Scott Wilks, A. Kemp, H. Chen LLNL

1. Intense Laser-Plasma Interactions in HED Science: Full Scale Simulations of Reduced Mass Targets & Laser Wakefield Accelerators LLNL-PRES-401398 Scott Wilks, A. Kemp, H. Chen LLNL C. Geddes, E. Esarey, W. Leemans, E. Michel LBNL J. Cary, D. Bruhwiler, P. Messmer Tech-X Physical Sciences

2. Outline • Intro: What is meant by intense laser? • I > 1x1018 W/cm2 (oscillatory velocity > c) • Physics is highly nonlinear • Relativistic plasma response • Self-focusing of laser light. • No analytic solutions -> simulation • Laser Wakefield Accelerator - LWFA • How does a LWFA work? • Experiment that was modeled. • Results obtained with GC time. • Reduced Mass Targets - RMT’s • Simple energy estimate. • LLNL Experiment using RMT. • Simulation results. LWFA RMT Physical Sciences

3. Laser Wakefield Accelerators may be the next generation of particle accelerators. Process: Laser radiation pressure (red) displaces electrons Space charge causes oscillating density ‘wake’ moving with the laser (blue-green) Wake electric fields of of ~ GV/cm accelerate particles (white) Laser v ~ c Similar to how a Boat displaces water, giving restoring force, resulting in a water wake. Wake velocity = boat velocity Trapped particles can get ~ 1 GeV in 1 cm ! Advantages: Accelerating fields 1000x conventional linacs - implies compact sources Intrinsically femtosecond bunches - ultrafast science, high current for FEL’s Typical experiment: Elaser ~ few J Ilaser ~ 1018-19 W/cm2 Ewake ~ 1 GV/cm 100 MeV in mm GeV electrons in cm Simulations required to: 1. Study emittance growth. 2. Understand experiments. 3. Help design for future multi-GeV experiments. Physical Sciences

4. 3 mrad divergence, E/E 4% Particle simulations reveal physics of LWFA experimentsand will help design the next generation accelerators • Capillary channels+low ne=GeV in 3 cm • Laser channeling: first low E/E beams ne ~ 4e18/cc, 1/4 of ‘04 density Geddes et al, Nature 2004 Leemans et al, Nature Phys 2006 • 10 GeV m-scale experiments proposed Simulation must resolve laser wavelength (µm) over cm of propagation, which takes 1 million hours in 3D. m-scale modeling challenging even in 1D. Physical Sciences

5. Start Laser modulates, particles trapped Particle dephasing & loading --> low E/E 20 a0 envelope Y(µm) Propagation max 2.4 max 4.5 max 4.3 -20 20 Plasma density Y(µm) max 4.1e19cm-3 max 3.6e19cm-3 max 2.9e20cm-3 -20 187 Phase space Px(MeV/c) 0 90 X(µm) 150 820 1030 X(µm) 880 972 X(µm) Particle simulations showed physics of LWFAsNow focus on detailed experimental modeling for optimization Goals: 1. Detailed optimization by understanding and fixing experiment/ simulation differences: divergence, guiding, stability 2. Design next generation multi GeV experiments Movie #1 shows time development of acceleration  Physical Sciences

6. High resolution modeling of 100 MeV experiments: Improved modeling of beam emittance Beam phase space Px-Py • 2D simulations proved new numerical approaches • ATLAS allows high resolution convergence studies - O(res4) • high resolution runs are 100x more costly ~ 100khour • used to benchmark higher-order particle shapes, smoothing: • improved modeling of emittance at reduced cost • beam Q,E vary weakly - close to expt. • 3D simulations verify noise reduction - add realistic physics • absorbing boundaries for outgoing waves (PML’s) • current smoothing along longitudinal axis • higher order weighting • Improved ability to model, optimize expt.’s • Further work in progress 14 Py[MeV/c] 2D 1st order -10 -2 Px[MeV/c] 147 14 2D 2nd order 3x resolution Py[MeV/c] -14 -2 Px[MeV/c] 162 16 Py[MeV/c] 3D 1st order -16 193 -2 Px[MeV/c] 16 3D 1st order smoothed Py[MeV/c] -16 -2 Px[MeV/c] 169 Physical Sciences

7. ATLAS providing first full scale modeling of proposed 10 GeV LWFAs & developing new tools • Next generation experiments to drive FEL’s, colliders • 10 GeV in meter -scale plasma • 100 x longer plasma, higher laser group velocity • Prohibitive in multi-dimensions using traditional PIC • 1 D ATLAS simulations: first full-scale evaluation • verified 10 GeV gain • evaluated laser evolution • verified scaled and envelope simulations used for multi-D • Benchmarked new Lorentz code • uses shifted calculation frame to minimize scale difference • 1000 x speedup • Allowed full scale modeling of 10 GeV • Develops method to model 10 GeV in 3D Lorentz Explicit 2e4 Laser Ez (GV/m) -2e4 1x104 8 GeV  8 GeV  8 GeV  x-px phase space x (mm) 413.3 x (mm) 413.7 19.3 41.4 Physical Sciences

