Abstract
The National Ignition Facility (NIF) provides a unique opportunity to study implosion physics with nuclear yield. The use of polar direct drive (PDD) [A. M. Cok, R. S. Craxton, and P. W. McKenty, Phys. Plasmas 15, 082705 (2008)] provides a simple platform for the experimental studies without expensive optics upgrades to NIF. To determine the optimum PDD laser pointing geometry on NIF and provide a baseline for validating inertial confinement fusion codes against experiments for symmetric and asymmetric implosions, computer simulations using the 3D radiationhydrodynamics code hydra [M. M. Marinak, R. E. Tipton, O. L. Landen, T. J. Murphy, P. Amendt, S. W. Haan, S. P. Hatchett, C. J. Keane, R. McEachern, and R. Wallace, Phys. Plasmas 3, 2070 (1996)] were preformed. The upper hemisphere of a DTfilled CH capsule was imploded by 96 NIF beams in a PDD configuration. Asymmetries in both polar and equatorial directions around the capsule were observed, with the former dominating the latter. Analysis of the simulation results indicates that the lack of symmetry in the initial power density profile (during the first 200 ps of the implosion) is a primary cause of latetime asymmetry in the implosion as well as decreased yield. By adjusting the laser pointings, the symmetry and total neutron yield were improved. Simulations with dropped quads (four of the NIF laser system’s 192 beamlines) without repointing worsen the overall symmetry by a factor of 10 (with respect to rms radial variation around the capsule) and reduce neutron yield by a factor of 2. Both of these degraded implosion characteristics are restored by azimuthal repointing of the remaining quads.
This research was supported by US DOE/NNSA, performed at LANL, operated by LANS LLC under Contract No. DEAC5206NA25396. The authors are extremely grateful to Marty Marinak and the rest of the HYDRA team for making their code available to us to perform this work. We also would like to express our gratitude to Larry Suter and Ines Heinz for facilitating computational access to HYDRA.
I. INTRODUCTION
II. ASSESSING POLOIDAL SYMMETRY IN PDD IMPLOSIONS FOR NIF
III. AZIMUTHAL SYMMETRY
IV. CONCLUSIONS
Key Topics
 Neutrons
 8.0
 Inertial confinement
 7.0
 Laser beams
 5.0
 Implosion symmetry
 3.0
 Laser ablation
 3.0
Figures
(Color online) World graph of the NIF chamber identifying the four polar angle rings of quad ports at roughly (1) 23°, (2) 30°, (3) 44°, and (4) 50° from the poles.
(Color online) World graph of the NIF chamber identifying the four polar angle rings of quad ports at roughly (1) 23°, (2) 30°, (3) 44°, and (4) 50° from the poles.
(Color online) (a) Exploding pusher capsule parameters and the temporal laser power profile. (b) The beam pointing geometry showing the polar angle of a laser beam, θ_{0} , the laser beam offset angle, δθ, the polar angle where the offset beam centroid intercepts the outer radius, R, of the capsule, α, and the angle of incidence of the offset beam centroid (as measured from the normal to the capsule surface), ψ, onto the target. A laser beam offset of δθ produces a radial offset, Δr, from the capsule center. The beams also can be defocused by a distance d to increase their size at the target surface.
(Color online) (a) Exploding pusher capsule parameters and the temporal laser power profile. (b) The beam pointing geometry showing the polar angle of a laser beam, θ_{0} , the laser beam offset angle, δθ, the polar angle where the offset beam centroid intercepts the outer radius, R, of the capsule, α, and the angle of incidence of the offset beam centroid (as measured from the normal to the capsule surface), ψ, onto the target. A laser beam offset of δθ produces a radial offset, Δr, from the capsule center. The beams also can be defocused by a distance d to increase their size at the target surface.
(Color online) (a) Normalized centerofmass position and velocity as a function of the polar angle with their respective rms values at t = 1.3 ns. (b) Poloidal % rms variations for centerofmass position and velocity as a function of time. (c) Polar profile of the radially integrated normalized absorbed power density at t = 1.3 ns. All dashed lines correspond to old pointings given in Table I. All solid lines correspond to new pointings given in Table II. (d) Pseudocolor plot of radially integrated absorbed power density as a function of time and polar angle showing the enhanced laser absorption/imprinting at early time.
(Color online) (a) Normalized centerofmass position and velocity as a function of the polar angle with their respective rms values at t = 1.3 ns. (b) Poloidal % rms variations for centerofmass position and velocity as a function of time. (c) Polar profile of the radially integrated normalized absorbed power density at t = 1.3 ns. All dashed lines correspond to old pointings given in Table I. All solid lines correspond to new pointings given in Table II. (d) Pseudocolor plot of radially integrated absorbed power density as a function of time and polar angle showing the enhanced laser absorption/imprinting at early time.
