Integrated Modeling of Fast Ignition Experiments

Transcription

1 Integrated Modeling of Fast Ignition Experiments Presented to: 9th International Fast Ignition Workshop Cambridge, MA November 3-5, 2006 R. P. J. Town AX-Division Lawrence Livermore National Laboratory This work was performed under the auspices of the U.S. Department of Energy by the University of California Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48. Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, CA UCRL-PRES

2 Collaborators: H. Chung, D. S. Clark, L. A. Cottrill, A. Friedman, S. P. Hatchett, M. H. Key, A. B. Langdon, B. F. Lasinski, S. Lund, A. J. Mackinnon, B. C. McCandless, H. S. Park, P. K. Patel, B. A. Remington, C. H. Still, M. Tabak, LLNL, Livermore, CA, USA. UCRL-PRES

3 Summary We are developing an integrated simulation capability for fast ignition research To model the entire fast ignition implosion we need to resolve very different spatial and temporal scales. It is prohibitively expensive to simulate the entire process in one code. We have used python to glue the various codes together to model an entire NIF fast ignition experiment. We are using the new integrated simulation capability to optimize our design, in particular: Improved hydrodynamic fuel assembly; and Improved laser-to-electron coupling efficiency. UCRL-PRES

4 Outline Review our code capabilities NIF integrated simulation Assembling the fuel Optimizing laser-to-electron coupling efficiency Conclusions UCRL-PRES

5 We use the python scripting language to couple independent codes into one virtual code UCRL-PRES

6 The hydrodynamic evolution is modeled using LASNEX LASNEX is used to establish the plasma conditions: during the compression phase of a fast ignition target; and wrought by the pre-pulse of the short-pulse laser. LASNEX uses: 2-D cylindrical geometry; realistic equation of state models; 3-D laser ray trace with inverse bremsstrahlung absorption; and radiation transport. Frequent file dumps are made as the simulation progresses. These files are read by python to establish the initial conditions for the short-pulse laser interaction and electron transport calculations. UCRL-PRES

7 The laser-plasma interaction and the generation of hot electrons uses Z3, an explicit PIC code Z3 is a fully 3D explicit PIC code that runs on massively parallel computers. 2D simulations have experiment scale size and duration. 0.3 ps 0.6 ps Reflection becomes more diffuse with time A W/cm 2 laser incident on a 16 n c plasma at 30 o. 0.3 ps 0.5 ps Electrons produced at significant angles. UCRL-PRES

8 LSP 1 can model larger, more dense plasmas for longer simulation times than explicit PIC codes LSP uses: an implicit solution of the electromagnetic fields; an implicit particle push; hybrid fluid-kinetic descriptions for electrons with dynamic reallocation; and inter-and intra-species collisions based on Spitzer collision frequencies. The simulations reported here used: 2-D cylindrical geometry; a fixed ionization state throughout the simulation; an ideal gas EOS; and a hot electron beam created by promotion from the background plasma. 1 D. R. Welch, et al, Nucl. Inst. Meth. Phys. Res. A 242, 134 (2001). UCRL-PRES

9 Collisions between kinetic charged particles use Spitzer Drifting Maxwellians at each grid cell in each direction for plasma electrons and ions are constructed. Each plasma kinetic particle is first scattered isotropically in the center-of-mass frame off its own distribution. Collisions between different species are separated into an energy push and a frictional momentum push. The velocity for kinetic particles is modified in energy then rotated. Highly collisional time steps (ν m Δt >> 1) are possible for a single species (electron). UCRL-PRES

10 Hybrid fluid-kinetic descriptions for electrons with dynamic reallocation The equation of motion for the fluid electrons is identical to that of a kinetic particle, except for the scattering terms. Fluid particles are pushed with ensemble velocities. A pressure gradient force term and a frictional force between the electron and other species replace the elastic scattering events for kinetic particles. The fluid particles carry a temperature that is advanced using: 3 dt 2m n %" T e e e e n =! n T " #' + ( T T ) Q n e e e e $! + " # +! i e e e i 2 dt m& %" T i ie e PdV 1+ fq Ohmic Energy exchange When the directed energy is large compared to the internal energy the fluid electron is converted into a kinetic electron. th conduction de dt in inelastic UCRL-PRES

