Simulation Needs Overview

1. ANL Theory Institute on Production of Bright Electron Beams Argonne National Lab, Argonne, IL Sept. 22-26, 2003 Simulation Needs Overview John Petillo – SAIC, Burlington, MA Baruch Levush – NRL, Washington, DC Sept. 25, 2003

2. Outline • Generate discussion on codes & needs • Code needs • Needs by component • Inter-Code issues • Code availability • Selection of examples to motivate interest in needs/wants • Action items

3. Post Processing & Animation • Animation of physics • Provides the researcher with an ability to gain intuition into the dynamics of the complicated interactions between particles (primary and secondary), fields & the device structure Electron Gun Multistage Depressed Collector

4. Code Needs • What’s needed? • Space charge - under what circumstances? • Emission models – characterize , implement model & test • 2D/3D – when is what needed & with what model? • Time-domain • When can we get away with ES vs. EM? • Frequency-domain • Need for conformal surfaces – gridding • Usually means FE, but can be “FD” • FE usually means SLOW – but not necessarily • What models are needed for modeling efficiency? • Explicit PIC, implicit PIC, transport in smooth pipe, etc. • List these in detail

5. (1,2) (2,2) Structured: logical ordering Unstructured: no specific ordering 2 (0,2) (1,1) (0,1) (5) (2,1) (3) (4) 1 (1,0) (7) (10) (2) (0,0) 1 (2,0) 0 (9) 0 2 (6) (1) (0) (8) 3D Mesh Types - Illustration

6. Needs by component • Examples – what’s needed for… • Guns • Steady-state/thermionic vs. RF • Beamline components • Buncher – what’s needed here? • List like this needs to be prepared • Need input from everyone for this

7. Inter-Code data transfer • Different codes are needed/used in different regions • Incompatibilities may exist – some can be extreme • Need to transfer… • Between codes, often have different… • Data types • Computational type • Mesh type • Particle data, field data • Sometimes easy… But, may have tocharacterize from one code – process – then send to second code • Region overlap needed? • How to transfer & start ultra-relativistic beams in a PIC code with a soft start • What development is needed here? • What standards should be used, or defined?

8. What Codes Are People Using? • Government provided / home grown / “store bought” • These should be listed • Pros & cons listed • What/which codes can be made available to everyone? • Govt. can commission codes to be written and disseminated to community with training

9. Training • Proper training often overlooked • Introducing new codes to users… • Many codes are difficult to use – people avoid them • Difficult to use – or difficult to get the right answer? • For difficult codes, often need “Experts” to model first problem before transitioning code to a user • Modern codes are both easier and harder to use • Better meshes get better answers • Meshing is now the hard part • Are they faster? • Outside support from authors is important – must be available • USPAS tries to help here, but general-purpose codes like PIC codes not well represented in the curriculum

10. Availability… • Are the codes available, and to whom? • Who provides/pays-for codes that cost money? • Important Goal… • that everyone in the community has access to the same tools • that they are trained

11. Examples • Motivation for coming up with desired code capabilities

12. Parallel Plate CapacitorLimited Emission Region • D = 10 cm, R = 10 cm • R_beam = 1 cm • V = 10,000 V • Is Max right?????

13. Parallel Plate CapacitorLimited Emission Region • D = 10 cm, R = 10 cm • R_beam = 1 cm • V = 10,000 V

14. Parallel Plate CapacitorLimited Emission Region • Childs Law: I = 0.073 A • Code: I = 0.23 A – ratio of 3.18 • Max says ratio of sqrt(d/r) = sqrt(10) = 3.16!

15. Gridded Gun 3D Collector Multi-Beam Gun Example: Use of Conformal Meshes • Guns • Modulation Control grids can be analyzed • Electron surface emission models have improved • Multi-beam and Sheet beam devices can be analyzed • Multistage Depressed Collectors - limitations • Anisotropic Collectors – can be analyzed • Secondary emission models – improved models • Tolerance analysis – often 3D in nature • Most typical alignment and clocking errors can be analyzed • Fine structure representation • Multiple particle species – electrons/ions/charge-exchange • Multiple-Beam • No longer a 2D or a small 3D problem when many beams need to be modeled

16. Hybrid Multiblock Mesh Example interface • Building a Hybrid Multiblock Mesh • Build structured mesh with an interface to unstructured block. • Extract quads on interface. • Build unstructured mesh. • Merge. Create pyramid elements. + = => interface 3D Hybrid Mesh MICHELLE Result

