Simulating Monoenergetic Proton Radiographs of Inertial Confinement Fusion Experiments using the Gea

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Simulating Mono-energetic Proton Radiographs of
Inertial Confinement Fusion Experiments using the
Geant4 Monte Carlo Particle Transport Toolkit
M. Manuel, F. H. Séguin, C. K. Li, D. T. Casey,
J. R. Rygg, J. A. Frenje, R. D. Petrasso MIT
PSFC R. Betti, O. Gotchev, S. Hu, J. Knauer, F.
Marshall, D. D. Meyerhofer, V. A. Smalyuk, UR-LLE
American Physical Society 49th Annual Meeting of
the Division of Plasma Physics Orlando, FL Nov.
11th- Nov. 16th, 2007
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Abstract
Proton radiography has been used to image
Inertial Confinement Fusion (ICF) capsules during
their implosions as well as to quantitatively
measure magnetic fields generated by laser-plasma
interactions at the OMEGA laser facility. An
imploded, D3He-filled capsule provides
mono-energetic, 15-MeV protons for radiographing
another capsule. We are developing simulated
models of these experiments using the Geant4
Monte Carlo Particle Transport Toolkit (G4). Of
particular interest are the limitations on
spatial resolution caused by scattering effects.
Experimental and simulated results will be
presented for different experiments and models.
This work was performed in part at the LLE
National Laser User's Facility (NLUF), and was
supported in part by US DOE, LLNL, LLE and FSC at
Univ. Rochester.
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Proton Radiography is a powerful diagnostic tool
for ICF

• Proton radiography is being used to study B E
  fields and mass distributions in a range of ICF
  experiments (laser-plasma interactions, ICF
  implosions, magnetic reconnection, etc. )
• The information content of radiography images is
  affected by imaging system parameters, such as
  source size, slowing and scattering of protons
  in the imaged sample, existence of electric
  and/or magnetic field structures, etc.
• We need to know what the information content is
  and what applications are practical, so we are
  using Geant4 as a simulation tool to analyze
  experiments

Radiograph of a plasma bubble 2.4 ns after the
laser pulse began.
Radiograph of an ICF implosion 1.5 ns after the
laser pulse began.
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Protons interact in several ways with fields and
mattereach affects the information content of
images

• 1. Protons Lose Energy while Traversing Matter
• This can be good If we know the initial proton
  energy, an energy-sensitive detector tells us how
  much energy was lost along a proton trajectory
  and we can use that information to infer ??dl.
• 2. Protons Scatter while Traversing Matter
• This is usually bad Scatter limits image spatial
  resolution
• 3. Proton Trajectories are Affected by Electric
  and Magnetic Fields
• This is good Measurements of beamlet
  displacements, or fluence modulations, allow us
  to study field strengths.

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Outline

• Principles Proton Radiography
• Spectra
• CR39 System
• Sources of Scatter
• Experiments and Simulations
• Proton Beamlets in a Mesh Configuration
• Unimploded ICF Capsules
• Planar Rayleight-Taylor-Instability
• Future Work
• Summary

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Mono-energetic protons are created by a D3He
filled backlighter implosion capsule
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CR39 imaging system used for proton radiography
Backlighter Capsule
Imaging Protons

• Target can down shift protons in matter, and/or
  deflect them in E- and/or B-fields
• CR39 stores individual proton position and
  energy information
• Filtering ranges two distinct fusion product
  protons to the optimum energy range for CR39
  detection

Target
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What scattering effects affect the resolution of
the radiograph?
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Calculate scatter on proton beamlets passing
through a target foil
Metal Mesh

• Proton beamlets are formed by imposing a mesh
  on the incoming proton flux from the
  backlighter
• We will examine scattering of beamlets through
  different types of materials CH, Au, Be, Ta,
  etc.
• Scattering effects are turned on or off to see
  effects on images using G4

Al Filter
Scattering Foil
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G4 simulation of proton beamlets scattering
through a 25-µm mylar foil
CR-39 Detector
Scattering foil behind mesh (not visible)
Beamlet creating mesh
Backlighter Capsule
Not all particle tracks are shown. Sources are
isotropic, but the simulation samples only those
particles whose direction is toward the image
target.
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Simulated proton radiographs of beamlets through
a 25-µm mylar foil with and with out scattering
effects

• Energy biased non-scattered radiograph
• Radiograph is still blurry due to finite source
  size and number statistics

• Energy biased scattered radiograph
• The effects of scattering through 25-µm of Mylar
  can easily be seen

Both radiographs were created with identical
parameters, 150 - 75 µm Ni mesh, yield of 1.7108
protons, 1/e radius of 27 µm, 15-MeV average
energy with 8.7-keV Tion .
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By turning off scattering, we can see how the
amplitude modulation changes
Effective scattering in 25-µm Mylar at a spatial
frequency of 150-µm was reduced by 23 in
amplitude modulation
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Imaging an ICF implosion capsule with a D3He
filled backlighter

• The amount of scattering and down shift is
  dependent on areal density
• Temporal evolution of ?L can be evaluated by
  imaging at different times
• E- and/or B-field structure near the implosion
  capsule can be investigated

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Image of the geometric setup of a G4 simulation
of a proton radiograph experiment of an
unimploded ICF capsule
CR-39 Detector
Object Capsule (to be imaged)
Backlighter Capsule
Not all particle tracks are shown. Sources are
isotropic, but the simulation samples only those
particles whose direction is toward the image
target.
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Simulated radiographs of an unimploded ICF capsule

• Capsule Parameters
• 20.0-µm CH shell
• 15-atm H2 fill pressure
• 435-µm outer radius
• 0.9-cm source-to-
• capsule distance

Fluence
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Experimental proton radiographs of an unimploded
ICF capsule (OMEGA shot 46531)
Proton Fluence (protons/µm2 )
?L (mg/cm2 )
Radius (µm)
Not related specifically to simulation on
previous slide, only an example of data for an
unimploded ICF capsule
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Simulating images of a Rayleigh-Taylor-instability
experiment (using areal density profiles
derived from DRACO)

• Experiment
• Seed modulations are amplified by laser
  illumination
• Proton backlighter is used to image the foil at
  different times
• Simulation
• The 2-D rad-hydro code DRACO is used to simulate
  the evolution of the instability
• G4 is used to simulate images using DRACO
  density profiles

Seed Modulations for RT growth
Interaction Beam
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Number density profiles from a G4 simulation of
an RT foil using mass distributions from
different times given by DRACO
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Experimental radiographs of a 20 µm CH foil with
0.5 µm seed modulation at different times

• Using DRACO mass distributions, simulated
  radiographs indicate lower amplitude modulation
  than experimental data
• This amplitude modulation difference is used to
  infer order of magnitude field structures near
  the accelerating RT foil surface

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Future Work

• Expand Geant4 Implementation
• Add electromagnetic fields
• Add plasma stopping power/scattering physics
• Parameterize Imaging System Performance
• Modulation transfer function (MTF) for different
  materials
• Resolution limits for proton radiography in ICF
  experiments
• Optimize System Parameters for Future Experiments
• Analyze Experimental Data

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Summary
Proton radiography is an exciting new imaging
technology for use in various ICF experiments.
Through the use of Geant4, we will be able to
simulate complex experimental setups and
scenarios, including complex shapes, fields, and
environments (plasmas). Simulation will be used
to optimize imaging systems, evaluate their
performance, and analyze the data they
produce. Geant4 has already been used to
simulate simple experiments, and it has proven
useful for investigating the scattering and
energy loss of charged particles as they pass
through materials. We are in the process of
expanding our implementation to include other
processes.