11 A NIF indirectly-driven cryogenic capsule implosion is simulated using LASNEX An imposed P1 radiation asymmetry was used to form a dense fuel assembly. S. P. Hatchett, et al, Fusion Science and Technology, 49, 327 (2006). UCRL-PRES

12 The hydrodynamic conditions are taken from LASNEX calculations around peak compression A non-hollow dense and cold fuel assembly was achieved. The dense fuel is located about 5 core radii from the short-pulse laser deposition region. Short pulse laser UCRL-PRES

13 The gold tip was heavily deformed during the implosion The non-uniform drive ejected the hot central gas towards the cone tip pushing it away from the dense fuel. Initial position of the outer surface of the cone Final position of the cone Hot-spot ejected Short pulse laser UCRL-PRES

14 For these calculations, we used scaling laws to establish the hot electron parameters Conversion Efficiency η = I(W/cm 2 ) Hot Electron Energy Pondermotive scaling: T hot (MeV)= (Iλ 2 /(10 19 W/cm 2 µm 2 )) 1/2 Beg scaling: T hot (MeV)= 0.1(Iλ 2 /(10 17 W/cm 2 µm 2 )) 1/3 A thermal spread of 300keV was also added. UCRL-PRES

15 One full NIF ARC beam will create about 200J of hot electrons with an energy of 1.5 MeV The electron energy is fixed with time; The number density of hot electrons is varied to take account of the temporal laser profile. UCRL-PRES

16 The hot electrons are created at the critical surface on the inside of the deformed cone UCRL-PRES

17 At later times most of the hot electrons are confined to the gold cone UCRL-PRES

18 A simplified yield model 1 shows a high quality implosion is needed 1 M. Tabak, D. S. Clark, L. J. Perkins, APS meeting (2006). UCRL-PRES

19 We need to assemble a high density core without a hot spot Clark is applying his self-similar theory to direct-drive capsule designs for the NIF µm 1440 µm 1000 µm DT/CH DT gas Intensity (W/cm 2 ) Time (ns) 1 D. S. Clark, M. Tabak, APS meeting (2006). UCRL-PRES

20 The imploded core is nearly isochoric with 300 g/cm 3 and a small hot spot region Density(g/cm 3 ) 45.3 ns 45.4 ns 45.5 ns radius (µm) Achieves ρr of 2.7g/cm 2 for 650kJ of laser energy UCRL-PRES

21 We are using Z3 to optimize hot electron production efficiency in cone targets 2D (x, z) Z3 simulations in p-polarization at W/cm 2, 15 cone half angle, n e = 16n c, T e = 10 kev, Zm i /m e = 3600, and ZT e /T i = 20. Initial beam profile Static Poynting flux (P z ) s Time = 0.08ps Amplitude (a.u.) z(µm) x(µm) x(µm) UCRL-PRES

22 The laser-plasma interaction is predominantly at the cone tip 2-D, 1019 W/cm2, p-polarization, 16nc, Te = 10 kev (Pz)s Electron density z(µm) z(µm) Time = 0.7ps x(µm) Field enhancement. Reflection along the sides of the beam. x(µm) Relativistic critical surface is heavily distorted. UCRL-PRES

23 Over 50% of the incident energy is absorbed into hot electrons 2-D, W/cm 2, p-polarization, 16n c, T e = 10 kev Energy / Incident In system Hot electrons Reflected light time(ps) This is about twice the absorption of a planar target. UCRL-PRES

24 However, the energetic electrons have a wide angular spread 2-D, 1019 W/cm2, p-polarization, 16nc, Te = 10 kev Electron position with energy > 0.8 MeV (γ > 2.6) t = 0.5 ps t = 0.55 ps z(µm) t = 0.4 ps x(µm) x(µm) x(µm) UCRL-PRES

25 Summary We are developing an integrated simulation capability for fast ignition research To model the entire fast ignition implosion we need to resolve very different spatial and temporal scales. It is prohibitively expensive to simulate the entire process in one code. We have used python to glue the various codes together to model an entire NIF fast ignition experiment. We are using the new integrated simulation capability to optimize our design, in particular: Improved hydrodynamic fuel assembly; and Improved laser-to-electron coupling efficiency. UCRL-PRES