1. Where Do We Stand With Start-End Simulations
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UCLA
ICFA-Workshop - Sardinia 07/02
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Motivation
FELs at short wavelength (UV to X-ray)
Do not achieve complete longitudinal coherence,
are sensitive to beam parameter fluctuations,
rely on beam manipulation prior to the FEL device (bunch compression).
The explicit longitudinal dependence of the beam parameters is a crucial aspect of modeling the FELs. +
High-brightness beams are more sensitive to degradation during the generation, acceleration and transport to the undulator.
Start-end Simulations
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Examples
Start-end simulations have been successfully used to analyze and understand FEL experiments in great detail:
VISA FEL
LEUTL FEL
TTF FEL
Based on these benchmarks start-end simulations are used for the two x-ray FEL projects
LCLS
TESLA FEL
to estimate a more realistic performance. New projects will most likely adopt the start-end simulation method.
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4. Experience From the Past
Codes can be regarded as an expert system, so are currently the users!
Often one experienced user for each code, most of the time the author itself.
Still manual conversion between different file formats, although there is a strong trend for standardized formats (SDDS).
Extended I/O capability of the codes to simplify the start-end simulations.
Modular approach, very easy to extend by adding new components.
First success in a fully automated, scripted execution.
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The Start-End Model
Physical Layout
Drive laser
Bunch Compressor
FEL Beam Optics
RF Gun
Beam line
FEL
Flow Model
Working start-end simulations
Laser
Compressor
Multi-stage FEL
Gun
Linac
FEL
FEL Optics
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6. Features of Start-End Codes
Runs typically in background (time-consuming calculation)
Runs in passive mode (no interaction with user, requires input from an external file)
N=1024,
g=500, …
I/O
Supports data streams
Postprocessing (data analysis) applied to output stream and is separated from the main code
Plotting
Support for scripted execution
Run
Core
Algorithm
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7. Data Streams & Files Particle Distribution (6D Phase Space)
Sliced Beam
Parameters
Projected Beam
Parameters
Radiation Wavefront
(sliced grid)
Sliced Radiation
Parameters
Projected Radiation
Parameters
Raytrace distribution
Beam Line Lattice
Simulation Control
Parameters
detail level
data stream
input file
output file
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File Format
Application’s native file format
Need conversion from/to other program format.
Difficult to maintain.
Often fixed format.
Pre- & postprocessing must be part of the code distribution.
Self-Describing Data Set (SDDS)
Library function available for code developer.
Existing toolset for processing and plotting data.
Simplifies the start-end flow.
Good for homogeneous data sets (particle distribution).
XML
Files are well-formatted, valid and platform-independent.
Various libraries and editor available.
Flexible transformation to other formats .
Overhead for long similar data sets.
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Drive Laser
Codes: ???
Features:
Temporal and spatial profile at photo cathode
(System jitter)
Comments:
Suitable code does not exist yet. Useful only if gun codes support dynamic emission models.
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Gun
Codes: Astra, Parmela
Features:
Space charge dominated beam propagation
Wide span of particle energy
Emittance compensation.
Comments:
Specialized set of codes to accommodate the vast change in particle energy (adaptive algorithm). Emission profile is the combination of the laser profile & local quantum efficiency. The generation of the initial distribution and the tracking or the particles are separated in the algorithm of the codes. Validity of non-uniform emission is limited by the space charge algorithm.
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Linac
Codes: Elegant
Features:
Beam transport
System tolerances
Comments:
Elegant is the ‘seed’ for most of the start-end projects, defining the infrastructure (SDDS) of the simulations. The beamline lattice follows the ‘MAD’ standard. In principle other codes (e.g. MAD, Sixtrack) are suitable as well, but no initiative has been shown so far.
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Bunch Compressor
Codes: Traffic4, (Elegant)
Features:
Bunch compression
CSR effects
Comments:
Self-consistent simulation of the interaction between electron beam and synchrotron radiation.
Exchange of particle distribution back and forth between Traffic4 and Elegant.
Elegant includes an analytical model (1D) for the CSR effects but might show artificial effects such as an enhanced fluctuation in the beam current due to the finite number of macro particles.
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FEL
Codes: Genesis 1.3, Ginger, Fast
Features:
Time-dependence
Resolve beam on a sub-resonant wavelength scale
Comments:
Codes typically support wide span of ‘resolution’ (projected & sliced beam parameters, distribution) for input. Additional effects such as undulator wake fields can be added at a preprocessing step.
Currently the electron beam data flow ends with the FEL simulation. Sliced parameter or entire distribution as output for both radiation field and electron beam.
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FEL Optics
Codes: SRW, Phase, R. LLNL, Shadow
Features:
Wavefront propagations or raytracing (Shadow)
Limited support for frequency dependent devices such as gratings or monochromator.
Comments:
Most programs are still in development phase. Beside a few (successful) tries FEL optic codes are not used yet for start-end simulations.
FEL code output is well suitable for wavefront propagation although no format to exchange data has been specified. Raytracer requires conversion to a distribution of rays.
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Multi-Stage FEL
Codes: FEL optic codes, electron tracker
Features:
Two-stage FELs (bandwidth control)
Higher harmonic generation
Comments:
Not established yet. Requires to track the fine longitudinal resolution of the particle distribution (conversion from and back to the ponderomotive phase, used in FEL codes). Needs time-dependent FEL optic codes.
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16. Improvement in the Future
Add FEL optics codes to start-end simulation.
Improved support of standardized format.
Fully self-describing file formats for all major data streams (SDDS almost perfect).
Easier/automated set-up of the control input file for each code.
Scripted execution.
User-friendly GUI, built on top of the ‘scripting’ shell.
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Conclusion
Core set of codes established
Successful simulation of various experiments
Support by the authors
Start-end simulations are an evolving process
Still a high expertise level is required.
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