Ji Qiang

Parallel index and query for large scale data analysis

Modern scientific datasets present numerous data management and analysis challenges. State-of-the-art index and query technologies are critical for facilitating interactive exploration of large datasets, but numerous challenges remain in terms of designing a system for processing general scientific datasets. The system needs to be able to run on distributed multi-core platforms, efficiently utilize underlying I/O infrastructure, and scale to massive datasets. We present FastQuery, a novel software framework that address these challenges. FastQuery utilizes a state-of-the- art index and query technology (FastBit) and is designed to process massive datasets on modern supercomputing plat- forms. We apply FastQuery to processing of a massive 50TB dataset generated by a large scale accelerator modeling code. We demonstrate the scalability of the tool to 11,520 cores. Motivated by the scientific need to search for interesting particles in this dataset, we use our framework to reduce search time from hours to tens of seconds.

Advanced Visualization Technology for Terascale Particle Accelerator Simulations

This paper presents two new hardware-assisted rendering techniques developed for interactive visualization of the terascale data generated from numerical modeling of next-generation accelerator designs. The first technique, based on a hybrid rendering approach, makes possible interactive exploration of large-scale particle data from particle beam dynamics modeling. The second technique, based on a compact texture-enhanced representation, exploits the advanced features of commodity graphics cards to achieve perceptually effective visualization of the very dense and complex electromagnetic fields produced from the modeling of reflection and transmission properties of open structures in an accelerator design. Because of the collaborative nature of the overall accelerator modeling project, the visualization technology developed is for both desktop and remote visualization settings. We have tested the techniques using both time-varying particle data sets containing up to one billion particles per time step and electromagnetic field data sets with millions of mesh elements.

Parallel 3D Poisson solver for a charged beam in a conducting pipe

In this paper, we present a parallel three-dimensional Poisson solver for the electrostatic potential of a charged beam in a round or rectangular conducting pipe with open-end boundary conditions. This solver uses an eigenfunction expansion in the transverse direction and a finite difference method in the longitudinal direction. The computational domain in the longitudinal direction contains only the beam since only the potential inside the beam will be calculated. The potential on both ends of the beam is matched into the source-free region for each eigenmode. This method avoids the use of a large computational domain outside the beam to implement the open boundary condition. This saves unnecessary computational time and memory storage that would be required if a large computational domain was used to simulate the open boundary. Parallel implementation using a two-dimensional domain decomposition approach and a message passing paradigm shows good scalability on both distributed memory machines and distributed shared-memory machines. This solver has important applications in accelerator physics studies that involve modeling high-intensity beam dynamics. As a specific example, we present results from large-scale simulations of beam halo formation in a linear accelerator.

Experimental study of proton-beam halo induced by beam mismatch in LEDA

We report measurements of transverse beam halo in mismatched proton beams in a 52-quadrupole FODO transport channel following the 6.7-MeV LEDA RFQ. Beam profiles in both transverse planes are measured using beam-profile diagnostic devices that consist of a movable carbon filament for measurement of the dense beam core, and scraper plates for the halo measurement. The gradients of the first four quadrupoles can be independently adjusted to mismatch the RFQ output beam into the beam-transport channel.

Design Studies for a VUV–Soft X-ray Free-Electron Laser Array

Several recent reports have identified the scientific requirements for a future soft X-ray light source [1, 2, 3, 4, 5], and a high-repetition-rate free-electron laser (FEL) facility responsive to them is being studied at Lawrence Berkeley National Laboratory (LBNL) [6]. The facility is based on a continuous-wave (CW) superconducting linear accelerator with beam supplied by a high-brightness, high-repetition-rate photocathode electron gun operating in CW mode, and on an array of FELs to which the accelerated beam is distributed, each operating at high repetition rate and with even pulse spacing. Dependent on the experimental requirements, the individual FELs may be configured for either self-amplified spontaneous emission (SASE), seeded high-gain harmonic generation (HGHG), echo-enabled harmonic generation (EEHG), or oscillator mode of operation, and will produce high peak and average brightness X-rays with a flexible pulse format ranging from sub-femtoseconds to hundreds of femtoseconds. This new light source would serve a broad community of scientists in many areas of research, similar to existing utilization of storage ring based light sources.

