Particle Accelerator Design
PI: Panagiotis Spentzouris
ITAPS Personnel: Mark Shephard, Roman Samulyak, Jim Glimm, Tim Tautges
Particle accelerators are a significant part of the DOE science goals, accounting for seven of the top twenty facilities priorities over the next two decades. Simulation will play an increasingly important role in the design of these accelerators because of its impact on the performance improvements and operating cost reductions required to make new facilities successful. Accelerator performance is sensitive to geometric shape because of the high-frequency operating regime of beam cavities in large-scale accelerators and placement of waveguide elements in reduced-scale accelerators. This sensitivity drives the need for sophisticated geometric modeling and body-fitted mesh generation services from ITAPS.
ITAPS geometry and mesh services are already playing a crucial role in design and optimization of accelerator cavities used in the International Linear Collider (ILC), the Continuous Electron Beam Accelerator Facility (CEBAF) upgrade, Spallation Neutron Source (SNS), and other near- and mid-term priority accelerator facilities. We will continue to work with accelerator scientists to provide shape optimization services that allow exploration of the design space in a more automated way. Moreover, there are a number of additional areas where ITAPS can continue to improve the state-of-the-art in accelerator modeling. For example, important accelerator design codes such as VORPAL (Tech-X) require new open source geometry tools that provide fast geometric queries for embedded boundary mesh generation and to track the motion of particles through accelerators with geometrically complex boundaries. In addition, more efficient and faithful solutions of the underlying physics can be achieved by incorporating parallel algorithms for the construction of adaptive grids to concentrate degrees of freedom where they have the most impact. Improved partitioning and load balancing services will be critical to obtaining maximal parallel performance of these adaptive methods. Future accelerator designs are currently analyzed using manual design evaluations, which are impractical or impossible to perform efficiently. for future accelerator designs, maximizing the performance using simulation where manual design evaluations would be impractical or impossible.
Mesh Generation and Problem Set Up
Our initial efforts with SLAC focused on the widely recognized issue of reducing the time needed to create a mesh starting with a CAD (computer-aided design) model giving the physical geometry for the simulation. There are two main bottlenecks involved in this process. One is the clean-up of the initial geometry such that it can be used for mesh generation. The second bottleneck (for SLAC) concerns the generation of high quality meshes as it relates to accuracy and convergence of the simulation code. Currently, many meshes may be generated before a successful simulation is obtained.
The geometry clean up process consists of removing unwanted detail, healing gaps between surfaces and volumes, and removing non-physical overlaps. This process can be quite tedious and time-consuming, often delaying simulations for months. ITAPS members have been assisting SLAC in the use of these tools. For example, toward the end of FY02, SLAC presented a very complex tapered waveguide geometry that needed to be cleaned up and meshed as quickly as possible (see Figure 1). The challenge in this geometry is that the position of the beam axis relative to the centroid of the geometric cross-section varies along the waveguide which makes it difficult to achieve sufficient mesh quality. Tim Tautges at ANL/ITAPS has been working with the SLAC analysts to clean up this geometry and mesh it using CUBIT.
Figure 1 An all-hexahedral mesh generated for the SLAC waveguide geometry
Omega3p Adaptive Loop Creation
For this work we utilize functionality developed in the ITAPS adaptive loops research to develop an adaptive loop for Omega3p simulations. In order for Omega3p to be a useful design tool, extremely accurate solutions needed to be computed with less than 0.01% error. This had to be done in complex CAD models such as shown in Figure 2 where tools such as automatic mesh generation and general mesh adaptation which maintains a proper approximation of the underlying geometry are required.
Figure 2: SLAC CAD Model
Results for the adaptive loop included obtaining accurate simulation results at a much lower cost. For the original test problem, one third the number of unknowns were required on the adaptively refine mesh as compared to previous calculations. In addition, the process for obtaining accurate solutions became more reliable. Some results for the adaptive loop constructed to work with Omega3p are shown in Figure 3.
|Initial Mesh Simulation Results||Intermediate Mesh Simulation Results||Final Mesh Simulaton Results|