ * This work was supported by the U.S. Department of Energy, Office of Nuclear Physics, under Contract No. DE-AC02-06CH11357. jin_xu@anl.gov ∑ ∑ ∑ −
125427818
Anisotropic flows and dihadron correlations in heavy ion collisions
Using a multiphase transport model that includes both initial partonic and final hadronic scatterings, we have studied anisotropic flows and dihadron correla tions as functions of pseudorapidity and azimuthal angular differences between trigger and associated particles in heavy ion collisions at LHC. With parameters in the model determined from fitting t he measured multiplicity density of mid-pseudorapidity charged particles in central collis ions and their elliptic flow in mid-central collisions, the calculated higher-order anisotropic flows from the two-particle cumulant method reproduce approximately those measured in experiments. Similar to experimental results, the two-dimensional dihadron correlations at most central collisions show a ridge structure at the near side and a broad structure at the away side. The short- and long-range dihadron azimuthal correlations, corresponding to small and large pseudorapidity differences, respectively, are studied for triggering particles with different transverse mom enta and are found to be qualitatively consistent with experimental results.
118666530
Triangular flow in heavy ion collisions in a multiphase transport model
We obtain a new set of parameters in a multiphase transport (AMPT) model that are able to describe both the charged particle multiplicity density and the elliptic flow measured in Au+Au collisions at center-of-mass energy {radical}(s{sub NN})=200 GeV at the Relativistic Heavy Ion Collider, although they still give somewhat softer transverse momentum spectra. We then use the model to predict the triangular flow due to fluctuations in the initial collision geometry and study its effect relative to those from other harmonic components of anisotropic flows on the dihadron azimuthal correlations in both central and midcentral collisions.
118494468
Transition Density and Pressure at the Inner Edge of Neutron Star Crusts
Using the nuclear symmetry energy that has been recently constrained by the isospin diffusion data in intermediate-energy heavy ion collisions, we have studied the transition density and pressure at the inner edge of neutron star crusts, and they are found to be 0.040 fm$^{-3}$ $\leq \rho_{t}\leq 0.065$ fm$^{-3}$ and 0.01 MeV/fm$^{3}$ $\leq P_{t}\leq 0.26$ MeV/fm$^{3}$, respectively, in both the dynamical and thermodynamical approaches. We have also found that the widely used parabolic approximation to the equation of state of asymmetric nuclear matter gives significantly higher values of core-crust transition density and pressure, especially for stiff symmetry energies. With these newly determined transition density and pressure, we have obtained an improved relation between the mass and radius of neutron stars.
56309123
Effects of triangular flow on di-hadron azimuthal correlations in relativistic heavy ion collisions
Using the AMPT model for relativistic heavy ion collisions, we have studied the di-hadron azimuthal angular correlations triggered by emitted jets in $\text{Au}+\text{Au}$ collisions at center-of-mass energy $\sqrt{{s}_{\mathit{NN}}}=200$ GeV and impact parameter $b=8$ fm. A double-peak structure for the associated particles at the away side of trigger particles is obtained after subtracting background correlations due to the elliptic flow. Both the near-side peak and the away-side double peaks in the azimuthal angular correlations are, however, significantly suppressed (enhanced) in events of small (large) triangular flow or triangularity in the initial collision geometry as a result of fluctuations. After subtraction of background correlations due to the triangular flow, the away-side double peaks change into a single peak with broad shoulders on both sides. The away side of the di-hadron correlations becomes essentially a single peak after further subtraction of higher-order flows.
55652297
Effects of hadronic potentials on elliptic flows in relativistic heavy ion collisions
Within the framework of a multiphase transport (AMPT) model that includes both initial partonic and final hadronic interactions, we show that including mean-field potentials in the hadronic phase leads to a splitting of the elliptic flows of particles and their antiparticles, providing thus a plausible explanation of the different elliptic flows between $p$ and ${\bar p}$, $K^+$ and $K^-$, and $\pi^+$ and $\pi^-$ observed in recent Beam Energy Scan (BES) program at the Relativistic Heavy-Ion Collider (RHIC).