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Crossed-beam energy transfer in direct-drive implosionsa)
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Image of FIG. 1.
FIG. 1.

(a) A 1.5-MJ direct-drive NIF ignition design.3 This design utilizes a triple-picket pulse and releases an energy gain of about 50. (b) Typical cryogenic OMEGA target. This target is a scaled-down version of the design in (a) and optimized for a laser energy up to 30 kJ. (c) Example of a warm OMEGA target (shot 63912). Such targets are a less-expensive alternative to cryogenic OMEGA targets. The warm targets are used to study laser coupling, hydrodynamic stability, hot-spot formation, and other aspects of implosion physics.

Image of FIG. 2.
FIG. 2.

Schematic illustration of a laser-ray geometry with the most energetically efficient CBET in a corona of an implosion target. An incident edge-beam ray (shown in blue) in beam 1 is refracted and turned outward above the critical radius. On its outgoing trajectory, this ray seeds perturbations to an incoming center-beam ray (shown in red) in beam 2 that results in energy transfer from the latter ray to the outgoing ray (also shown in red). The energy transfer occurs near the Mach 1 radius, which is typically located at from 0.2 to 0.3 . As the result of CBET, center-beam rays deliver less energy to the maximum absorption region near the critical radius.

Image of FIG. 3.
FIG. 3.

Reflected light power history measured (thick black line) and simulated using flux-limited transport (green dashed–dotted line), nonlocal transport (blue short-dashed line), and nonlocal transport with CBET (red long-dashed line). The thin black line shows the incident laser power. Note good agreement between the measured power history and the simulated one with CBET.

Image of FIG. 4.
FIG. 4.

Measured (a) and simulated scattered-light spectra for a warm plastic-shell implosion (OMEGA shot 63912). lilac predictions using nonlocal transport and CBET are shown in (b), and simulations without CBET using flux-limited and nonlocal transports are shown in (c) and (d), respectively. The white contours in (a) indicate the shape of the simulated spectrum in (b). The incident light wavelength is presented by the dashed line.

Image of FIG. 5.
FIG. 5.

(a) Neutron-production history measured (black solid line) and simulated with flux-limited transport (green dashed–dotted line), nonlocal transport (blue short-dashed line), and nonlocal transport and CBET (red long-dashed line). The measurements and simulations with CBET show good agreement between bang times, which are estimated as the rise time of the neutron rate. (b) Ablation-front trajectory inferred from x-ray framing camera images20 (black dots), and the trajectories simulated using nonlocal transport with and without CBET (red solid and blue dashed lines, respectively). The simulations with CBET show good agreement with measurements.

Image of FIG. 6.
FIG. 6.

(a) Reflected light power history in a glass-shell implosion (OMEGA shot 51856). For notations, see Fig. 3. (b) Neutron-production history measured and simulated. For notations, see Fig. 5(a). Note good agreement of the measured scattered light and bang time in (a) and (b) with the simulations using CBET and poor agreement with the simulations not using CBET.

Image of FIG. 7.
FIG. 7.

Distributions of time-integrated energy transferred between crossing beams as functions of the relative ray impact parameter in a plastic-shell implosion (OMEGA shot 63702). Distribution of the incident energy is shown by the black solid line. Distribution of the transferred energy for the incoming trajectories is shown by the green dashed–dotted line, for the outgoing trajectories by the blue short-dashed line, and for whole trajectories (including the incoming and outgoing parts) by the red long-dashed line. The negative corresponds to energy losses, and the positive one to energy gains.

Image of FIG. 8.
FIG. 8.

Predicted scattered energy and deposition nonuniformities (rms) as functions of in plastic-shell implosions. The scattered energy is normalized to the incident energy. The simulated energies with and without CBET are shown by the blue solid and long-dashed lines, respectively. The deposition nonuniformities (red short-dashed line) are calculated using the OMEGA beam-port geometry and time averaging over the whole laser pulse.

Image of FIG. 9.
FIG. 9.

Density distributions at maximum compression from 2-D hydrodynamics simulations of implosion targets illuminated by different-sized laser beams: (a) , (b) 0.8, and (c) 0.7. Beam-overlap nonuniformities in the case of small result in asymmetric implosions and degradation of neutron yield. Each simulation shows yield over clean (YOC, which is 2-D yield normalized to 1-D yield).

Image of FIG. 10.
FIG. 10.

Measured profiles of beams with small DPPs at different defocus offsets. The beam profile at best focus is shown by the solid line, and wider beams have increasing defocus offsets. These profiles correspond to , 0.65, 0.74, 0.88, 1.0, and 1.09 (from narrow to wide, respectively).

Image of FIG. 11.
FIG. 11.

Measured and simulated scattered-light spectra for plastic-shell implosions using wide and narrow laser beams ( and 0.5, respectively). The implosion with narrow beams recovers the red-shifted part of the spectrum (indicated by the red ovals), which corresponds to rays that deeply penetrate into the target corona. These rays are not present in the implosion with wide beams () because of CBET. Note good agreement between measured and simulated spectra.

Image of FIG. 12.
FIG. 12.

Scattered-light fractions in implosion experiments using variable-diameter beams. Measurements corresponding to implosions with different are shown by the red solid circles with error bars. Simulation results with and without CBET are shown by the blue open triangles and orange open squares, and approximated by the solid and dashed lines, respectively. The measured fractions are in good agreement with the simulated ones including CBET.

Image of FIG. 13.
FIG. 13.

Bang times in implosion experiments using variable-diameter beams. For notations, see Fig. 12. The measured bang times are in good agreement with the simulated ones including CBET.

Image of FIG. 14.
FIG. 14.

(a) Ablation-front trajectories inferred from x-ray framing-camera images (dots) and simulated (lines) in implosions with wide and narrow beams ( and 0.75, respectively). (b) Measured (black square dots) and simulated with (red line and triangles) and without CBET (blue line and diamonds) implosion velocities as functions of . Higher implosion velocities are achieved with smaller beams in both measurements and simulations.

Image of FIG. 15.
FIG. 15.

Relative neutron yields in experiments using uniform beams and variable-diameter targets. (a) Measured yields normalized to simulations with CBET (circles). (b) The same yields as in (a) but normalized to simulations without CBET and having (squares). The solid lines in (a) and (b) approximate the data points. The dashed line in (a) shows an expected constant relative yield in the case of similar uniformity. The drop of relative yields at is due to beam-overlap nonuniformities.

Image of FIG. 16.
FIG. 16.

Simulated absorption fraction (solid line) as a function of the wavelength separation in a plastic-shell implosion using two-color light. The upper dashed line corresponds to 50%-reduced CBET using one-color light.

Image of FIG. 17.
FIG. 17.

Simulated absorption fractions with (blue solid line and circles) and without (blue long-dashed line and triangles) CBET for imploded plastic shells with different fractions of doped Ge. The effect of CBET is reduced in implosions with a higher-Ge dopant. Hydrodynamic efficiency in implosions with CBET (red short-dashed line and diamonds) is decreased with increasing-Ge dopant.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Crossed-beam energy transfer in direct-drive implosions<sup>a)</sup>