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A polar-drive–ignition design for the National Ignition Facilitya)
a)PaperKI3 2, Bull. Am. Phys. Soc. 56, 180 (2011).
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View: Figures


Image of FIG. 1.
FIG. 1.

(Color online) (a) Baseline target design, consisting of a DT-ice shell surrounded by a solid-CH ablator. The ice layer is shimmed to be thinner at the equator than at the pole. (b) Laser pulse shapes used for the various groups of laser beams. Beams irradiating the mid-latitude region require lower laser power. The groups are identified by their port ring (1–4) and irradiation region [(a)–(c)], as defined in Fig. 2.

Image of FIG. 2.
FIG. 2.

(Color online) Beam-pointing strategy for the polar-drive–ignition design, illustrated with respect to the initial target surface. (a) The four rings of ports per hemisphere (shown as open ellipses) are pointed to the three irradiation regions (polar, mid-latitude, and equatorial, shown as solid squares) as indicated by the red arrows. (b) The beams from port ring 3 are split so that half the quads illuminate the mid-latitude region and half illuminate the equatorial region.

Image of FIG. 3.
FIG. 3.

(a) The illumination pattern for beam group 2b (θ =  30° → 44°) is shown, before (solid) and after (dashed) repointing, at the start of the first picket. Also shown is the illumination pattern after repointing for a super-Gaussian exponent of 5 (dotted). (b) The illumination pattern is shown for beam groups 3c (44.5° → 83°) and 4c (50° → 83°), before (solid) and after (dashed) repointing. These patterns include contributions from all beams in the group and from both hemispheres.

Image of FIG. 4.
FIG. 4.

(Color online) Representative phase-plate envelope intensity profiles in the target plane for (a) the polar, (b) the mid-latitude, and (c) the equatorial regions. Lineouts are given in the vertical direction through the center of the profile. The equatorial spot shape is designed to mitigate the loss of coupling near the equator attributed to oblique incidence and to reduce the amount of energy spilled over the target horizon.

Image of FIG. 5.
FIG. 5.

(Color online) Schematic of the 1-D multi-FM SSD system proposed for the triple-picket portion of the laser pulse. The bandwidth is applied to the pulse in a fiber front end and is distributed across three phase modulators. The bandwidth is dispersed by a grating in the PAM, giving rise to a temporal shear of 360 ps in the y direction across the final 40-cm aperture of the beam, as shown by the simplified plots in the (y, t) plane. (The actual color variation across the beam, depending on the incommensurate modulator frequencies, is more complex than shown.)

Image of FIG. 6.
FIG. 6.

(Color online) Angular spectra of (a) the baseline SSD system and (b) a similar system with the same Δλ1 + Δλ2 + Δλ3 and the same grating dispersion but slightly different modulation frequencies (ν1, ν2, ν3). (Here, Δλi denotes the bandwidth of the ith modulator.) The larger effective half-angle divergence of 100 μrad in (a) smoothes to a lower asymptotic level. The wavelength scale on the top is obtained from the divergence scale using the grating dispersion Δθ/Δλ = 29.3 μrad/Å. The divergence Δθ is referenced to the final beam aperture.

Image of FIG. 7.
FIG. 7.

(Color online) Target gain as a function of the duration of multi-FM SSD beam smoothing relative to the polar laser pulse shape. Triangles indicate the simulated gain that results from terminating the multi-FM SSD smoothing at the indicated time, normalized to a simulation for which the SSD is applied during the entire pulse. These simulations include target and illumination nonuniformities.

Image of FIG. 8.
FIG. 8.

(Color online) Density contour plots at 9.8 ns, the end of the acceleration phase of the current design, simulated by DRACO for three different SSD systems: (a) the 185-GHz (UV) 1-D SSD system present on the NIF, (b) the 0.5-THz multi-FM, 1-D SSD system, and (c) a 1-THz, 2-D SSD system using a single modulator in each direction. The simulations include laser imprint, mispointing, mistiming, power imbalance, and inner and outer ice roughness. The level of imprint reduction is quantified by the rms surface outer-surface perturbation in modes 30–100. The poor smoothing performance of (a) caused the target to fail, whereas the targets in (b) and (c) ignite with gains within 1% of each other.

Image of FIG. 9.
FIG. 9.

(Color online) Density contours in the (r, θ) plane at 4.8 ns, shortly before shock breakout, showing the three shocks generated by the three pickets. The nonuniformities in the initial inner-surface shape are determined by the shim and the ice-roughness spectrum.

Image of FIG. 10.
FIG. 10.

(Color online) Density contours from a DRACO simulation of the polar-drive–ignition design including imprint, power imbalance, ice roughness, beam mispointing and mistiming, and surface roughness at (a) 9.9 ns, the end of the laser pulse, and (b) 10.4 ns, at ignition. In (b) ion-temperature contours are indicated as white lines, labeled in keV.

Image of FIG. 11.
FIG. 11.

(Color online) The Fourier spectrum of the outer shell surface at 9.9 ns, the end of the acceleration phase.

Image of FIG. 12.
FIG. 12.

(Color online) (a) Dependence of 1-D gain of the polar-drive design on the relative drive powers of the five portions of the laser pulse, normalized to their nominal values. (b) Dependence of the 1 D gain on timing errors between various portions of the laser pulse. (If the time interval between picket 1 and picket 2 is increased, the error is positive.)

Image of FIG. 13.
FIG. 13.

(Color online) Sensitivity of target gain to ring pointing error (transverse displacement of the beam axis for all the beams in the ring, in the target plane relative to the design pointing) for each of the polar, mid-latitude, and equatorial regions. The results for the mid-latitude and equatorial regions are given by the averages of their two component port rings.

Image of FIG. 14.
FIG. 14.

(Color online) Normalized target gain as a function of DT-fill-gas temperature. The corresponding DT-gas density is given and is specified to be 0.26 ± 0.06 mg cm−3 for this design.

Image of FIG. 15.
FIG. 15.

(Color online) Target gain as a function of rms ice roughness for the polar-drive design, assuming an ℓ−1 power spectrum (red; “Polar-drive design”) and for the (non-polar-drive) all-DT direct-drive design assuming spectra ∼ℓ−3/4 and ℓ−3/2 (blue; “All-DT”). The polar-drive simulations include surface and laser nonuniformities. The all-DT simulations include only ice roughness.

Image of FIG. 16.
FIG. 16.

(Color online) Dependence of target gain on (a) time-independent beam power imbalance, (b) beam mispointing, and (c) beam mistiming. The symbols represent 2-D DRACO simulations. Each simulation includes at least the nominal NIF power imbalance (8%), mispointing (50 μm), and mistiming (30 ps), as well as the effects of Legendre modes with m ≠ 0. The horizontal scales start from these values.

Image of FIG. 17.
FIG. 17.

(Color online) The dependence of target gain on target offset. Target offset is along the polar axis of symmetry for the simulations (z). Due to the increased computational domain, the perturbations modeled in these simulations include only drive perturbations attributed to beam-repointing port locations.

Image of FIG. 18.
FIG. 18.

(Color online) Schematic of the NIF laser system indicating the upgrades (shown in red) required to accommodate direct-drive ignition. These include a 1-D multi-FM SSD system for the triple pickets preceding the main drive laser pulse and accompanying dispersion gratings, phase plates, polarization smoothing optics, and a cryogenic target-handling system compatible with direct drive.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: A polar-drive–ignition design for the National Ignition Facilitya)