Advanced Beam-Dynamics Simulation Tools for RIA

We are developing multi-particle beam-dynamics simulation codes for RIA driver-linac simulations extending from the low-energy beam transport (LEBT) line to the end of the linac. These codes run on the NERSC parallel supercomputing platforms at LBNL, which allow us to run simulations with large numbers of macroparticles. The codes have the physics capabilities needed for RIA, including transport and acceleration of multiple-charge-state beams, beam-line elements such as high-voltage platforms within the linac, interdigital accelerating structures, charge-stripper foils, and capabilities for handling the effects of machine errors and other off-normal conditions. This year will mark the end of our project. In this paper we present the status of the work, describe some recent additions to the codes, and show some preliminary simulation results.

Self-Consistent Langevin Simulation of Coulomb Collisions in Charged-Particle Beams

In many plasma physics and changed-particle beam dynamics problems, Coulomb collisions are modeled by a Fokker-Planck equation. In order to incorporate these collisions, we present a three-dimensional parallel Langevin simulation method using a Particle-In-Cell (PIC) approach implemented on high-performance parallel computers. We perform, for the first time, a fully self-consistent simulation, in which the friction and diffusion coefficients are computed from first principles. We employ a two-dimensional domain decomposition approach within a message passing programming paradigm along with dynamic load balancing. Object oriented programming is used to encapsulate details of the communication syntax as well as to enhance reusability and extensibility. Performance tests on the SGI Origin 2000, IBM SP RS/6000 and the Cray T3E-900 have demonstrated good scalability. As a test example, we demonstrate the collisional relaxation to a final thermal equilibrium of a beam with an initially anisotropic velocity distribution.

Visualizing high-resolution accelerator physics

Particle accelerators play an increasingly important role in basic and applied science. Several countries are involved in efforts aimed at developing accelerator-related technologies to support different application domains, including high-energy and nuclear physics, material science, biological science, and military use. The technological challenges associated with designing the next generation of accelerators will require numerical modeling capabilities far beyond those normally used within the accelerator community. In 1997 the US Department of Energy initiated a Grand Challenge in Computational Accelerator Physics, primarily to develop a new generation of high-performance accelerator modeling tools and apply them to projects of national importance. These tools will have a major impact on reducing the cost and technical risk of future projects, as well as maximizing the performance of present and future accelerators. In addition, they will enable the simulation of problems three to four orders of magnitude larger than ever done before. The use of algorithms and software optimized for high-performance computing will make it possible to obtain results quickly and with very high accuracy. This work is being done in collaboration between Los Alamos National Laboratory (LANL), Stanford Linear Accelerator Center, the National Energy Research Scientific Computing Center, Stanford University, and the University of California at Los Angeles. This article focuses on the accelerator simulation model and the current techniques used to visualize the project results.

Design Studies for a High-Repetition-Rate FEL Facility at LBNL

Lawrence Berkeley National Laboratory (LBNL) is working to address the needs of the primary scientific Grand Challenges now being considered by the U.S. Department of Energy, Office of Basic Energy Sciences: we are exploring scientific discovery opportunities, and new areas of science, to be unlocked with the use of advanced photon sources. A partnership of several divisions at LBNL is working to define the science and instruments needed in the future. To meet these needs, we propose a seeded, high-repetition-rate, free-electron laser (FEL) facility. Temporally and spatially coherent photon pulses, of controlled duration ranging from picosecond to sub-femtosecond, are within reach in the vacuum ultraviolet (VUV) to soft X-ray regime, and LBNL is developing critical accelerator physics and technologies toward this goal. We envision a facility with an array of FELs, each independently configurable and tunable, providing a range of photon-beam properties with high average and peak flux and brightness.

Space-charge driven emittance growth in a 3D mismatched anisotropic beam

In this paper we present a 3D simulation study of the emittance growth in a mismatched anisotropic beam. The equipartitioning driven by a 4th order space-charge resonance can be significantly modified by the presence of mismatch oscillation and halo formation. This causes emittance growth in both the longitudinal and transverse directions which could drive the beam even further away from equipartition. The averaged emittance growth per degree of freedom follows the upper bound of the 2D free energy limit plus the contributions from equipartitioning.