8. -40 -80 Y[m] Y[m] 40 80 1.16mm X 1.28mm 1.12mm X 1.24mm Realistic physical modeling of GeV experiment to understand waveguide and bunch stability effects Experimental laser transmission (....) sensitively depends on plasma current(__) • Simulations did not reproduce • stable 0.5 GeV experiments • need for channel in expt. • Simulate closer to experiments • Higher order laser modes • Self guiding not as effective - explains need for channel • Affects electron trapping From Leemans et al, Nature Physics V 2 p 697, 2006 Plasma density Laser field: realistic higher order modes Realistic mode Gaussian Physical Sciences

9. Reduced Mass Targets: Are these good high energy density physics (HEDP) targets? Is the efficiency high? Simple idea: Fixed E, reduce mass of target, to get target hotter. Target: 30x30x30 mm3 Cu Ka image “Small” number of ions in a target ~ 2 x 1015 total ions. V ~ 105mm3 “Instantaneously” ionize entire target while solid. Energy balance argument over time tpulse gives: e ~ GJ/cm3 Elaser ~ 300 J Ehots ~ fabsElaser Assuming ideal gas and LTE, target heats up to 330 mm x 330 mm x 7 mm Te = 1.5*Eback/Ne For the NIF-ARC laser and a 100x100x7 micron Copper target, we estimate a bulk Te of ~ 2 keV! G, Gregori, et. al., Phys Plasmas Comments (2003) Temperature vs. Transverse Target Size Applications: 1. HEDP opacity experiments (Schneider/Shepherd) 2. X-ray Backlighters for Nat’l Ignition Campaign (Park) 3. Proton acceleration: Highly efficient ion accelerators? Simulations are required to: 1. Understand laser absorption and electron transport. 2. Estimate temperature and density profiles. 3. Help interpret effects seen in experiment. Physical Sciences

10. Experiments performed by Hui Chen at LLNL in the JUPITER facility on the Callisto laser in February of 2007 surprised us. Proton Acceleration (Patel, Snavely) Target Normal Sheath Acceleration Laser: I = 1018-1020 W/cm2 t ~ 100 fs E = 10 J Reduced Mass Target Standard, Large Target S. C. Wilks, et. al., Phys Plasmas (2001) 6.5 MeV 5.6 MeV 5.6 MeV 4.6 MeV 4.6 MeV 3.4 MeV 3.4 MeV 1.5 MeV 1.5 MeV Question: Why was the proton signal so dramatically different between the reduced mass and the standard large target cases? Physical Sciences

11. Our ATLAS time allowed us to simulate this experiment. This shows the accelerating field, Ey , that develops on the front and back of the target. Eacc back Solid Target side side front Laser This was expected, and is the usual outcome of large targets. Movie #2 shows how field develops: high values early, low late. Physical Sciences

12. Because target was finite size, large fields on all sides of target are generated. Amplitude plot of transverse field (Ex) that develops on the sides of the target. This explained the large, low level signal all around the target. “Enhanced energy conversion from short pulse laser to protons using RMT’s” H. Chen, S. C. Wilks, A. J. Kemp, Y. Ping, R. Shepherd, D. Offermann, A. J. Link, L. Van Woerkom. Physical Sciences

13. ATLAS also showed large B-field on back of target. Existence of ~ 10 MegaGauss magnetic fields on back of targets has yet to be confirmed by experiment. Example of PIC simulations providing insight into things not easily measured in experiments. Physical Sciences

14. 3-D RMT simulations run on ATLAS also helped explain puzzling Chen data from Callisto. Simulation gave Emax ~ 5 MeV out the back, and Emax ~ 1.5 MeV, in agreement with experiment. Sides acted like “thick” proton acceleration targets: protons are still accelerated via TNSA, however, they have lower energies, and are more uniform over entire surface. (Movie #3) Physical Sciences

15. Another question regarding RMT’s was this: How isochoric is the heating? • Rutherford Appleton Laboratory experiments • A. Kemp APS-DPP (2007.) 500 mm PSC simulations on ATLAS explained bottled up energy by showing large electron and ion density gradients were present. Physical Sciences

16. Significance and impact of access to computing resources • Plasma response is highly non-linear: 3-D PIC code is the ideal tool to use for both LWFA and RMT’s, but needs ATLAS • LWFA: Impossible to model either problem in the traditional “static” laboratory frame (distances too great.) ATLAS allows for time intensive runs (~ 10,000,000 time steps) using “moving window” frame simulations not possible elsewhere. • RMT: High electron currents cause self-focusing, which is highly non-linear & needs 3-D to accurately simulate experiments. • ATLAS allows us to: • do several “physics” 2-D simulations that could feedback to experiment in a timely fashion. • do a few 3-D runs, allowing scaling studies for GeV accelerators and realistic RMT’s. More to come. Physical Sciences

17. What did we learn? • Non-ideal laser beam transverse profile require channeling. • 10 GeV gain obtainable with 1 PW. • New methods improve emittance modeling. • Proton/ion acceleration important absorption mechanism for RMT. • Collisional PIC explained electron transport inhibition seen in experiment. • ATLAS allowed large scale (~ 2,000 cpu) runs, and lots of smaller “physics” runs (~ 512 cpu’s.) • John Cary and David Bruhwiler helped LC identify bug involving use of parallel I/O via HDF5 on the Lustre file system. • John and others at Tech-X developed VisIt plugin for VORPAL, installed and tested on ATLAS by Mark Miller. Physical Sciences