(Color online) NIF capsule parameters and the temporal power profile.
(Color online) NIF capsule parameters and the temporal power profile.
(Color online) Pseudocolor plot of initial absorbed power density as a function of azimuthal and polar angles for (a) clean case, (C); (b) pulled Q11T quad case with power equally distributed to the remaining seven quads in the 50° ring, (P); and (c) repointed version of (b) with only seven 50° quads, (R).
(Color online) Pseudocolor plot of initial absorbed power density as a function of azimuthal and polar angles for (a) clean case, (C); (b) pulled Q11T quad case with power equally distributed to the remaining seven quads in the 50° ring, (P); and (c) repointed version of (b) with only seven 50° quads, (R).
(Color online) 3D Normalized pseudocolor plots of density (right half), radial speed (left half), and mesh profiles at t = 3.3 ns for (a) clean case, (C); (d) Q11T quad pulled with power equally distributed to the remaining 7 quads in 50° ring, (P); and (g) same as (d) but with remaining 7 quads repointed, (R). Side view (middle column) and top view (right column) of normalized pseudocolor surface density plots for bc) clean case, (C); (e) and (f) pull out case, (P). The bulge marks the location of the missing quad; and (h) and (i) repointed case, (R).
(Color online) 3D Normalized pseudocolor plots of density (right half), radial speed (left half), and mesh profiles at t = 3.3 ns for (a) clean case, (C); (d) Q11T quad pulled with power equally distributed to the remaining 7 quads in 50° ring, (P); and (g) same as (d) but with remaining 7 quads repointed, (R). Side view (middle column) and top view (right column) of normalized pseudocolor surface density plots for bc) clean case, (C); (e) and (f) pull out case, (P). The bulge marks the location of the missing quad; and (h) and (i) repointed case, (R).
(Color online) (a) Azimuthal rms variation (%) for centerofmass shell position as a function of time for the three cases. (b) Azimuthal rms variation (%) for centerofmass shell velocity as a function of time for the three cases. (c) Polaraveraged centerofmass shell radial position in microns as a function of azimuthal angle at t = 3.3 ns for the three cases with corresponding %rms values. (d) Polaraveraged centerofmass radial velocity in cm/s as function of azimuthal angle at t = 3.3 ns for the three cases with corresponding %rms values. (e) Azimuthally averaged (solid) and attheazimuthal positionofthedroppedquad (dashed) centerofmass radial position in microns as a function of polar angle at t = 3.3 ns for the three cases with corresponding %rms values. (f) Azimuthally averaged (solid) and attheazimuthalpositionofthe droppedquad (dashed) centerofmass radial velocity in cm/s as a function of polar angle at t = 3.3 ns for the three cases with corresponding %rms values.
(Color online) (a) Azimuthal rms variation (%) for centerofmass shell position as a function of time for the three cases. (b) Azimuthal rms variation (%) for centerofmass shell velocity as a function of time for the three cases. (c) Polaraveraged centerofmass shell radial position in microns as a function of azimuthal angle at t = 3.3 ns for the three cases with corresponding %rms values. (d) Polaraveraged centerofmass radial velocity in cm/s as function of azimuthal angle at t = 3.3 ns for the three cases with corresponding %rms values. (e) Azimuthally averaged (solid) and attheazimuthal positionofthedroppedquad (dashed) centerofmass radial position in microns as a function of polar angle at t = 3.3 ns for the three cases with corresponding %rms values. (f) Azimuthally averaged (solid) and attheazimuthalpositionofthe droppedquad (dashed) centerofmass radial velocity in cm/s as a function of polar angle at t = 3.3 ns for the three cases with corresponding %rms values.
(Color online) Total neutron yield as a function of time for the three cases.
(Color online) Total neutron yield as a function of time for the three cases.
Tables
Laser pointings from Ref. 47.
Laser pointings from Ref. 47.
New laser pointings. Please note that all quads in the 44.5° ring were split in two to improve the symmetry of the implosion.
New laser pointings. Please note that all quads in the 44.5° ring were split in two to improve the symmetry of the implosion.
Laser pointings for backlighting study. Please note that all quads in the 44.5° ring were split in two to improve the symmetry of the implosion.
Laser pointings for backlighting study. Please note that all quads in the 44.5° ring were split in two to improve the symmetry of the implosion.
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