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Status of BDSIM Simulation
L. Nevay, S. Boogert, H. Garcia-Morales, S. Gibson,
R. Kwee-Hinzmann, J. Snuverink
Royal Holloway, University of London
17th October 2014
Beam Delivery Simulation
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Beam Delivery Simulation (BDSIM)
Geant4 and C++ particle tracking code
Developed for linear collider background simulation
Combines fast in-vacuum tracking with Geant4 physics
Simulates interaction of primaries with accelerator
Tracks all secondary particles
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Previous State of BDSIM
• BDSIM being adapted for use with rings
― Namely LHC & HL-LHC
• Existing LHC Lattice partially converted to BDSIM format
― Some elements missing / not implemented
• No turn control -> Geant4 models don’t typically have rings
• Offset between ends of model (~100μm x & y)
• Single turn optics shown
L. Nevay et. al, In MOPRO045,
Proceedings of IPAC 2014
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Progress Since Then
• All elements in LHC lattice implemented
― With rotation
― Using collimator database
― All collimator materials set properly
• Turn control
― Dynamic volume tracking user limits
― Terminate all particles once N turns completed
• Offset solved
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Inherent to MADX model of LHC
More accurate survey model investigated
Over 250 extra items
Naming and lengths inconsistent between the two
• Lattice solved as periodic boundary
― Offset represents extremely small inaccuracy in model
― Insignificant compared to misalignments of all components
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4TeV Model
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2012 4TeV B1 in collision optics used as test machine
Generic Geometry
3.5um rad emittance x & y
6σ Halo in one dimension, Gaussian in other
― No cropping at 3σ
• Vertical halo as example here
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Simulation Details
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650000 primaries
1000 jobs
~ 1.5 days on 150 node farm
~3 Gb output in root format
20GeV energy cut-off
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4TeV Model Loss Maps
E (GeV / 10cm)
650000 primaries
S (m) from IR 1
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E (GeV / 10cm)
IR 7 Detail
S (m) from IR 1
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Comparison
E (GeV / 10cm)
BDSIM
S (m) from IR 1
R. Bruce et. al, Phys. Rev. ST Accel. Beams 17, 081004 (2014)
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Entries 1e+09
Mean 1.943e+04
RMS
2391
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S (m)
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Number of Particle Impacts
As A Function of S
Models in Preparation
• Aiming to reproduce 4TeV LHC with higher statistics
• 4TeV 2012 for B2 as well
• HL-LHC model
• In all cases, would like numerical comparison with
Collimation Team results
• Underway with help of Hector & Regina at RHUL
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Improving The Statistics
• Profiling shows significant time for Geant4 processes
― Intersection with next volume
― Volume searching
• Only <1% of CPU time spend on tracking functions
― NB. our tracking functions are much quicker than a Runge-Kutta integrator
― Not unexpected
• Geant4 processes cannot be improved upon (by us)
• Geant4 is primarily designed for detector simulation
― Omnidirectional / no directional preference
― Highly repetitive structure can be described programmatically
― Hierarchical
• An accelerator does not make a good detector
― Repetitive, yet sufficiently unique throughout
― Flat hierarchy – gives very poor geometry navigation / volume searching
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Tracker Factorisation
• BDSIM creates a Geant4 model of an accelerator
― Uses generic geometry classes
― Model built dynamically based on input file / lattice description
• Normal Runge-Kutta integrators / steppers are replaced
― By custom functions
― Known solutions for known magnetic fields
• Geant4 handles all tracking management
― Great for scattered particles, secondaries etc
― Highly inefficient for primaries that spend long time tracking
• Factorise tracking routines
― Use only routines until particle close to aperture or in collimator
― Switch to Geant4
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BDSIM Tracker
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Tracking routines separated from Geant4 Model
Particles initially tracked using tracker
When close to aperture -> passed to normal BDSIM (G4)
BDSIM uses exactly the same tracking routines
― Wrapped in Geant4 steppers for Geant4 model of accelerator
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Tracker development implementation in final stages
Testing underway
Interaction with BDSIM also being tested
Expect significant improvement in speed -> statistics
• Leads to more generic plug and play tracking
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Geometry Improvement
• Currently using generic geometry
― Cylindrically symmetric
― One material
• Adding Geant4 geometry for an LHC Dipole & Quad
― Slightly simplified design
― Correct materials & therefore cross-sections
• Also more realistic collimator design
• Naturally leads to geometry libraries for BDSIM
― Truly generic
― Normal conducting
― Superconducting / LHC
• Hector G-Morales working on this
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Summary
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LHC Model fully generated
BDSIM developed for use with rings
First loss maps from 4TeV LHC produced
BDSIM loss maps include primaries and secondaries
Close qualitative match between BDSIM simulation &
measured losses
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Other models in preparations
Quantitative comparison underway
Improvements for increased statistics nearly complete
More realistic generic geometries being implemented
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