18. Alignment with the Laboratory science and technology strategic vision: HEDP Laser Wake Field Accelerator • Table-top laser-based electron accelerators would benefit the laboratory mission, as drivers for compact light sources. • Explores channel guiding of intense lasers in plasmas relevant to Fast Ignition. Reduced Mass Targets • Fundamental understanding of RMT’s will benefit: • NIF-ARC small back-lighter effort. • Current opacity experimental effort • Fast Ignition and proton acceleration effort. • “3-D PIC to 3-D rad-hydro code” for integrated modeling. New ATLAS Proposals • Two separate GC proposals have been submitted on each topic: • “Next Generation Laser Accelerators and Amplifiers” (Wilks/Geddes) • “Fast Ignition Laser Plasma Interactions” (Kemp/Divol) Physical Sciences

19. List of Papers, Presentations, and Proposals that included these ATLAS results. 1. C.G.R. Geddes, et. al., "Laser wakefield simulations towards development of compact particle accelerators," J. Phys. Conf. Series V 78 pp. 12021/1-5 (2007) 2. C.G.R. Geddes et al, "High fidelity simulation of experiments: 100 MeV to 10 GeV," presented at the BELLA Proposal Review (Berkeley Lab, Oct. 11, 2007), DOE Office of High Energy Physics, status CD0. 3. Cameron G.R. Geddes et. al, "Simulation results from INCITE : Full scale modeling of LWFA experiments," LBNL AFRD Division Review, 9 May 2007 4. C.G.R. Geddes, E. Esarey, et. al., "Laser wakefield simulations towards development of compact particle accelerators," , Boston SciDAC June 26, 2007 5. Estelle Cormier-Michel, C.G.R. Geddes,et. al., “Simulation of 1 GeV laser wakefield accelerator experiments and scaling to 10 GeV” APS-DPP07 meeting, Orlando,Fl. Nov. 2007. 6. D.L. Bruhwiler, K. Paul, P.J. Mullowney, J.R. Cary, P. Messmer, C. Nieter, G. Werner, E. Cormier-Michel, C.G.R. Geddes, E.H. Esarey and W.P. Leemans, "Toward Quieter PIC Simulations of LWFA Experiments with the VORPAL Code," presented at the Laser and Plasma Accelerators Workshop (Azores, July 10, 2007). 7. K. Paul, D.L. Bruhwiler, J.R. Cary, P. Messmer, P.J. Mullowney, E. Cormier-Michel, E.H. Esarey, C.G.R. Geddes, W.P. Leemans and C. Schroeder, "Benefits of Higher-Order Particle Shapes in the Electromagnetic PIC Code VORPAL," presented at the 22nd Particle Accelerator Conference (Albuquerque, June 29, 2007). 8. K. Paul, D.L. Bruhwiler, J.R. Cary, P. Messmer, P.J. Mullowney, E. Cormier-Michel, E.H. Esarey, C.G.R. Geddes, W.P. Leemans and C. Schroeder, "Benefits of Higher-Order Particle Shapes in the Electromagnetic PIC Code VORPAL," presented at the 49th Annual Meeting of the Division of Plasma Physics (Orlando, Nov. 12, 2007). 9. D.L. Bruhwiler, "High-Order Particle-in-Cell Algorithms for Laser Plasma Simulations," Phase II SBIR proposal, DOE Office of High Energy Physics, (April, 2007), not funded. 10. E. Esarey, ``GeV electrons from channel-guided laser wakefield accelerators,''invited talk at 38th Colloquium on The Physics of Quantum Electronics Snowbird, Utah, January 6-10, 2008. 11. S. C. Wilks, A. J. Kemp, D. S. Hey, P. K. Patel, et. al. "Isochoric Heating of Reduced Mass Targets by Ultra-Intense Lasers", APS-DPP talk JO6.00003 (2007) 12. H. Chen, Y. Ping, S. Wilks, A. Kemp, et. al. "Angular distribution of fast electrons and protons in short pulse laser target interaction", APS-DPP talk CO6.00011 (2007) 13. A. Kemp, 'Laser heating of solid matter by light pressure-driven shocks at ultra-relativistic intensities', presented at Anomalous Absorption Conference, Maui August 2007 14. A. Kemp, 'Laser heating of solid matter by light pressure-driven shocks at ultra-relativistic intensities', presented at APS-DPP meeting, Orlando, FL, November 2007 15. K.Akli, S.Hansen, A.Kemp et al., “Laser heating of solid matter by light pressure-driven shocks at ultra-relativistic intensities”, submitted to PRL (2007) We would like to thank: Don Correll for ERI-LDRD support, the VisIt team help desk, LLNL visualization group, the VORPAL development team for continuing support, testing and development, and US Department of Energy, Office of Science, Office of High Energy Physics, for support of work at LBNL and Tech-X Corp Physical Sciences