17. Hexahedra Tetrahedra Models, Meshing & Algorithms • Finite Element Approach – linear, quadratic • Grid System Supported - conformal • Within Voyager GUI with ICEM-CFD mesher • Supports most high-end CAD modelers – we use SolidWorks • Structured Mesh • 3D Multi-block, Hexahedral • Unstructured Mesh • 2D - Triangle, Quadrilateral • 3D - Tetrahedral, Hexahedral, Prism, Pyramid • Hybrid Mesh • Single run Structured mesh & Unstructured mesh • Makes use of compact data storage of a structured mesh for computational efficiency • Within STAR’s ANALYST code • Adaptive Mesh Refinement • Unstructured Mesh • 2D – Triangle • 3D - Tetrahedral Coarse Mesh Fine Mesh

18. Cold beam starting point trajectory crossing point Thermal beam Example: Advanced Models & Algorithms • Particle Tracking – adaptive time step model • Boris push – Structured • Nelson push – Structured, Unstructured • Field Solutions • Self-consistent Electrostatic field solution (CG) • Off-axis B-field expansion • Import externally calculated B-fields (Maxwell 3D) • 2D self-magnetic field model • Emission • Child’s law, Longo-Vaughan Temperature-limited, Thermionic • Secondary emission • Spent Beam particle launch • Periodic, Reflection • Ion beams / charge exchange - Applied Voltage + Emitter Surface

19. Example: GUI & Post-Processing • Graphical User Interface (GUI) • Problem control – setup and run • New model creation with Setup Wizard • Batch control • Direct mesh visualization • Multiple job queuing • Export of results for post-processing • Embedded Python support for comprehensive scripting capabilities • Hybrid mesh support • Unification and simplification of translator interfaces • Post Processor • Structure, mesh, fields & particles visualization • Calculation of beam quantities • Beam profiles. • Collection power,current, efficiency table & visualization. • Alpha, emittance, velocity spread. • User-defined quantities (via Python). • Effort • Creation of useful tools to maximize understanding and design intuition

20. Split Split Split Split Start Beam Pipe: Full & Segmented (5 segments) Full Segmented Run 1 Run 2 Run 3 Run 4 Run 5 End :Beam Transport issues • Modeling of beam transport in long, thin beamlines often suffers from numerical instability or very slow convergence. • Under some conditions, the beamline can be broken up into n segments, and each segment run separately, patching the solutions together across the interfaces with good results. • Sufficiently short segments are always stable, and each has faster convergence. • In the examples shown with 5 segments, the speed increase was a factor of 2 to 4. Beam Pipe and Beam: uniform beam not matched to B-field

21. Long, Thin Beamlines:Performance Comparison • This case represents beam transport where the beams are very laminar. The segmented method works well in these cases. • Emittance Comparison • Shows overall excellent agreement between single and segmented cases • Tracking Error Comparison • Shows a maximum of < 2.0% tracking difference at end of trajectory between single and segmented cases • Indications that segmentation is a suitable solution to a difficult problem over a wide problem class • Automating the segmentation task would be a useful development task

22. CPI XK-8050 Example3D Structured, 2D/3D Unstructured Comparison • Area convergence ratio ~71 – difficult problem • Same CAD Model - ICEM-CFD produced meshes • "Same” MICHELLE input file 2D Unstructured 3D Structured 3D Unstructured 0.277 A 0.2804 A 0.2852 A • Good agreement with DEMEOS • Expect better agreement when 3D unstructured mesh is refined 2D Unstructured Mesh 3D Structured Mesh 3D Unstructured Mesh

23. Embedding MICHELLE in a Commercial Design Environment • MICHELLE Standardized mesh, result, and solver configuration file formats enable embedding. Existing Design Environment Interface Layer MICHELLE CAD File translation interface Solver setup Config Meshing MICHELLE Mesh Result Post-processing

24. Integrated parametric CAD. Automated meshing. Adaptive mesh refinement. Python. Support for field solvers Statics Eigenmodes Driven frequency Optimization. MICHELLE has been embedded in STAR’s Analyst product Python-based scripting provides important flexibility for problem specific numerical post-processing.

25. Action Items • Come up with a roadmap for code capability • Characterize codes and availability (cost) • Many codes in other disciplines can be used – list what’s available • Distribute to this group • List of names & contact info of the group members • Characterize what it would take to do a start-to-end simulation • Gather information from community • List development level of desired capability/upgrades • What do you want? • Some development is important and possibly can be made available right now • Will provide the basis for the roadmap • Start by e-mailing me code names, your application for that code & suggested upgrades/needs • jpetillo@bos.saic.com