Ionizing Radiation from Optical Laser
Light Interacting with Matter:
Simulations with
Particle in Cell Code and FLUKA
Johannes Bauer, James Liu, Sayed Rokni, Ted Liang*
SLAC National Accelerator Laboratory
* also Georgia Institute of Technology
RadSynch17, NSRRC, Hsinchu, Taiwan, April19-22, 2017
RadSynch11
April 28, 2011
Outline
• Overview on Hazards
• PIC-based Predictions for Radiation Dose from
Solid-Target Interactions
–
–
–
–
–
Simulation of Hot Electron Spectra
Dose Yield from FLUKA
Comparison to Measurements
Measurement of Spectrum
TVL of Shielding
• Gas Target Experiments: Measurements vs. Expectation
RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
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SLAC and LCLS
Laser Facility at
SLAC National Accelerator Laboratory,
part of Linac Coherent Light Source (LCLS),
at its Matters in Extreme Conditions instrument (MEC)
Laser hazards unrelated to X-rays from LCLS
RadSynch11
RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
April 28, 2011
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MEC Instrument (2)
MEC Laser Parameters
/

Irradiance [W/cm2]
= power per area
energy
=
time * area
RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
Three Types of Experiments
• Solid Targets:
•
Metals (Au, Cu, Ni, etc.); plastics
•
Mainly study of Matter in Extreme Conditions (warm dense matter like in
Jupiter, confinement for NIF)
•
Also proton acceleration
• Liquid (Frozen) Targets
Hot electrons to MeV level
creating Bremsstrahlung
Protons to 10s MeV
•
Stream of Liquid H2, D2, etc., freezes in vacuum
•
Main goal ion (proton) acceleration
• Gas Targets
•
Small gas cells; gas jets
•
Mainly electron acceleration with Betatron X-ray generation
Laser Wakefield Acceleration of
electrons to few 100 MeV
RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
Photons at 10s of keV
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Ionizing Radiation from Laser on Solid Targets
Plasma
Target
Laser
X ray
eeions
Laser creates plasma
 Electrons accelerated
by strong electric field of laser light  PIC code
 Hot electron energy distribution
described with temperature Th
 Bremsstrahlung from
interaction with target material  FLUKA
RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
I (W/cm2)
Th (MeV)
1x1018
0.1
1x1019
0.7
1x1020
3.0
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Targets inside Chamber, Laser Beam
Gaussian peak = good beam
Target in center
Many small peaks = not so good
Intensity (W/m2)
RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
Spot size (m)
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Particle-In-Cell Code: EPOCH
• Particle-in-cell (PIC)
• Simulates Maxwell’s equations: EM fields + charged particles
• Physical particles represented by macro-particles containing many
physical particles
• Implemented in 2D version of EPOCH
Step 1:
Code moves charged particles in space under influence of
EM fields, calculates currents from particle motion
Step 2:
Code determines EM field based on new positions
and currents
Repeated in 0.1 fs steps over time from 0 to 600 fs
Plot direction and energy of (macro)particles
both inside area and those that have left the area
Arber, T. D. et al., Contemporary particle-in-cell approach to
laser-plasma modelling, Plasma Phys. Control. Fusion 57, 113001 (2015)
https://cfsa-pmw.warwick.ac.uk/EPOCH/epoch
RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
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Example of PIC Simulation
1020 W/cm2
Electron density per grid,
snapshots every 10 fs
Laser direction
Density ramp of pre-plasma
RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
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Example of PIC Simulation
1020 W/cm2
Laser direction
Density ramp of pre-plasma
RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
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Results from PIC Simulation (1)
Sample
spectrum of
hot electrons
Angular
distribution
around 0o
RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
Angular
distribution
around 180o
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Results from PIC Simulation (2)
Optimization of
parameters for
highest dose yield
Hot Electron
Temperature vs
Irradiance
Fraction of laser
energy converted
to ionizing radiation
vs irradiance
RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
Forwardbackward hot
electron yield
vs irradiance
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Particle In Cell Code: EPOCH
Target
Laser
Energy distribution +
Angular
distribution +
EPOCH:
Laser absorption +
Geometry
____________________________
 FLUKA dose yield calculation
Simulation from EPOCH
Simulation from FLUKA
3D geometry in FLUKA
RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
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Example of FLUKA Simulations
Backward dose mainly
from interaction with
target chamber
Taking sum of dose outside target chamber
RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
Forward dose mainly
from interaction with
target in center
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RP Dose Yield Model & SLAC Measurements
previous model
0o direction
180o direction
90o direction
“Revised Model” is
basis to determine
controls
Includes 2.54 cm Al
Measurements at various
angles
RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
Published in
Radiation Protection Dosimetry
doi:10.1093/rpd/ncw325
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Detailed Comparison to SLAC Measurements
Above model is simplification.
Now comparing full FLUKA
simulation with measurements
at correct angles
Measurements follow shape from
simulation, but consistently lower
 o.k. since PIC code optimized for
maximum yield
RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
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Confirming Electron Spectrum by Measurement
FLUKA simulation for both nonrelativistic and relativistic Maxwellian
 much better agreement with nonrelativistic Maxwellian, same as found
from PIC simulation
Placed Landauer nanoDot passive
dosimeters in stacks, separated by
Plexiglas
Set about 30 cm from target, various
angles
RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
Plotting dose versus thickness
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Tenth Value Layers for Shielding Material (1)
How much shielding do we need?
 FLUKA simulations using spectra as source terms
}
TVL1
TVL
}
TVL1
RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
Here for concrete, also
for
• glass
• aluminum
• iron
• lead
• tungsten
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Tenth Value Layers for Shielding Material (2)
TVL1
TVL
TVLe
RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
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Gas Targets: Experiments
 electron acceleration
 Betatron oscillation in electric field
 short-pulse X-rays used in
experiments (similar to FEL X-rays, weak)
Measured
electron
spectrum
RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
number of electrons / MeV
Laser Wakefield Acceleration
Average electron spectrum
assumed for
2015 experiment
~1.5% conversion efficiency
1.5 mW at 0.1 Hz
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Gas Targets: Shielding
2015 Experiment: 1 J, 12,000 shots in few weeks
8 cm tungsten
10 cm lead
Substantial shielding,
but only in forward direction
RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
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Simulations around Target Chamber
50 mrem
20 mrem
200 mrem
75 mrem
2000 mrem
250 mrem
5000 mrem
RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
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Simulations and Measurements outside Hutch
Measurements
outside hutch
(5 m, 0° behind shielding)
• Maximal 84 μSv
total for experiment
Goal: 100 μSv
(no dosimeters)
Inside hutch (no access)
• Several times
2 mSv Pocket Ion
Chamber over
range in one day
• Active Radiation
Monitor (1 n, not in forward
direction) was 12 mSv
RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
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Conclusion
• Ionizing Radiation from lasers need to be addressed
• Solid Targets
– Determined dose yield from PIC & FLUKA simulations
 agreement with measurements
– Confirmation of spectrum shape (non-relativistic Maxwellian)
– TVL for various shielding material
• So far most radiation from gas target experiments
– Understood source term
– Need good shielding
• Various upgrades and new facilities considered
– Perhaps more at next RadSynch!
RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
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Thank You!
RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
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