* Work supported by the Office of Naval Research.
Simulation Needs OverviewJohn Petillo – SAIC, Burlington, MA
Baruch Levush – NRL, Washington, DC
Sept. 25, 2003
ANL Theory Institute on Production of Bright Electron BeamsArgonne National Lab, Argonne, IL
Sept. 22-26, 2003
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
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
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
3D Mesh Types - Illustration
Structured: logical ordering
00
1
12
2
(1,0)
(1,1)
(1,2)
(2,0)
(2,1)
(2,2)
(0,0)
(0,1)
(0,2)
Unstructured: no specific ordering
(0)
(6)
(7)
(1)
(8)
(2)
(3)(5)
(4)(10)
(9)
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
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 to
characterize 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?
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
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
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
Examples
Motivation for coming up with desired code capabilities
Parallel Plate CapacitorLimited Emission Region
D = 10 cm, R = 10 cm R_beam = 1 cm V = 10,000 V
Is Max right?????
Parallel Plate CapacitorLimited Emission Region
D = 10 cm, R = 10 cm R_beam = 1 cm V = 10,000 V
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!
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
3D Collector
Multi-Beam Gun
Gridded Gun
Hybrid Multiblock Mesh Example
Building a Hybrid Multiblock Mesh
1. Build structured mesh with an interface to unstructured block.
2. Extract quads on interface.
3. Build unstructured mesh.
4. Merge. Create pyramid elements.
+
=>
=
interface
interface 3D Hybrid MeshMICHELLE Result
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
HexahedraTetrahedra
Coarse Mesh Fine Mesh
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
iRtrajectory
startingpoint
crossingpoint
- Applied Voltage +EmitterSurface
Cold beam
Thermal beam
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
: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
Split Split Split Split
Beam Pipe: Full & Segmented (5 segments)Full
SegmentedRun 1 Run 2 Run 3 Run 4 Run 5
Start End
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
2 10
-74
10-7
6 10
-78
10-7
1 10
-61.
2 10
-61.
4 10
-6
0 0.02 0.04 0.06 0.08 0.1
Emittance of full simulation vs. segmented simulation as a function of axial position
Emittance - Full SimulationEmittance - Segmented Simulation
Em
itta
nce
Axial Position
0.5
11
.52
0 0.0001 0.0002 0.0003 0.0004 0.0005 0.0006
No
rma
lize
d P
erc
en
t E
rro
r in
Ra
diu
s
Radius
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
3D Structured Mesh 3D Unstructured Mesh2D Unstructured Mesh
Embedding MICHELLE in a Commercial Design Environment
CADCAD
MeshingMeshing
Post-processingPost-processing
MICHELLEMICHELLEMesh
Result
File translationinterface
ConfigSolver setupSolver setup
MICHELLEExisting Design Environment Interface Layer
MICHELLE Standardized mesh, result, and solver configuration file formats enable embedding.
MICHELLE has been embedded in STAR’s Analyst product
Integrated parametric CAD. Automated meshing. Adaptive mesh refinement. Python. Support for field solvers
» Statics» Eigenmodes» Driven frequency
Optimization.
Python-based scripting provides important flexibilityfor problem specific numerical post-processing.
Python-based scripting provides important flexibilityfor problem specific numerical post-processing.
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– [email protected]
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