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 2 RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
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 3 RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers April 28, 2011
MEC Instrument (2)
MEC Laser Parameters / Irradiance [W/cm 2 ] energy = power per area = 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) Hot electrons to MeV level • Also proton acceleration creating Bremsstrahlung Protons to 10s MeV • Liquid (Frozen) Targets • Stream of Liquid H 2 , D 2 , 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 Photons at 10s of keV electrons to few 100 MeV 6 RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
Ionizing Radiation from Laser on Solid Targets Target Plasma Laser X ray e - e - ions Laser creates plasma Electrons accelerated by strong electric field of laser light PIC code Hot electron energy distribution I (W/cm 2 ) T h (MeV) described with temperature T h 1x10 18 0.1 Bremsstrahlung from interaction with target material FLUKA 1x10 19 0.7 1x10 20 3.0 7 RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
Targets inside Chamber, Laser Beam Gaussian peak = good beam Many small peaks = not so good Target in center Intensity (W/m 2 ) Spot size (m) 8 RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
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 9 RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
Example of PIC Simulation 10 20 W/cm 2 Electron density per grid, snapshots every 10 fs Laser direction Density ramp of pre-plasma 10 RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
Example of PIC Simulation 10 20 W/cm 2 Laser direction Density ramp of pre-plasma 11 RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
Results from PIC Simulation (1) Sample spectrum of hot electrons Angular Angular distribution distribution around 0 o around 180 o 12 RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
Results from PIC Simulation (2) Optimization of Hot Electron parameters for Temperature vs highest dose yield Irradiance Forward- backward hot Fraction of laser electron yield energy converted vs irradiance to ionizing radiation vs irradiance 13 RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
Particle In Cell Code: EPOCH Target Energy distribution + EPOCH: Angular distribution + Laser absorption + Geometry ____________________________ Laser FLUKA dose yield calculation Simulation from EPOCH Simulation from FLUKA 14 3D geometry in FLUKA RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
Example of FLUKA Simulations Backward dose mainly Forward dose mainly from interaction with from interaction with Taking sum of dose outside target chamber target chamber target in center 15 RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
RP Dose Yield Model & SLAC Measurements previous model 0 o direction 180 o direction 90 o direction “Revised Model” is basis to determine controls Includes 2.54 cm Al Measurements at various angles Published in Radiation Protection Dosimetry 16 RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers doi:10.1093/rpd/ncw325
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 17 RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
Confirming Electron Spectrum by Measurement FLUKA simulation for both non- relativistic and relativistic Maxwellian much better agreement with non- relativistic Maxwellian, same as found from PIC simulation Placed Landauer nanoDot passive dosimeters in stacks, separated by Plexiglas Plotting dose versus thickness Set about 30 cm from target, various angles 18 RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
Tenth Value Layers for Shielding Material (1) How much shielding do we need? FLUKA simulations using spectra as source terms } Here for concrete , also TVL 1 for } • glass TVL • aluminum TVL 1 • iron • lead • tungsten 19 RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
Tenth Value Layers for Shielding Material (2) TVL TVL 1 TVL e 20 RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
Gas Targets: Experiments Laser Wakefield Acceleration electron acceleration Betatron oscillation in electric field Average electron spectrum short-pulse X-rays used in number of electrons / MeV assumed for experiments (similar to FEL X-rays, weak) 2015 experiment ~ 1.5% conversion efficiency 1.5 mW at 0.1 Hz Measured electron spectrum 21 RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
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 22 RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
Simulations around Target Chamber 50 mrem 20 mrem 200 mrem 75 mrem 5000 mrem 250 mrem 2000 mrem 23 RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
Simulations and Measurements outside Hutch Measurements outside hutch (5 m, 0° behind shielding) Goal: 100 μSv • Maximal 84 μSv (no dosimeters) total for experiment 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 24 RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
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! 25 RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
Thank You! 26 RadSynch17: Johannes Bauer, Ionizing Radiation from Optical Lasers
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