Kwok Ko

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.

Omega3P: A Parallel Finite-Element Eigenmode Analysis Code for Accelerator Cavities

Omega3P is a parallel eigenmode calculation code for accelerator cavities in frequency domain analysis using finite-element methods. In this report, we will present detailed finite-element formulations and resulting eigenvalue problems for lossless cavities, cavities with lossy materials, cavities with imperfectly conducting surfaces, and cavities with waveguide coupling. We will discuss the parallel algorithms for solving those eigenvalue problems and demonstrate modeling of accelerator cavities through different examples.

Adjoint methods for electromagnetic shape optimization of the low-loss cavity for the International Linear Collider

We formulate the problem of designing the low-loss cavity for the International Linear Collider (ILC) as an electromagnetic shape optimization problem involving a Maxwell eigenvalue problem. The objective is to maximize the stored energy of a trapped mode in the end cell while maintaining a specified frequency corresponding to the accelerating mode. A continuous adjoint method is presented for computation of the design gradient of the objective and constraint. The gradients are used within a nonlinear optimization scheme to compute the optimal shape for a simplified model of the ILC in a small multiple of the cost of solving the Maxwell eigenvalue problem.

Shape determination for deformed electromagnetic cavities

The measured physical parameters of a superconducting cavity differ from those of the designed ideal cavity. This is due to shape deviations caused by both loose machine tolerances during fabrication and by the tuning process for the accelerating mode. We present a shape determination algorithm to solve for the unknown deviations from the ideal cavity using experimentally measured cavity data. The objective is to match the results of the deformed cavity model to experimental data through least-squares minimization. The inversion variables are unknown shape deformation parameters that describe perturbations of the ideal cavity. The constraint is the Maxwell eigenvalue problem. We solve the nonlinear optimization problem using a line-search based reduced space Gauss-Newton method where we compute shape sensitivities with a discrete adjoint approach. We present two shape determination examples, one from synthetic and the other from experimental data. The results demonstrate that the proposed algorithm is very effective in determining the deformed cavity shape.

Solving large sparse linear systems in end-to-end accelerator structure simulations

Summary form only given. We present a case study of solving very large sparse linear systems in end-to-end accelerator structure simulations. Both direct solvers and iterative solvers are investigated. A parallel multilevel preconditioner based on hierarchical finite element basis functions is considered and has been implemented to accelerate the convergence of iterative solvers. A linear system with matrix size 93,147,736 and with 3,964,961,944 nonzeros from 3D electromagnetic finite element discretization has been solved in less than 8 minutes with 1024 CPUs on the NERSC IBM SP. The resource utilization as well as the application performance for these solvers is discussed.

Solving Large Scale Nonlinear Eigenvalue Problem in Next-Generation Accelerator Design

A number of numerical methods, including inverse iteration, method of successive linear problem and nonlinear Arnoldi algorithm, are studied in this paper to solve a large scale nonlinear eigenvalue problem arising from finite element analysis of resonant frequencies and external Q{sub e} values of a waveguide loaded cavity in the next-generation accelerator design. They present a nonlinear Rayleigh-Ritz iterative projection algorithm, NRRIT in short and demonstrate that it is the most promising approach for a model scale cavity design. The NRRIT algorithm is an extension of the nonlinear Arnoldi algorithm due to Voss. Computational challenges of solving such a nonlinear eigenvalue problem for a full scale cavity design are outlined.

X-BAND LINEAR COLLIDER R&D IN ACCELERATING STRUCTURES THROUGH ADVANCED COMPUTING ∗

This paper describes a major computational effort that addresses key design issues in the high gradient accelerating structures for the proposed X-band linear collider, GLC/NLC. Supported by the US DOE’s Accelerator Simulation Project, SLAC is developing a suite of parallel electromagnetic codes based on unstructured grids for modeling RF structures with higher accuracy and on a scale previously not possible. The new simulation tools have played an important role in the R&D of X-Band accelerating structures, in cell design, wakefield analysis and dark current studies.

State of the art in electromagnetic modeling for the Compact Linear Collider

SLACs Advanced Computations Department (ACD) has developed the parallel 3D electromagnetic time-domain code T3P for simulations of wakefields and transients in complex accelerator structures. T3P is based on state-of-the-art Finite Element methods on unstructured grids and features unconditional stability, quadratic surface approximation and up to 6 th -order vector basis functions for unprecedented simulation accuracy. Optimized for large-scale parallel processing on leadership supercomputing facilities, T3P allows simulations of realistic 3D structures with fast turn-around times, aiding the design of the next generation of accelerator facilities. Applications include simulations of the proposed two-beam accelerator structures for the Compact Linear Collider (CLIC) - wakefield damping in the Power Extraction and Transfer Structure (PETS) and power transfer to the main beam accelerating structures are investigated.

Three-gap klystron output cavity at X-band

A high-power X-band klystron employing a double-gap output cavity has been operating at SLAC. Multi-gap output circuits have lower surface gradients at the interaction gaps than single-gap ones but are prone to self-oscillate due to negative beam loading and trapped higher-order modes. In the double-gap circuit design, considerable attention had been directed to deal with these stability problems. The performance of the present tube appears to be limited by gap breakdown and beam interception particularly at long pulses. A three-gap output cavity is currently under development to further reduce the gap surface gradient. Another new feature of the circuit is an enlarged downstream drift tube to improve on beam clearance. This paper discusses the considerations involved in designing a multi-gap output cavity and presents the cold test measurements on the three-gap circuit. The experimental data is compared with numerical results from the 3-D simulation code ARGUS.

Enabling technologies for petascale electromagnetic accelerator simulation

The SciDAC2 accelerator project at SLAC aims to simulate an entire three-cryomodule radio frequency (RF) unit of the International Linear Collider (ILC) main Linac. Petascale computing resources supported by advances in Applied Mathematics (AM) and Computer Science (CS) and INCITE Program are essential to enable such very large-scale electromagnetic accelerator simulations required by the ILC Global Design Effort. This poster presents the recent advances and achievements in the areas of CS/AM through collaborations.